# Idiots guide to electrics



## whitevanwoman

Please could someone either point me in the direction of a good website or post explaining about electrics to a complete dummy re electrics?

All I know is that houses run on 240 volts and car batteries run on 12volts but I don't know the difference between volts / amps / watts etc and don't know where to start to try to calculate how much electricity my various appliances would use on mains hook up. I know that the higher the wattage, the more elec is required eg old style 100w light bulbs use alot more elec cos they are brighter, than old style 40 w bulbs, and that low energy bulbs only use about 11 w and so are much more economical.

But how do I calculate my consumption when not all accessories say on them what wattage they require - sometimes it just says volts and amps. All I know about amps is that 3amps is the bog standard fuse in a house plug and that a kettle or hairdryer needs a higher fuse like 13 amps - or have i got that the wrong way round?

At the moment I have a leisure battery (no idea what size but it says its a heavy duty auto / marine battery, second hand donated by a friend with caravan so no idea of age / condition ) which isn't wired in and so I am just running 12 v appliances direct off that and am trying to work out how long it would last. 

The 12 volt various appliances I have are :

laptop - requires 19v, got an incar charger for it which works fine
mini car heater / fan - says 150 watts on the packing, works when plugged direct into cig lighter but it blew my 4 way adaptor
mobile - incar charger
AA & AAA high speed battery charger (charges flat batteries in approx 20 mins) - has both 240v lead and 12v incar charger
mini oven 
mini kettle
mini fridge - it works with a fan and can apparently used to keep food hot aswell but I guess is would use more elec, it has both 12v and 240 volt cables, it's effectively an elec cool box, fits a couple of milk cartons in or 4 pack of beer


Do I need to have the engine running for some of the things eg mini oven, mini kettle (I remember from school that things that heat up consume much more elec)?

The various items I would like to use if I had hook up (I have a hook up extension cable with 2 sockets, not wired directly into battery) are :

a small 2 bar elec heater which I had in my caravan (only 1 bar works, am guessing it's probably about 800w)
laptop charger
leisure battery charger (apparently it's a fast charger?)
a light with a low energy bulb (11w)
AA battery charger (fast charge)
mini fridge

I also have a small work top 240v oven which would be useful 

I have 2 sockets in my hook up adaptor, what combination of the above 240v items should I avoid using - presumably I shouldn't use the heater and oven at same time, but what about the leisure battery charger and /or mini fridge with other things?

I have a 4w solar panel trickle charger but from what I've read, this isn't going to be much use except to stop the battery from discharging if not used for a while. Is it worth getting a second one and can I use 2 clipped onto the leisure battery at the same time?

Any explanation needs to be very very simple - a friend once tried to explain using the analogy of a river and the width of the river, speed of the flow of water etc but I got very confused. 

Thanks.


----------



## kimbowbill

Jess
you will find a load of information on here, if you go to motorhome problems or knowledge and do a search there, maingate has posted many responses about this, if you cant find anything pm him, i cant help you cos i get confused about it too, lol


----------



## whitevanwoman

kimbowbill said:


> Jess
> you will find a load of information on here, if you go to motorhome problems or knowledge and do a search there, maingate has posted many responses about this, if you cant find anything pm him, i cant help you cos i get confused about it too, lol



I spent a couple of hours last night reading back through various threads but didn't find anything from within the past year or so. I'll try doing a search now, but I'm guessing that if I search for "electrics" I'll get loads of results. 

Any suggestions as to a more specific phrase to search for?


----------



## maingate

John Wickersham has written books on all aspects of doing as self build

Amazon sell them cheaper than elsewhere. Maybe some members have one they could sell you cheap.


----------



## Firefox

Hi Jess 

I bumped a thread which I posted on this,

http://img137.imageshack.us/img137/305/movanowiring.jpg




The diagram shows how the 240V system is separate from the 12V.

Also a formula you will find useful is Watts = Volts x Amps

For example at 240v   40W(Bulb) = 240V x 0.166A

at 12V     40W(Bulb) = 12V  x 3.25A

The way you can use this practically is take the wattage of your appliances and divide by the voltage and it gives the current drawn

eg 40W/12V = 3.25A So you know if you run this bulb for 8 hours you need 8x3.25Ah = 26 Ah of supply. You can compare this to the Ah capacity of your battery(s).

(Edit On the wiring diagram, if you prefer to have a split charge relay, just replace switch A with the relay, and if you want to wire your fridge so it works on 12V while the engine is running, replace switch C with a fridge relay. If I need to run my fridge while moving I usually just plug it into my inverter on 240V. I find it works better off 240V and gas)


----------



## whitevanwoman

maingate said:


> John Wickersham has written books on all aspects of doing as self build
> 
> Amazon sell them cheaper than elsewhere. Maybe some members have one they could sell you cheap.



Thanks for this, will check it out, sounds just the job. Would be very glad of any second hand copies.


----------



## whitevanwoman

Firefox said:


> Hi Jess
> 
> I bumped a thread which I posted on this,
> 
> http://img137.imageshack.us/img137/305/movanowiring.jpg
> 
> View attachment 4114
> 
> The diagram shows how the 240V system is separate from the 12V.
> 
> Also a formula you will find useful is Watts = Volts x Amps
> 
> For example at 240v   40W(Bulb) = 240V x 0.166A
> 
> at 12V     40W(Bulb) = 12V  x 3.25A
> 
> The way you can use this practically is take the wattage of your appliances and divide by the voltage and it gives the current drawn
> 
> eg 40W/12V = 3.25A So you know if you run this bulb for 8 hours you need 8x3.25Ah = 26 Ah of supply. You can compare this to the Ah capacity of your battery(s).
> 
> (Edit On the wiring diagram, if you prefer to have a split charge relay, just replace switch A with the relay, and if you want to wire your fridge so it works on 12V while the engine is running, replace switch C with a fridge relay. If I need to run my fridge while moving I usually just plug it into my inverter on 240V. I find it works better off 240V and gas)



Brilliant, thanks v much. 

But aaaggghhh, maths equations!!! I failed my maths o level (showing my age now) 4 times, I finally got a CSE grade 1 at 18. Dropped physics and chemistry at 13. Even Brian Cox's explanations of science stuff confuse me though I'm interested - did you see him on Jonathan Rosss last night, he put half a million volts? watts? amps? through Ross in a mini Hadron collider type thing and made his hair stand totally on end, amusing but I couldn't understand why a) ross wasn't electrocuted (was he standing on a rubber mat?) b) how touching Ross or the collider thing with a wooden stick decharged them c) why it was dangerous for Ross and Cox to touch each other whilst Ross was still charged. 

And Brian Cox is the best science teacher I've come across so if I can't understand his explanations, that's why I need an idiots guide. 

All I really know about H&S for electrics is that during a thunderstorm the safest place is in the van or lying flat in an open area away from trees, tent poles, brollies etc, and that I should do all my lawn mowing with the flymo and any DIY involving electric sockets / lights (eg, rewiring a lamp, plug, changing bulb etc) in my wellies. 

I once had to have all the wiring (the coil?) in my 2CV replaced because I was using silver foil instead of a fuse which had blown - lesson learned the expensive way but I still don't understand what appliance needs what type of fuse in a house plug. 

Tonights homework will be to sit down with pen and paper and work through this info you've given me - I might actually start putting together a "camper" file with info and notes etc about this sort of stuff plus some of the other hints  & tips from other threads - after the RH meet, I realised that I needed a "Pre-trip check list" and a "Post-trip check list", a Packing list, print outs of info about Trannies, tyre pressures, type of oil, etc. Also my dad told me when I got my first car that I should do a POWER check each week (I never have but its good advice which I'm going to try to start doing) POWER = Petrol, Oil, Water, Electrics, Rubber (although both my vans are Diesel but Dower doesn't work as well as POWER - and putting in petrol instead of diesel is another story for the next meet...)

Am quite proud of yesterday afternoon's efforts which saw a deluxe version of my "bucket with a lid on it" loo - it is now disguised in an old laundry box with a hinged wooden lid which can double up as a seat and, instead of an inflatable neck cushion for a seat, I have a plywood commode type arrangement and there is space in the laundry box to store spare wood shavings, loo roll and air freshener  

Have been giving some thought to a foldaway bunk instead of an air mattress on the floor... it's hard to explain if you haven't seen inside my van, but I'm thinking along the lines of adapting a camp bed / sunlounger, so am on the look out for a second hand one (I'm quite handy with a sewing machine so can replace / recover canvas) if anyone has got one destined for the tip / car boot sale / ebay - happy to give you a few quid for it, cheapest new one I've seen is about £20, and am not even sure that my idea will work so I don't really want to buy a new one just yet.


----------



## NicknClair

Hello,

I would also like to point out that some sites also have restrictions in how much they supply in Amps. Many a time have I witnessed a person hitting the switch on a kettle, then tripping the site!! However the best one by far I saw was someone in there caravan awning with a huge George Foreman Grill, wondering why every time he turned it up fully, he was throwing the trip.

Go with Maingate's suggestion with John's book for understanding the fundimentals with using your bus. 

The only alarm bells ringing from my point of view, is how you wish to use the van when wilding, without making sure you have enough power to do the jobs you want to do. The best calculation I have used (but this is purely a guide/guess and has worked for me so far) is take the wattage of the object in question (e.g a 700W Cooking Power microwave, which uses 920W total when in use), divide it by the supply voltage, divide again by 60 (this puts in a time element of consumption), then times it by the length of time you wish to run the object.
So for purpose of demonstration;

940W divide 12V = 78.3A (please note that a use of an inverter is used in this, so will use approx 20W for it's own operation based on a 1500W unit)
78.3A divide by 60 = 1.305
1.305 times by 10 mins (the average time for a Jacket Spud) = 13.05ah of power used.

Now you have a rough idea of consumption, PLEASE also bear in mind of battery capacity, as one 100ah battery will not give you 100ah of usable power. Most batteries only have a certain percentage of operation over 12v (the average is 25%), hence on all previous posts I have ever made, I've always dummed on about battery capacity (even before suggesting on how you charge them). Again as an example, one 100ah battery may only give 25ah of usable power, so as you can see from the above calculation, one jacket spud has killed half of your usable battery capacity!!!


----------



## oldish hippy

here is a zig wiring diagram 

as regard heater get a small oi;ed filled heater 700 watt then if you on hook up you can leave it on low overnight should warm up van nicely and it only small draw  i did a post on here about dunelm heater ony i think 15 pounds


----------



## whitevanwoman

oldish hippy said:


> here is a zig wiring diagram View attachment 4115
> 
> as regard heater get a small oi;ed filled heater 700 watt then if you on hook up you can leave it on low overnight should warm up van nicely and it only small draw  i did a post on here about dunelm heater ony i think 15 pounds



Cheers, I got one of the Dunelm heaters (and said thank you), not used it yet, it was for my bedroom but it's not been below zero here except once or twice this winter. 

I had the other heater in my caravan and it's really light, and I thought one bar would be sufficient to warm up the van pre bed time. I wouldn't have a heater on overnight because, as yet, the van isn't insulated so it would just be a waste, and I'm used to sleeping in an unheated bedroom in Cumbria, 200 m above sea level, and I have a mountaineering down sleeping bag so am not likely to be cold once in bed. 

Plus, the dog provides me with an XL "always on" hot water bottle!


----------



## Firefox

> Even Brian Cox's explanations of science stuff confuse me though I'm  interested - did you see him on Jonathan Rosss last night, he put half a  million volts? watts? amps? through Ross in a mini Hadron collider type  thing and made his hair stand totally on end, amusing but I couldn't  understand why a) ross wasn't electrocuted (was he standing on a rubber  mat?) b) how touching Ross or the collider thing with a wooden stick  decharged them c) why it was dangerous for Ross and Cox to touch each  other whilst Ross was still charged.




I didn't see the Ross item as I don't watch TV :lol-053: Current (amps) is what kills you. It sounds like Ross was charged up very slowly using a high voltage, and ended up with a static charge. Since the current wasn't flowing anywhere fast it is safe. I think if he was discharged quickly there could be a problem but the wood stick being a poor conductor could discharge him slowly. Even a stick can carry current if the pressure (voltage) is high enough.

Voltage is electrical pressure. It doesn't flow through anything but you put it across (either side of an object) it makes the current flow. The voltage can be high or low, but if the current is small it is safe in personal terms. Voltage = Current x Resistance is another equation, but the useful one for motorhome electrics is Watts = Volts x Amps.

You will often see newspaper journalists get it wrong when they say "the man was killed when 40,000V went through him". He was actually killed by the current going through him caused by the electrical pressure of 40,000V, but the trouble is that misleading statements are not much good to help understand the difference between current and voltage  :rolleyes2:


----------



## whitevanwoman

Firefox said:


> I didn't see the Ross item as I don't watch TV :lol-053: Current (amps) is what kills you. It sounds like Ross was charged up very slowly using a high voltage, and ended up with a static charge. Since the current wasn't flowing anywhere fast it is safe. I think if he was discharged quickly there could be a problem but the wood stick being a poor conductor could discharge him slowly. Even a stick can carry current if the pressure (voltage) is high enough.
> 
> Voltage is electrical pressure. It doesn't flow through anything but you put it across (either side of an object) it makes the current flow. The voltage can be high or low, but if the current is small it is safe in personal terms. Voltage = Current x Resistance is another equation, but the useful one for motorhome electrics is Watts = Volts x Amps.
> 
> You will often see newspaper journalists get it wrong when they say "the man was killed when 40,000V went through him". He was actually killed by the current going through him caused by the electrical pressure of 40,000V, but the trouble is that misleading statements are not much good to help understand the difference between current and voltage  :rolleyes2:



trying to digest this, word by word, to understand...


----------



## oldish hippy

this might be of some use to you it explains som,me of it most straitforwardly Leisure Battery Install - Goatboy's VW Site


----------



## Firefox

If it helps (or confuses!!), coulombs is charge (or electrons). What Ross had was a lot of charge. So this has to be let off slowly through the stick to keep the current low.

Current is the flow of charge (or rate over time). If 1 coulomb flows in 1 second that is called a current of 1 amp, or put another way 1 Amp = 1 coulomb per second.

Volts = Joules/coulomb  and Amps = coulombs/second . If you multiply J/C  X C/S, the C cancels out and you get Joules/second = power = watts.

Hence Watts = Volts X Amps

A current of 0.3A to 1A could kill someone if it went through the heart or brain as it messes up all the very small body currents which keep us going.


----------



## AuldTam

Firefox said:


> If it helps (or confuses!!), coulombs is charge (or electrons). What Ross had was a lot of charge. So this has to be let off slowly through the stick to keep the current low.
> 
> Current is the flow of charge (or rate over time). If 1 coulomb flows in 1 second that is called a current of 1 amp, or put another way 1 Amp = 1 coulomb per second.
> 
> Volts = Joules/coulomb  and Amps = coulombs/second . If you multiply J/C  X C/S, the C cancels out and you get Joules/second = power = watts.
> 
> Hence Watts = Volts X Amps
> 
> A current of 0.3A to 1A could kill someone if it went through the heart or brain as it messes up all the very small body currents which keep us going.




That explains it perfectly, thankyou.  So Sue Ellen didnt shoot JR then, who'd a thunk it!


----------



## scampa

You can still see last nights' Jonathan Ross Show on ITV Player, here....  (To see Brian Cox electrify him, fast-forward to approx 39 minutes).

Video - ITV Player

There's some useful info on this thread, but a lot of it will be much too confusing for a novice. As others have already said, the best thing would be to get a basic grasp of Ohms Law which states (in basic laymans terms):

Power (in Watts) is equal to Voltage (Volts) multiplied by Current (Amps).   

So Watts = Volts multiplied by Amps.

Volts = Watts divided by Amps.

Amps = Watts divided by Volts.

(The voltage, as you know, will normally be 240volts in your house (in UK), and 12volts (usually) in your car or motorhome).

If in any doubts about electrics it's always best to ask advice or let a competent person do the job.... They're not called "killer-volts" for nothing!

And remember, when you apply Ohms-Law to campervans, it's known as " Motor-Ohms Law"!!


----------



## Firefox

scampa said:


> There's some useful info on this thread, but a lot of it will be much too confusing for a novice. As others have already said, the best thing would be to get a basic grasp of Ohms Law which states (in basic laymans terms):
> 
> Power (in Watts) is equal to Voltage (Volts) multiplied by Current (Amps).
> 
> So Watts = Volts multiplied by Amps.
> 
> Volts = Watts divided by Amps.
> 
> Amps = Watts divided by Volts.



Actually the above *Watts* law.

Ohms law is the other equation I gave : V= IR or Voltage = current x resistance

Otherwise, all present and correct :lol-053:

For those who have problems with W = VxA,      V=W/A,    and A= W/V.... think off 6=3x2, 3=6/2 , and 2=6/3 

Sometimes putting in example numbers instead of letters helps to see the relationship :cool1:


----------



## scampa

Firefox said:


> Actually the above *Watts* law.



Yes, strictly you are correct, but "Motor-Watts Law" doesn't have the same ring to it!!

For anyone who really doesn't like having to work things out themselves, here's a link to an instant calculator for Ohms and Watts questions....

Ohm's Law / Watt's Law Calculator


----------



## solarman

here are some more links  which might help
20. Self-Build DIY Camper Van. 240V AC electrics, consumer unit, inverter, and sockets - YouTube
Basic Electronics
Electrics Help Tips and Advice | Diy Fix It
Campervan, Motorhome, Motorcaravan electrics shop.

Basic Electrical Theory

Basic Electrical Theory


----------



## kimbowbill

if i were you Jess, i would try the "trial and error" method go out in your van, use you power and roughly time it how long you get, for eg, when i went to RH i used my lights, water pump and tv over 4 days on 2 leisures


----------



## Firefox

kimbowbill said:


> if i were you Jess, i would try the "trial and error" method go out in your van, use you power and roughly time it how long you get, for eg, when i went to RH i used my lights, water pump and tv over 4 days on 2 leisures



This is also useful espec if you have older or second hand batteries and you don't know how much capacity is left in them. When trying it out don't let the leisure battery voltage drop much below 11.8-12v though as deep discharge is not good for the battery. That voltage is about 50% charge or a bit less.


----------



## Viktor

Ok Ladies... I know how you feel about electricity...feel much the same way and I have O level physics!


Assuming you have 1 x 12volt car battery (or leisure battery) fully charged:
Rough guide if you have a 300 Watt inverter such as the 'Powering RINV300' which is about £60 from a camping store -
(I would always suggest running the engine when using an inverter so as not to drain the battery just as quick as below)


Computer (monitor and printer) 200w to 400w           - 1.5 hours
Electric Blanket  200w - 400w                                   - 1.5 hours
Portable Fan 50w - 200w                                          - 3 hours
Laptop  60w - 95w                                                   - 4 hours
Small fridge (14cu. ft)  200w - 300w                         - 1.5 hours
TV (19") 100w - 160w                                              - 3 hours

Now you see why generators and EHU's are popular....an electric blanket on hookup thats luxury lol...most of us would use a hot water bottle!


----------



## whitevanwoman

Firefox said:


> Actually the above *Watts* law.
> 
> Ohms law is the other equation I gave : V= IR or Voltage = current x resistance
> 
> Otherwise, all present and correct :lol-053:
> 
> For those who have problems with W = VxA,      V=W/A,    and A= W/V.... think off 6=3x2, 3=6/2 , and 2=6/3
> 
> Sometimes putting in example numbers instead of letters helps to see the relationship :cool1:




:scared:  

I'm having problems with both numbers and letters!!  I think it's gonna take me some time and some reading to get my head round all this.


----------



## whitevanwoman

Thanks to everyone for the suggestions and comments and advice, much appreciated. I think I'm going to have to spend some time going right back to basics, some of the links look useful and I've found a few others too.

I'm not going to be doing any work on the electrics myself, but after blowing my incar 4 way adaptor by plugging a 12v mini fan heater through it, I realised that I am going to need to properly understand the basic principles of electricity so as not to do further damage or trip the elec when hooked up and I need to be careful about using high powered appliances etc. 

I'm not going to be wanting luxuries like microwave / hair dryer / straighteners etc but on the other hand, I don't want to have to grope around by torchlight if the leisure battery dies when simply changing the lights from florescent to led would double or triple the length of time the battery will last. 

I will eventually be getting my local garage to wire the leisure battery in using this relay switch thing that is apparently what I need. But not until I've decided on the layout and where the battery will be best located. But in the meantime it is like Kimbowbill said, a good way of testing the various different gadgets etc.


----------



## scampa

whitevanwoman said:


> :scared:
> 
> I'm having problems with both numbers and letters!!  I think it's gonna take me some time and some reading to get my head round all this.



It can be difficult trying to explain the basics in only a few written sentences or posts without causing confusion.   Maingates' suggestion of getting hold of one of the books by John Wickersham would be a good idea. The books explain it with diagrams, pictures and real-life examples to make it easier to understand. Plus of course, other chapters in these books cover many other areas such as gas, water systems, accessories and self-build etc.

Look online at Amazon, Ebay etc.  Here's an example on Ebay......

Haynes The Motorcaravan Manual by John Wickersham. Motorhomes.. | eBay


----------



## Tony Lee

> that's why I need an idiots guide.



Sorry, there is no idiot's guide to motorhome electrics and nothing makes that plainer than the simple fact that a large proportion of automotive electricians haven't much of a clue about the inner workings of a decent MH system, and an even larger proportion of normal electricians are similarly ignorant - and they do know Ohms law.

If you are serious about wanting to learn a bit more, then I strongly suggest you ignore most of this thread and instead do some research on the internet and be prepared to learn slowly at first and then faster as you pick up some basic stuff -- 

or do what most RVers do and just hope that nothing breaks down and if it does, call in the experts.

Psssst  The 12volt Side of Life (Part 1) might get you started.


----------



## Firefox

You don't need to know that much about electricity  to wire up a van.

The problem is is knowing exactly which bits you do need to know. That's why a good read round everything will be helpful. The most important things are seperating the 240v and 12v systems, knowing what size of currents are expected and hence how thick to make the various wires, and also providing fuses, RCD's, earths and insulation to protect your various circuits.


----------



## whitevanwoman

I have a 100 W inverter which plugs into cig lighter for normal 3 pin house plugs and am trying to figure out the wattage of my new android tablet, I'm guessing it will be under 100w but just wanted to be sure. But I can't work out what numbers to use in the W = V x A equation that Firefox posted earlier (I am really dumb with maths, failed O level 4 times). 

On the tablet plug it says Input : 100-240VAC0.3A, Output 9V---2000mA

so would this be right... watts = 240 x 0.3  therefore watts = 72  ?


----------



## Viktor

That's what I would have thought WVW 72 watts.....but I would also need a second opinion.


----------



## discostewie

72 watts is correct.

If you have an iPhone or Android phone, there are resistance calculators available which can make life a lot easier.


----------



## whitevanwoman

discostewie said:


> 72 watts is correct.
> 
> If you have an iPhone or Android phone, there are resistance calculators available which can make life a lot easier.



Thank you. Resistance calculator - is that an app?


----------



## discostewie

Yes it is, here's a link to an iPhone app:
App Store - Resistor Calculator


----------



## whitevanwoman

discostewie said:


> Yes it is, here's a link to an iPhone app:
> App Store - Resistor Calculator



Cheers, I'm on Android so I'll check out android market.


----------



## scampa

Here's a link to a handy little online auto-calculator that I posted earlier.

Ohm's Law / Watt's Law Calculator

You just enter the two values that you know (in WVWs' example above, the voltage and current) and it will give you the answer, with no maths needed!


----------



## whitevanwoman

scampa said:


> Here's a link to a handy little online auto-calculator that I posted earlier.
> 
> Ohm's Law / Watt's Law Calculator
> 
> You just enter the two values that you know (in WVWs' example above, the voltage and current) and it will give you the answer, with no maths needed!



Thanks, just had a look. What is ohms?


----------



## scampa

whitevanwoman said:


> Thanks, just had a look. What is ohms?



Ohms is the unit of Electrical Resistance in a circuit.

The Resistance very very basically is a measure of how easily a current will flow through a cable, and depends on things such as type of material (eg copper), length and diameter of the cable, temperature etc.

For example, the longer the length of cable, the higher the Resistance.

(I'm tempted to repeat my Motor-Ohms joke, but I won't!)
_
PS.  For the calculation that you needed earlier, you can just ignore the Ohms figure, it was just the Watts that you were looking for._


----------



## Canalsman

There's an analogy for resistance that's quite good ...

Imagine the wire is a length of water pipe.

Small pipes will only carry small amounts of water before you have to apply pressure to increase the rate of flow. The reason you have to apply pressure is because there's resistance to flow.

To make it easier for the water to flow, make the pipe bigger. This means there's less resistance.

The same with wires or cables. Make them bigger, and there's less resistance ...


----------



## oldish hippy

well i  put in 50 mm armoured cable got a chunk of that lying around do u think that will do to wire up les battery lol


----------



## Bigpeetee

For us peasants without a posh fone, you can download the Electronics Assistant

Electronics 2000 | Downloads - FREE software including Electronics Assistant & EPE Index

The power and resistance parts are particularly useful.


----------



## Bigpeetee

whitevanwoman said:


> I have a 100 W inverter which plugs into cig lighter for normal 3 pin house plugs and am trying to figure out the wattage of my new android tablet, I'm guessing it will be under 100w but just wanted to be sure. But I can't work out what numbers to use in the W = V x A equation that Firefox posted earlier (I am really dumb with maths, failed O level 4 times).
> 
> On the tablet plug it says Input : 100-240VAC0.3A, Output 9V---2000mA
> 
> so would this be right... watts = 240 x 0.3  therefore watts = 72  ?



Sorry guys but wrong!!!

As it's a universal charger the 0.3A is at 100v, the lower the voltage the higher the current.

therefore it's 30w input and 18w output (9v x 2A) it will dissipate 12w as heat on full load:dance:


----------



## whitevanwoman

Bigpeetee said:


> Sorry guys but wrong!!!
> 
> As it's a universal charger the 0.3A is at 100v, the lower the voltage the higher the current.
> 
> therefore it's 30w input and 18w output (9v x 2A) it will dissipate 12w as heat on full load:dance:



I think I understand, except for "it will dissipate 12w as heat on full load". But basically it's safe to use. 

Thanks Bigpeetee. So for other appliances, instead of using 240v in the equation, I use 100v?


----------



## Firefox

Generally use 12V and the wattage if you can.

This gives you the current drawn at 12V from your battery.

Add on 20% for inverter losses (for a small inverter) and 20% for adapter losses (if appropriate)

eg 9V at 2A = 9x2 = 18 W (or use the table)

18 + 40%(18) = 25W (But 30 in this case!)

30 W at 12V = 30/12 = 2.5 Amps ( or use the table)

Now compare that against your Amp hour for battery. eg 60Amp hours x 50% discharge = 30 Amp hours .... 30/2.5 = 12 hours use


----------



## Bigpeetee

Not so simple!!

If it's a universal ie about 100-240 use the lowest number, if it's 220-240 use 240

Most wall warts as they've been named only drawer lowish power ie less than 100w as a rule, the power supply on a cable sometimes drawer more. But say my laptop, it claims to be 85w but it's only when flat and you're trying to run it as well. Most times much less.

When I worked on a large job in London, if you added together the wattage on the plate of all the PC's it was astronomical, but they usually ran at 30% of the plate value, temporarily going to a higher power.

As a designer, I used to over rate the power supplies to ensure they lasted and weren't running flat out.

It's like most things, take a car for instance, it might do 120 mph, but you wouldn't run it at that speed all day long. (unless you were a German!!)


----------



## Firefox

This was a bit of a confusing example because the charger listed a range of input voltage and an inverter too.

But usually you will have an appliance of known wattage

eg 60W bulb

Divide by 12 to get Amps

60W/12V = 5A (or use the table 12V X 5A = 60W)

Add something in for inverter losses (20% for small inverter)

1.2 x 5 = 6Amps

You have for example a 110Ah battery fully charged

110 x 0.5 = 55Ah capacity

55Ah/6Amps = 9 hours or so


Same kind of calculation every time 

It's only a guideline but no sense getting much more complicated than this, cos you don't know for certain how good/old your battery is and what the charge level is. An alternator (split relay) charge very rarely gives 100% charge.


----------



## whitevanwoman

Thanks both of you for your replies but I'm sorry I am now completely baffled. I need to sit down and try to work it out with a piece of paper. Not quite sure where the 2A comes from in Firefox's first post (I am stupid where numbers are concerned!) 

I've ordered an in car charger for the android tablet but it is coming from China so 3-4 weeks delivery and I'm away in the van next weekend. I wasn't planning on using the inverter on the leisure battery (which is 85mA and second hand so am only really using it to power LED lights - will be getting a new one when finances permit and the more pressing jobs on the Tranny have been done) but plugged into the cig lighter to the vehicle battery (which incidentally I've now discovered is located under the driver's seat!) when driving to charge and to use as sat nav and music player. 

(Sssshh, don't tell anyone but when I'm away next weekend, I'll be on a campsite and will have hookup....  it's closed for the season but competitors in the dog comp I'm going to can use it for £5/night. Hook up and water only though)


----------



## Firefox

whitevanwoman said:


> Thanks both of you for your replies but I'm sorry I am now completely baffled. I need to sit down and try to work it out with a piece of paper. Not quite sure where the 2A comes from in Firefox's first post (I am stupid where numbers are concerned!)




Don't worry you are not stupid... it's all part of the cryptic world of electrics!

The output was 9v at 2000mA ( = 2000 milli Amps = 2 Amps). 1000ma = 1 Amp,  like 1000ml = 1 litre etc 

Once you have done a few you get use to the size of the numbers:


Assuming a 100Ah battery (50Ah practical capacity)

An Led bulb at 6W = 0.5 A .... nothing hardly... could run for days

A small TV at 24W = 2 A... can run that for ages, well  a day or so

A halogen bulb at 60W = 5A... that's beginning to be a draw on your battery, 12 hours?

A fridge at 100W = 10 A...  that could flatten your battery in 6 hours if it is a warm day (this has happened to many!)

A fan heater at 500W = 42 A   :scared:  :scared: :scared:   Poor mister battery will be flat in 20 mins. In theory you can get an hour but this draw is so high the battery will give up sooner and perhaps recover a bit.


----------



## whitevanwoman

Thanks, that makes sense now. But why use the output numbers on the tablet charger plug instead of the input numbers? Surely it should be the input as that is what is coming into the tablet through the inverter from the battery?  And if this is the case, why are there output numbers? Where would this output be going if there are no accessories plugged into the tablet?


----------



## Firefox

You can use either, kind of , because if the transformer and and inverter were perfect then

*Watts output* (= Volts output x Amps output)  *=  Watts input* (= Volts input x Amps input)


In practice their are losses and 

Watts output + losses = Watts input

As long as you allow for some losses you'll be OK 

What you really want to get at is the Amps input at 12V to see how long you can use your thingie for

Amps input = (Watts input )/12  or  Amps input =   (Watts output + losses)/12


*But in really simple terms if you just take the wattage and divide by 12, and add something extra for inverter/charger losses as above you'll get the amps to measure against your battery capacity.

If you don't have any inverter or charger, just divide the wattage by 12 and you won't go far wrong!*


----------



## whitevanwoman

I think I get that, it's filtering through slowly....





Firefox said:


> What you really want to get at is the Amps input at 12V to see how long you can use your thingie for



For next weekend's trip I'm only planning to use the tablet with the inverter whilst driving, the rest of the time I can charge the tablet through hookup.  Presumably I can use it for as long as I want whilst the engine's running...?

But I'm also presuming that in future, once I've got an in car charger and don't need the inverter, I'll still need to do the maths if I want to charge/power the tablet direct off the leisure battery using a 12v adaptor and croc clips. And not using the inverter will mean a little less drain but not significantly?


----------



## Firefox

Yes and yes and yes.

If I want to use my fridge when running, I just switch on the inverter and plug into the 240v. The fridge does work direct off 12v too, but I find it's a little better on 240 (or gas).

Eventually if you use your van a lot, two leisure batteries are cool, cos then you can park up for several days and not really have to worry about tv, gadgets, lights or accessories


----------



## whitevanwoman

Thank you O Wise One  :bow:


Roll on summer and solar power.


----------

