Converting mains current draw to 12v

[BillyPants]

One thing I never got the hang of was converting the draw at mains, to what will be likely draw at 12v when run through an inverter.
My laptop charger says it draws 1.5 amps at 230 volts, and puts out 4.7 at 19 volts. It draws about 6 amps when added in with the inverter, so maybe 5.2 amps by itself?
So, when looking at any mains device, how do you work out what draw it will pull at 12 volt? Anyone know?

 

[FULL TIMER]

do you not just divide the wattage of the mains powered item by 12 instead of 240, should be close enough to give you the amps

 

[ivecotrucker]

Add on at least +15% for power loss on conversion, preferably at least 15% & maybe more.

 

[BillyPants]

do you not just divide the wattage of the mains powered item by 12 instead of 240, should be close enough to give you the amps

Well in the example, 240 volts and 1.5 amps is 360 watts, so 360 divided by 12 volts is 30 amps, which is clearly wrong?

 

[Smaug]

Add on at least +15% for power loss on conversion, preferably at least 15% & maybe more.

An inverter going from 12v DC to 240v ac will be around 15%, but transforming from 240 > 12 (plus rectification & posibly some smoothing) is probably 10% loss, possibly less. Anyone using an inverter & then deriving a DC supply will have BOTH losses added.

You are far better going for a DC-DC converter unit & best of all, select devices that are designed to work off 12v or possibly a 5v USB supply.

But simply speaking thw wattage in & out are similar (ignoring losses) so divide by 240 for mains amps & by 12v for DC amps. You will then start to understand why leisure batteries go flat so quick with inverters running TV's or laptops.

 

[shortcircuit]

Well in the example, 240 volts and 1.5 amps is 360 watts, so 360 divided by 12 volts is 30 amps, which is clearly wrong?

And how is it wrong? Add a bit for losses and it would be about 35>40A

 

[BillyPants]

And how is it wrong? Add a bit for losses and it would be about 35>40A

Because when running, I measure a 7 amp drain with both laptop and inverter. The length of time my battery lasts shows that to be about right too.

 

[BillyPants]

An inverter going from 12v DC to 240v ac will be around 15%, but transforming from 240 > 12 (plus rectification & posibly some smoothing) is probably 10% loss, possibly less. Anyone using an inverter & then deriving a DC supply will have BOTH losses added.

You are far better going for a DC-DC converter unit & best of all, select devices that are designed to work off 12v or possibly a 5v USB supply.

But simply speaking thw wattage in & out are similar (ignoring losses) so divide by 240 for mains amps & by 12v for DC amps. You will then start to understand why leisure batteries go flat so quick with inverters running TV's or laptops.

So dividing the mains 1.5 amps, by 12 volts, gives 8amps, although current draw with inverter is about 7, but at least it's close enough.
So, if a device is rated at 195 watts, that's 1.25 amps, yet that gives a higher reading at 12 volts, of 9.6 amps?

 

[maingate]

If you put an Ammeter in the circuit you will see what the draw is.

I have just had a look on the Maplin website but they no longer seem to sell the Car Current Tester. It is a really useful bit of kit. You remove the blade fuse in the circuit you are checking, put that fuse into the tester and then plug the tester into the fuse holder (where you took the blade fuse from). It gives a digital readout of the amps being used.

 

[BillyPants]

If you put an Ammeter in the circuit you will see what the draw is.

I have just had a look on the Maplin website but they no longer seem to sell the Car Current Tester. It is a really useful bit of kit. You remove the blade fuse in the circuit you are checking, put that fuse into the tester and then plug the tester into the fuse holder (where you took the blade fuse from). It gives a digital readout of the amps being used.

Hi I have one of those, I use it with croc clips on the end though for testing form battery to inverter so I can see the whole amount coming out. I'm after a new computer but I'm always wondering how to determine what the draw will be through the inverter. I didn't know a way of measuring or predicting it.

 

[shortcircuit]

I would suspect the 360 watts is the power supply at full output and is a lot of consumption for a laptop. 7amps at 12 volts is 84 watts and nearer the power used by the laptop.

 

[maingate]

Hi I have one of those, I use it with croc clips on the end though for testing form battery to inverter so I can see the whole amount coming out. I'm after a new computer but I'm always wondering how to determine what the draw will be through the inverter. I didn't know a way of measuring or predicting it.

Why use a 240/12 volt inverter in the first place?

It is more efficient to use a 12/19 DC volt inverter, there are plenty of them about. I stopped using a laptop and use a netbook which is rated at 40 W as opposed to a 65 W laptop.

A lot of people now only use Smartphones, Ipads etc these days.

 

[Smaug]

Get yourself a 12v lappy or netbook & power it direct from the ciggy lighter, by far the lowest power needed as there are no conversion losses.

 

[BillyPants]

I use a laptop for work 40 hours a week so I need a decent one, and I process photos so I need a fair size and decent screen. I really fancy an imac but I don't know what sort of power it pulls. I've got confused already with the formulas here!! lol

 

[maingate]

I use a laptop for work 40 hours a week so I need a decent one, and I process photos so I need a fair size and decent screen. I really fancy an imac but I don't know what sort of power it pulls. I've got confused already with the formulas here!! lol

Turn it over and it should state the output in watts. My last one was 65W and I think most would be around that figure. The main thing you need to consider is how long the battery life is. That was part of my reasoning to get a netbook because they generally are much better than the average laptop. They do just about everything a laptop does except have a DVD drive built in. An external drive is quite cheap though.

 

[BillyPants]

Turn it over and it should state the output in watts. My last one was 65W and I think most would be around that figure. The main thing you need to consider is how long the battery life is. That was part of my reasoning to get a netbook because they generally are much better than the average laptop. They do just about everything a laptop does except have a DVD drive built in. An external drive is quite cheap though.

There's no wattage marked, just 4.75 amps at 19 volts. I can't run a netbook because of the interface I use at work. I've got an 18.5 inch lappie and Ideally I'd like bigger, hence thinking of an imac. The imac says it draws 170 watts at mains but I've no idea what that is through an inverter.

 

[maingate]

There's no wattage marked, just 4.75 amps at 19 volts. I can't run a netbook because of the interface I use at work. I've got an 18.5 inch lappie and Ideally I'd like bigger, hence thinking of an imac. The imac says it draws 170 watts at mains but I've no idea what that is through an inverter.

Just working on a 12 volt supply (and discounting efficiency losses), if you use a 12V/19V inverter then the amps used will be 7.5 amps. This seems quite high but there again 4.75 amps @ 19 volts seems high. You will need a hookup , a big bank of batteries or plenty of solar power if you are going to spend long periods on the laptop.

My advice ........ buy a caravan and use sites for unlimited power. :raofl:


Sorry for being cheeky but most of use only use computers to get on line for short spells. In your case, you will always have to keep an eye on your leisure battery situation.

 

[BillyPants]

Just working on a 12 volt supply (and discounting efficiency losses), if you use a 12V/19V inverter then the amps used will be 7.5 amps. This seems quite high but there again 4.75 amps @ 19 volts seems high. You will need a hookup , a big bank of batteries or plenty of solar power if you are going to spend long periods on the laptop.

My advice ........ buy a caravan and use sites for unlimited power. :raofl:


Sorry for being cheeky but most of use only use computers to get on line for short spells. In your case, you will always have to keep an eye on your leisure battery situation.

No worries, you don't me so you don't know the trouble I've had getting power sorted in this van. I've now got 2 x Trojan 225 ahrs monobloc's which run through a 1000 watt pure sine wave inverter. I've been doing this for more than 6 years so used to dealing with the power, the problem I've got is simply understanding how to work out power draw from a device.
If a device uses 1.5 amps from mains, or, if it uses 170 watts at 230 volts, how to work out what it will draw when put through an inverter. That's my only stumbling block at the moment. I was hoping there'd be a simple calculation.

 

[shortcircuit]

The simple calculation is to divide the watts by 12 and that will be roughly near the current. Remember the load Watts is the same in or out of the inverter (forgetting losses) Current for 170w @ 230 volts = 170/230=0.74amps. Current @ 12 volts = 170/12= 14.17amps

If you are using the 1000watt inverter you will be wasting a lot of power just running the inverter and reducing battery life having said that you have a large battery bank 2@225 A/h= 450A/h (@12volt?? they are not 6volt batteries?). I have a 300w inverter and it runs tv, laptop etc

 

[Smaug]

Billy,

I think you need to look at a 240v genny to support a powerful mains supply for that sort of requirement, or work in a pub or hotel all day using their power!

Back in the day when I was working we used to arrange meetings in hotels so that everyone could get together, all the lappies were plugged in via a multisocket or two & we ordered a couple of rounds of coffee & buns to keep the manageement happy. It was much cheaper than having an office or hiring a meeting room & was warm & with free power. Coffee was pricey tho! :lol-053: