Going from 15v to 12v

[MonkehBoi]

Hi,

I'm new to the forum and not too knowledgeable about electronics (hence the question) so I'll be as detailed as I can, but bear with me....

I have an electric cooler box which is powered from the cars 12volt DC power supply. The thing is I want to be able to just plug it in to a normal mains outlet as I work away a lot during the year with a group of people and we like to have cold beers waiting for us back at the the crappy hotels we stay in.

Anyway, I also have an old laptop power supply with the connector at the end snapped off so I thought this would be ideal to use. The problem is it only goes as low as 15volts DC.

I thought of just buying a DC transformer that you can buy from anywhere for a a couple of pounds but they only provide 500mA or 1000mA of current. The electric cooler box is rated at 60watts (and if my maths are correct that means it requires something like 5 to 6 amps???) so that wasn't feasible.

The laptop power supply has a peak output of 77 watts so should be able to cope with the cooler box.

My question is what can I solder in between the socket for the cooler to plug into and the laptop power supply. I thought of just looking up a resistor(s) to knock it down from 15 to 12 volts but a friend suggested that because of the high current they'd probably burn out very quickly.

As I said, I'm not very knowledgeable about electronics so would appreciate the help. I'm just hoping to nip into Maplins, get the part(s) I need, come home and solder them together.

Thanks in advance.

 

[ericgibbs]

hi,
If the cooler is designed for 12Vdc and its rated at 60W that is 5 amps'

To drop from 15v to 12V at 5amps , that is drop 3volts at 5amps requires a 0.6R resistor, rated at LEAST 15Watts.

To be sure it didnt get overheated I would use a 25Watt or higher resistor.

 

[MikeMl]

With the engine running, the alternator in modern cars gets the battery voltage up to 14.7V. The cooler was designed with this in mind. I think the cooler will work just fine on 15V.

 

[Hero999]

Yes, if it's designed to be connected to an automotive system then it will be fine on 15V, as long as it's no more than 15V.

 

[ericgibbs]

hi MonkehBoi
The question was 'can I run a cooler box' designed to operate a vehicles 12V supply from 15Vdc.
As pointed out, the cooler will most likely accept the 14.5Vmax that occurs when the battery is being charged.

BUT.
Its a 12V 60Watt cooler, so it will draw 5 amps from a 12V supply.
Assuming the cooler absorption heater is resistive, that makes the resistance of the heater approx 2.4R.

If we now power the cooler 2.4R from 15Vdc it will try to draw 6.25 amps from the supply, this works out as 15V * 6.25A = 93 Watts.
You stated that the 'peak' wattage quoted for the old laptop power supply is only 77 Watts.!

IMHO the old psu will not be suitable for powering the cooler directly.

If you add the 0.6R as I suggested earlier making a total resistance of 3R, that gives 5 amps at 15V, which is 75 watts... which I consider too close to the peak wattage quoted. So IMO its not suitable

I appreciate the resistance of the cooler heating element would rise a little due to the extra heating of the element.

 

[MonkehBoi]

Hi,

Thanks for the help so far...

Just realised that the cooler is actually rated at 12volts 5.5amps. I'm not sure how that would affect things.....

 

[MonkehBoi]

Hi all,

I've been rummaging around in a cupboard on a completely unrelated subject and have just realised I have two old PC PSUs lying around doing nothing.

These are of course capable of providing 12volts. Both are rated @ 300watt max but one states that it provides 140watt max @ 12 volts / 12.0amps and the other 180 watts max @ 12volts / 15.0amps.

Does anyone have any thoughts on using one of these instead? I think it would simplify things a lot and also has the added benefit of a built in cooling fan for the PSU.

 

[bigal_scorpio]

Hi mate,

I use a lot of laptop PSUs and they are very robust with features like short circuit protection and overheat cut off.

Since the PSU you have is spare, just connect it up and try it, worst that can happen is the PSU will get too warm and shut down.

Just keep your eye on it for an hour or two and if its still ok then it should be fine.

Al..

PS the cooler I have says the same wattage but never draws anything near that, its probably just a max rating as variables affect things, just try it!

 

[ericgibbs]

Hi all,

I've been rummaging around in a cupboard on a completely unrelated subject and have just realised I have two old PC PSUs lying around doing nothing.

These are of course capable of providing 12volts. Both are rated @ 300watt max but one states that it provides 140watt max @ 12 volts / 12.0amps and the other 180 watts max @ 12volts / 15.0amps.

Does anyone have any thoughts on using one of these instead? I think it would simplify things a lot and also has the added benefit of a built in cooling fan for the PSU.

hi,
Look here: Converting a PC Power Supply

 

[MonkehBoi]

Hi,

Thanks for all your help everyone.
I've rigged up an old PSU from a PC and it's all working fine. Left it running for a few hours to test it out and no problems.