Author Topic: Car fridge 12v why 14.2v PSU for mains  (Read 438 times)

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Offline mdronic

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Car fridge 12v why 14.2v PSU for mains
« on: April 22, 2021, 12:18:41 pm »
I bought a Brass monkey 36l portable 12v fridge freezer. I want to run on mains 240. The supplied power supply is 14.2v 6amp. Why is the PSU 14.2v not 12v. ? Wouldn't this higher voltage damage the fridge. I thought maybe wrong PSU but I checked website and a replacement PSU for the fridge is the same 14.2v 6amp. Why. Thanks
 

Offline Robert Smith Eco Warrior

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Re: Car fridge 12v why 14.2v PSU for mains
« Reply #1 on: April 22, 2021, 12:52:52 pm »
Cars are nominally 12 volt but that varies with the situation.
A 12v lead acid battery is pretty flat at 12 volts, something like 50% charged.
A fully charged battery at rest is about 12.7v
While on charge they are up around 14.4v.

Stick your multimeter in the car cigarette lighter / 12v output while driving. It will be up over 14v most of the time.
 :-+
 

Online Ice-Tea

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Re: Car fridge 12v why 14.2v PSU for mains
« Reply #2 on: April 22, 2021, 12:59:48 pm »
It will work fine with a 12V supply, too, but it will most likely be less powerfull. In this case, I'm guessing the "fridge" is probably peltier powered, these are pretty linear I believe. So, say the PSU is a bit overrated and the current is more like 5A at 14.4V then that means the "load" is 2.88 ohms, power is 72W. At the nominal battery voltage (12V) P= V²/R or 50W.

Offline kjpye

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Re: Car fridge 12v why 14.2v PSU for mains
« Reply #3 on: April 23, 2021, 08:46:23 am »
Why would you guess peltier powered? Most serious 12V fridges are in fact standard compressor type fridges. A peltier-powered freezer is going to need to drop the temperature significantly, and so would need multiple stages, and be quite inefficient. Some devices are sold as coolers, and they are often peltier powered.

A quick search for Brass Monkey fridges shows that they are indeed not peltier powered, but contain a compressor.
 

Online Ice-Tea

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Re: Car fridge 12v why 14.2v PSU for mains
« Reply #4 on: April 23, 2021, 09:53:39 am »
Well, my main reason for assuming it was peltier is because mine is :D

That said, looking a bit further, it looks like:

very small: peltier
medium & cheap: peltier
medium & expensive: compressor
big: compressor

That said, the reasoning is still valid: voltage vs current is also rather linear for electric motors.

Offline BrokenYugo

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Re: Car fridge 12v why 14.2v PSU for mains
« Reply #5 on: April 23, 2021, 05:24:26 pm »
The "12 volt" bus on a typical car is held at ~14.4 whenever the engine is running and spinning the alternator.
 

Offline Jwillis

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Re: Car fridge 12v why 14.2v PSU for mains
« Reply #6 on: April 25, 2021, 04:44:50 am »
12 volts rating is the nominal voltage . Most electronics will operate at a few volts higher or lower than  the rating . Also many power supplies are matched to the appliance with a voltage drop in mind . They do this to keep cost down  . So under load that power supply might drop in voltage under load . Having a voltage higher without load is normal .
 

Offline JustMeHere

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Re: Car fridge 12v why 14.2v PSU for mains
« Reply #7 on: April 25, 2021, 07:52:23 am »
Check out this.  It shows all of the stuff you have to do in order to make it "safe" to use a car's power supply.

https://www.ti.com/lit/an/snva681a/snva681a.pdf

 


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