DC current vs AC current

Thread Starter

Lumenosity

Joined Mar 1, 2017
478
I have a IGLOO car peltier cooler that I carry in my car.
It runs off of DC 12v but it comes with an AC adapter to plug it into 115AC outlet or an optional cigarette lighter cord for in vehicle use.

When I had it plugged in with the AC adapter, it pulled 65 Watts.
When I plugged it into the cigarette lighter in my car, it pulled 65 Watts

I was expecting a significant decrease in watts used running it directly off of DC ?
I thought without the AC adapter having to convert AC to DC I would see a large drop in power usage.

Where did I go wrong?

Thanks
 

Papabravo

Joined Feb 24, 2006
15,471
When you go to a lower voltage the current increases. Power is power. 115VAC(rms) times 0.5652 AmpsAC(rms) gives 65 watts. similarly 12VDC times 5.417 Amps DC is .... you guessed it 65 watts. A load will always consume the same power regardless of the power source. That's where you went wrong. If it were otherwise we'd have perpetual motion and free energy, and we can't have that.
 

Thread Starter

Lumenosity

Joined Mar 1, 2017
478
What I cannot understand is that I eliminated the very current intensive AC converter that converted AC to DC and went with a straight cord from DC to DC....and still the current draw was the same.

How could the cord with no adapter use the same current as the cord with a large complex AC adapter?

Seems like the AC adapter would use quite a bit of power all by itself in the conversion process.

It's basically an inverter and they consume power. Let's say 15%.

That should be 15% more to run the Ac adapter or 65watts + 9.75watts = 74.75watts total for the AC adapter (or 55.25 with the direct DC to DC cord)
 

Papabravo

Joined Feb 24, 2006
15,471
What I cannot understand is that I eliminated the very current intensive AC converter that converted AC to DC and went with a straight cord from DC to DC....and still the current draw was the same.

How could the cord with no adapter use the same current as the cord with a large complex AC adapter?

Seems like the AC adapter would use quite a bit of power all by itself in the conversion process.

It's basically an inverter and they consume power. Let's say 15%.

That should be 15% more to run the Ac adapter or 65watts + 9.75watts = 74.75watts total for the AC adapter (or 55.25 with the direct DC to DC cord)
Converting AC to DC is not an inverter - it is a transformer and they are pretty efficient. An inverter is going from DC to AC an yeah they're not quire as efficient.
 
Last edited:

crutschow

Joined Mar 14, 2008
26,737
still the current draw was the same.
That makes no sense if you measuring the current draw from the 110Vac, since that would be a lot less.
It makes sense if you are measuring the DC current to the cooler.
In that case you would expect the currents to be the same if the two DC voltages are the same.
 

Yaakov

Joined Jan 27, 2019
2,362
If you measure at the output of the AC power supply you will find exactly the same current and voltage as from the output of the DC supply. If you want to measure losses, you need to measure at the input to the AC supply which is the power to operate both the device and the supply as opposed to the power consumed only by the device.
 
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