PSU Question

[CycloWizard]

I bought a PSU to run a thermoelectric assembly (TEA) in my lab. The TEA nominally needs about 4 A at 12 V to give 30 W of heating/cooling, so according to the ATX 2.01 specs, this PSU should have no problem providing that. However, when I connect the 12V2DC connector and corresponding ground to the two terminals of the TEA, the voltage drops to around 9.6 V and very little heating/cooling goes on. I know just enough about electronics to be dangerous, so I'm not sure why the PSU is dropping so much voltage. The TEA vendor indicated that it can be modeled electrically as a resistor, but other than that I'm not really sure how to proceed to get this thing working at full capacity. Any ideas would be appreciated.

 

[bobdole369]

You need a load on nearly all legs of the PSU in order to operate correctly. Supply enough load to pull half an amp on all sections (5V, 12V1, -5V, -12V, 3.3V), etc.

In modern PSU's there is one or more output transformers that are tapped in order to supply these legs - usually they are all common (eg 1 toroid that supplies +5, +12V1 +12V2, -12V, etc) WIth no load on the other legs and a big load on 1 of the legs the supply is in an unknown state - Not surprised the circuit protection kicks in.



Also - switching PS are notoriously bad at supplying inductive loads full power (think fridge motors and electromagnets) Don't think a peltier junction is inductive though (I'm assuming thats what you are doing).

The stupid radio shack has a 3A 13.8V supply on the shelf, and they also might have a larger one than that. You could always build your own supply.

 

[CycloWizard]

You need a load on nearly all legs of the PSU in order to operate correctly. Supply enough load to pull half an amp on all sections (5V, 12V1, -5V, -12V, 3.3V), etc.

In modern PSU's there is one or more output transformers that are tapped in order to supply these legs - usually they are all common (eg 1 toroid that supplies +5, +12V1 +12V2, -12V, etc) WIth no load on the other legs and a big load on 1 of the legs the supply is in an unknown state - Not surprised the circuit protection kicks in.



Also - switching PS are notoriously bad at supplying inductive loads full power (think fridge motors and electromagnets) Don't think a peltier junction is inductive though (I'm assuming thats what you are doing).

The stupid radio shack has a 3A 13.8V supply on the shelf, and they also might have a larger one than that. You could always build your own supply.

Thanks, I didn't think to load the other outputs... The vendor's engineer warned me about overvolting it and I haven't been able to find a cheap 4A 12VDC supply, so I thought I could fake the above PSU into doing the job... I guess I faked myself out instead.

edit: The nearby Shack has the 13.8VDC 15A supply for $80. That should get the job done.

 

[Aluvus]

Thanks, I didn't think to load the other outputs... The vendor's engineer warned me about overvolting it and I haven't been able to find a cheap 4A 12VDC supply, so I thought I could fake the above PSU into doing the job... I guess I faked myself out instead.

A few power resistors on the other outputs should do the trick. Assuming you aren't worried about efficiency.

 

[PottedMeat]

Thanks, I didn't think to load the other outputs... The vendor's engineer warned me about overvolting it and I haven't been able to find a cheap 4A 12VDC supply, so I thought I could fake the above PSU into doing the job... I guess I faked myself out instead.

edit: The nearby Shack has the 13.8VDC 15A supply for $80. That should get the job done.

maybe you're better off getting some beefy power resistors (pretty cheap actually) for the other outputs on the atx supply so you get 12V+/-5%. the radioshack supply is more for a high current car battery replacement ( ~13-14V vehicle running ).

 

[CycloWizard]

Thanks again guys. I bought a kit with 10 W resistors a while ago, so those should do the trick. I guess I could drop them in the same box and let them offset thermal losses in the system, then pretend like that's what I intended to do all along...

 

[Modelworks]

All electronics has several supplies that would work and are cheap.
http://www.allelectronics.com/make-a-store/category/480/Power-Supplies/1.html

This one would be perfect, only $16.00
http://www.allelectronics.com/make-a-store/item/PS-1248/12-VDC-4A-SWITCHING-POWER-SUPPLY/1.html

 

[Mark R]

Most cheap ATX PSUs regulate the 12 V and 5 V supplies together.

In effect, the PSU contains a transformer with a 12 V winding and a 5 V winding - a voltage sensor is arranged such that it measures a weighted mean of the actual 12V and 5V voltages, and that is used as feedback for the input to the transformer.

(The other voltages are usually supplied from independent regulators, except for 3.3V which is usually obtained by taking the 5V output and running it through a step-down regulator. So, you don't need to load them down.)

If you load down only one of the outputs, the system may not regulate correctly, because the other output will rise in voltage (as input to the transformer is upregulated).

To get around this, you could load-down the 5V output. Try and match the level of loading so that it is proportional to the level of loading on the 12 V output. E.g. if you are running the 12V output at 75% capacity, try and aim for about 75% capacity of the 5V output.