Hi all,

I'm working on a project using a 150W peltier thermoelectric cooler. I'm powering it from a 24V 6A power supply and would like to introduce a "dimmer" so that I can regulate the power drawn by the pelt from say 10W to 120W.

I think for this sort of thing I'd need a MOSFET circuit right? I'm a little out of my experience here - I'd like to isolate the high-current circuitry from the control circuitry (pot and switches) for obvious safety reasons.

Any help appreciated, will post circuit diagram in a sec.

 

So here is the current circuit. The lamps are just used in my circuit simulator to simulate the peltier load.

Circuit turned off.

Circuit turned on in "heat" mode - note direction of current through load.

And circuit on in "cool" mode - note opposite current through load.

So the problem is to introduce a current limiting device, controlled by a low voltage pot, probably into the top left of the circuit to limit the power to the load to about 10W-120W.

Hope this makes sense!

 

Try brushing up by searching for and reading up on "H-bridge" and "pulse width modulation." The combination of those two techniques should do exactly what you want.

 

AH I hadn't even thought of using PWM. Something simple with a 555 driving a MOSFET?

The heat/cool DPDT relay is acting as an H-Bridge, is there something wrong with this approach?

Cheers

 

So the following works ok in the simulator:

(thanks to http://www.dprg.org/tutorials/2005-11a/index.html for the PWM 555 circuit).

So the problem now is that the current draw is down from 5A to 2.8A max, giving a max of about 65W. Probably not a bad thing, but is this just due to inefficiency in the MOSFET? I noticed that changing the voltage to the 555 changes the current through the MOSFET; so I assume that proper MOSFET choice will be a big issue?

 

An NE555 has a maximum Vcc of 16v. I suggest 12v is a good Vcc for your application.

An N-ch power MOSFET generally requires Vgs=0 to turn it off, and Vgs=10 to turn it fully ON. You will need to check the particular MOSFET you use for it's particular specifications. An IRFZ34 or IRFZ44 would likely work well for this application.

 

Thanks folks, I'll check these suggestions out and let you know how I get on. I am thinking too I should add some diodes around the relay coils?

 

I should add some diodes around the relay coils?

Yes.

 

Shouldn't the temperature of whatever the Peltier device is heating or cooling be the controlling factor?

Why not regulate the output with a thermostat controlling a soft-start circuit to bring the power on gently over a second or two and then off when the target temperature is achieved? The soft-start may prolong the life of the device by avoiding thermal shock associated with on/off cycling.

Just another thought, 2 SPDT relays could be used to do the same job of switching power and polarity reversal.

 

I did exactly the same thing about a month ago, but I don't have the circuit handy. I just found some nice high and low side fet drivers and created an h-bridge with some low rds-on fets. The drivers were able to sink/source 2A and the fets had about 5mOhm on-resistance. I used D2PAK fets with a large unsoldermasked pour and it didn't require any additional heatsinking.

For a PWM source, I used a PSoC microcontroller to do it. The PSoC accepts a voltage ( 0 for 100% cold, 2.5V for nothing, and 5 for 100% heating. This was done so that it could be easily controlled via a PID. I also have an enable line and overcurrent protection.

If you're interested in schematic/code, I can post it tomorrow.

Steve