PWM not putting out full current...

Hey guys, I’m hoping someone here can tell me what’s up with this. I am running a peltier off of one of these, http://www.robotshop.com/productinfo.aspx?pc=RB-Pol-23&lang=en-US
and I’m supplying 15V from a PSU that is capable of up to 35 amp @ 15V. The Peltier is a 15 amp module at 15.4V. However, I can’t seem to get full current to the peltier. The ammeter on the PSU has it reading maybe 7 amps max… I believe this to be because of the PWM not running fast enough to deliver the full current. Is this true? By fast enough I mean the PWM frequency.

Any thoughts on this?

Thanks!

I believe this to be because of the PWM not running fast enough to deliver the full current. Is this true? By fast enough I mean the PWM frequency.

No, a PWM at 100% on duty cycle is a constant high, no matter what the frequency of the PWM generator is. So not sure where your voltage drop is coming from, but the easy way is to set the PWM to 100% (full on) and measure the voltage drop across the Peltier device. If it’s less then +15vdc you may find that the size and length of your main DC wiring is dropping volts, or it maybe dropping some in the motor drive output transistors. Raising the source voltage higher then +15vdc may be your only option if you must have 100% current rating for the Peltier.

There is a small voltage drop... Less than 1v from the posts on the ps to the output terminals on the motor driver. I set the PWM to 100% (255) and I even left the loop run with no delay... I don't know? lol

FlyingSteve:
There is a small voltage drop... Less than 1v from the posts on the ps to the output terminals on the motor driver. I set the PWM to 100% (255) and I even left the loop run with no delay... I don't know? lol

You need to try and measure as close to the pelter terminals as possible to see what the total voltage being dropped. Your petler device has a internal resistance of around 1 ohm (if it's ratings are honest) so you can see if you have even 1 ohm of additional resistance in the power/driver wiring that would cut the current flow to 7.5 amps.

Actually you are kind of up against the fact that you are trying to run the motor driver over it's 100% ratings (14 amps) and I'm sure there is some voltage drop across the motor drivers switching transistors. It's normal to size motor drivers and power supplies at around 150-200% of the actual power desired for the load, so as not to be running components at their hairy 100% ratings. You don't size a car engine such that you need pedal to the metal to go 55mph, motor won't last long that way. :wink:

Lefty

The motor driver is good for up to 30 amps with proper cooling... It's barely warm to the touch now. I'll check the voltage as close to the peltier as possible, but from the output of the motor driver board to the peltier is only a few inches of wire.

OK, I'm getting 14.93V at the PSU and 14.33V right at the peltier. I connected the power supply directly to the peltier and found no difference in current consumption.... I ordered some new peltiers today, I should have them in a day or two. Maybe it's the peltier? I don't know...

I have a second peltier, this one is rated @ 9 amps at 15.4V and it also only picks up 7amps from the power supply... So both the 15 amp peltier and the 9 amp peltier only run at a max of like 7 or 7.5 amps separately (together they do draw about 15 amps from the PSU) connected directly to the PSU, receiving 14.3VDC... And they are from different manufactures. Makes me think that I just need more voltage?

I have a Pyramid PS36KX and my buddy also has the same one and he did a mod which removed the voltage crowbar and changed some other things around and was able to safely mod the unit to put out 15-18V adjustable, rather than the stock 13-15V. I'm gonna try that I guess...

See if you can locate a datasheet for the device, perhaps it can only run at 15 amps with proper heatsinking.

Lefty

Datasheet for the PSU or the motor driver? It seems to have nothing to do with the motor driver because I connected the peltiers directly to the PSU and got the exact same results.

OK, I'm getting 14.93V at the PSU and 14.33V right at the peltier. I connected the power supply directly to the peltier and found no difference in current consumption....

When you have the power supply directly supplying the peltier, what is the measured output voltage of the power supply and the measured (with a multimeter) current being supplied to the peltier? Just verifying you measured the power supply output voltage when loaded.

Datasheet for the PSU or the motor driver?

Neither, a datasheet for the peltier is what I was interested in to see if it's 15 amp rating is conditional on some other variable.

If you wire the peltier directly to the power supply set to 15 volts and don't get 15 amps of current flow then the the only variables are wire size and length and the peltier device itself. I suspect they are hedging on the 15 amp rating.

Lefty

This is all I could find...

http://new-electronics.gr/peltier/CP1-127_SERIES-PERFORMANCE_SPECIFICATIONS.pdf

I have the CP1-12715

Voltage loaded is 14.93 @ PSU and 14.33 @ peltier.

Personally, I think it needs more juice because even the other peltier isn't reaching it's full current draw and it's from a different company.