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I've made a cooler box for a DSLR to keep it cool while doing long exposure astophotography. It uses 2x 12706 12V 5.8A peltiers which are water cooled. Peltier spec on amazon says VMAX 15V IMAX 6A QMAX 72W.

The box works really well and I'm able to cool 25 Degrees Celsius below ambient temperature in 15 minutes but I'm seeing something strange..... When I power the peltiers on, the Voltage drops to around 6-7V drawing around 4A. I am using an 800w ATX PSU with a 5-60V DC input 20A PWM speed controller to control the temperature. The label on the PSU states a total load of 125A/648W on the 12V rail.

I think I understand from some other reading on here that if the psu cannot deliver enough current it drops the voltage. I'm sure that's a very simplistic way of looking at it but if that is the case. Why am I getting a drop in voltage when the PSU clearly is capable of delivering enough current? Am I missing something obvious?

With nothing connected to the PSU it is giving me 12.2V. With only one peltier connected I get a drop to around 9V.

I have tried connecting one and both of the peltiers directly to the PSU without the PWM controller and get the same readings. Anyone any ideas please ?

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    \$\begingroup\$ 1. If you break up your question into paragraphs it will be easier to read and you'll be more likely to get an answer 2. Do you have a datasheet for the PWM controller? 3. Where did you measure the 6-7 V at 4 A? \$\endgroup\$
    – The Photon
    Aug 24, 2017 at 20:26
  • \$\begingroup\$ Please draw a schematic and clear up your wall of text. \$\endgroup\$
    – winny
    Aug 24, 2017 at 20:42
  • \$\begingroup\$ 1.sorry about the layout of my question. It's my first time using this site and wasn't sure how to format it etc... \$\endgroup\$
    – donhonking
    Aug 24, 2017 at 21:38
  • \$\begingroup\$ 2. I do not have a data sheet for the pwm controller i'm afraid. It is a DROK one from amazon. The pwm frequency has been bumped up to 25kHz to avoid thermal cycling of the peltier and the rotary knob adjusts the duty cycle to adjust the amount of power going to the peltiers. \$\endgroup\$
    – donhonking
    Aug 24, 2017 at 21:40
  • \$\begingroup\$ 3. I'm measuring the 6-7V at 4A on the output of the PWM controller \$\endgroup\$
    – donhonking
    Aug 24, 2017 at 21:41

2 Answers 2

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Why am I getting a drop in voltage when the PSU clearly is capable of delivering enough current? Am I missing something obvious?

When you power the peltiers on they are not only an active load, but can also generate power. If there exists a temperature gradient on the peltiers before the supply is powered up, then they could actually be generating power, and if this power is working against your supply then that could explain the need for more current.

Or the peltiers could be shorted out from improper handling. Check them with an ohm meter, there shouldn't be a direct short.

I have tried connecting one and both of the peltiers directly to the PSU without the PWM controller and get the same readings. Anyone any ideas please ?

Don't connect the modules in parallel, make sure you handle them properly. When you start them up, make sure there are no temperature gradients across the modules. 7V @ 4A is only 28W so if you are indeed having trouble (and actually measured the 7V across the load), I would suspect the supply, get a real power supply instead of an ATX supply.

Keep in mind peltiers have self heating, so when they have the full voltage and you don't have heatsinks, this is not an optimal condition.

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  • \$\begingroup\$ The peltiers are working and cooling pretty well so I don't think there's a direct short on them. They give a reading of 3.2Ohms when checked with my multimeter. I checked the temperature of the cold plate and the hotsides' water blocks and these both give me very similar temps and very close to ambient too. makes me think the temperature gradient is a dead end maybe. \$\endgroup\$
    – donhonking
    Aug 24, 2017 at 21:50
  • \$\begingroup\$ I've read that the peltiers should be connected in parallel so could you clarify on that point please ? \$\endgroup\$
    – donhonking
    Aug 24, 2017 at 21:51
  • \$\begingroup\$ I have also tried a 120W/10A LED transformer and have also tried using a 12V 4A psu which came out of a minifridge (it was originally powering a peltier). same result. \$\endgroup\$
    – donhonking
    Aug 24, 2017 at 21:54
  • \$\begingroup\$ re. the last part of your reply.....the hot sides are being cooled pretty efficiently with water cooling \$\endgroup\$
    – donhonking
    Aug 24, 2017 at 22:00
  • \$\begingroup\$ @donhonking You'll get double the cooling power, but your supply can't handle it. If you have a controller it will take longer to reach your intended temperature as long as you have enough cooling power to reach the intended set point. Try them in series. PWM controllers are also less efficient because of self heating in the peltiers. Self heating might also be working against you with the peltiers in parallel. \$\endgroup\$
    – Voltage Spike ♦
    Aug 24, 2017 at 22:18
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your power supply is delivering 12v but at how many Amps? you failed to state. i pulled one psu and checked it was giving 12v but at 2a. if this is what you had it would explain the v drop as it didn't have enough amps to maintain load. v dropped as it is supposed to like you stated.

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