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I am in the process of building a DSLR cooler (I use it for astrophotography) using a couple of TEC1-12703s (12V 3.3A) or TEC1-12704s (12V 4A). The ultimate idea is to run the TEC's at a higher power when initially cooling to the target temperature, then throttle them back to maintain the temperature as best as possible.

The bulk of the cooler will use an Arduino (~15mA), a power-on LED (~20mA), a couple of sensors (~1-1.5mA each when measuring, ~40µA standby), and a couple of fans (~65mA). So total power draw from most of the parts won't be very high (<200mA).

A couple of the PWM outputs on the Arduino will be used to switch IRBL8748PbF N-channel MOSFETs to deliver power to the TECs. The TEC power will be LC filtered, and a schottky diode will be used on each filter to make sure I don't fry the FETs (not sure if that would really be a problem, but I threw them in just to make sure I don't have weird voltage spikes at the FET drains). Ultimately the fan speed would also be controlled with PWM outputs the same way as the TECs are, with a MOSFET, however to start they will just be connected to the main 12V source and run at full power.

enter image description here

With two TECs, that is potentially up to 8.2A of total current. Even with the lower power TECs, that is still 6.8A of total current. I am beginning to wonder if I can send that much power through a standard 5.5mmx2.1mm power jack. I am also beginning to wonder if I can find a DC power switch that could handle over 8A.

If finding a switch that could handle that kind of power is unlikely, might it be possible to power the TECs directly off the power from the jack, and put the switch after that power line? Since the TECs won't actually power up until the MOSFETs are switched on, they should remain off until the power switch is flipped. At that point, only a few hundred mA would be pulled through the switch. Which would leave only the power jack in question.

Thanks!

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2 Answers 2

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It is very unlikely that whatever DC "wall wart" jack you have can handle the current required by the coolers. The 200 mA or so for everything else should be fine. However, switches to handle 8 A are readily available.

You could directly connect the coolers to the battery because in theory the coolers shouldn't come on until controlled to do so. A solidly off MOSFET should have little enough leakage to not matter. I'd feel better with a master shutoff switch though.

Either way, you should put pulldown resistors on the FET gates. You want to make sure the FETs are off when the processor outputs are high impedance. 10 kΩ to 100 kΩ is good enough and won't get in the way of the processor driving the FET gates. The pulldowns only need to be low enough to overcome leakage and provide some impedance against external noise pickup.

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  • \$\begingroup\$ I agree that a master off switch would be ideal. From a safety standpoint if no other. I hadn't thought of pull-down resistors on the FETs...I could give that a try. My biggest problem is space limitations...I can't fit a huge switch on the device. It's 100x40x40mm in size, and the electronics, I am hoping, will all fit on a 2x8cm PCB with an Arduino Micro. \$\endgroup\$
    – jrista
    Dec 30, 2015 at 19:45
  • \$\begingroup\$ Any ideas for a power jack that could handle the current? I would rather not resort to something odd, like requiring two power connections or something like that. \$\endgroup\$
    – jrista
    Dec 30, 2015 at 19:48
  • \$\begingroup\$ @jrista: The connector between the battery and your device doesn't need to be a wall wart style power jack. There are plenty of connectors that can handle 10 A. Go to a distributor (like Mouser) or manufacturer (like Molex) web site and look around for a suitable connector. You can wire several pins in parallel to get more current, with a little be of derating. \$\endgroup\$
    – Olin Lathrop
    Dec 30, 2015 at 20:59
  • \$\begingroup\$ I was trying to keep things as standard as possible. "Wall warts" are pretty darn standard in the astrophotography world, and I was hoping that if this thing actually worked, I might be able to set up shop to make them for people who want them, and they could power the device however they pleased (12V DC battery, some kind of power supply, power adapter so long as it could deliver enough amps, etc.) \$\endgroup\$
    – jrista
    Dec 31, 2015 at 1:15
  • \$\begingroup\$ FYI, for my camera (astro-camera actually), I was planning on using this one: CUI PJ-063BH (rated to 8A).(mouser.com/ProductDetail/CUI/PJ-063BH/…). \$\endgroup\$
    – Pål-Kristian Engstad
    Dec 31, 2015 at 2:09
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Standard DC Barrel Connectors (Coaxial Power Connectors, IEC 60130-10 and similar) will not do 8 amps. There are high power/current versions, but they are not standard.

Most common power switches can do 10 Amps easy, especially at 12V. Nothing special about them, you can even find them in Retail Auto part shops for a few bucks.

enter image description here

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  • \$\begingroup\$ When you say "common power switch", do you mean AC? \$\endgroup\$
    – jrista
    Dec 30, 2015 at 19:34
  • \$\begingroup\$ @jrista I mean like the one pictured or similar, but a standard home light switch should handle 10 Amps as well. Maybe 15 Amps. You'd need to check it's rating on the box/bag/light switch. \$\endgroup\$
    – Passerby
    Dec 30, 2015 at 19:38
  • \$\begingroup\$ I have limited space, no where near enough to use a standard home light switch. The cooling unit itself is about 100mmx40mmx40mm in size. Maybe slightly larger once some insulation is put on the cold plate. The electronics all need to fit along one side. I plan to use an Arduino Micro, and probably a 2x8cm PCB. I need something that will fit on that. \$\endgroup\$
    – jrista
    Dec 30, 2015 at 19:42
  • \$\begingroup\$ @jrista the switch I have in the image is 25 x 12.5mm or smaller. It is not a home light switch. \$\endgroup\$
    – Passerby
    Dec 30, 2015 at 19:43
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    \$\begingroup\$ I am no expert, but in order to save space, you could also consider a P-Channel Power MOSFET, for instance the Si7461DP. This particular one has V_DS = -60V and V_GS = +/- 20V, and drain current up to -8 A. The actual switch could then be at a much smaller amperage rating. \$\endgroup\$
    – Pål-Kristian Engstad
    Dec 30, 2015 at 20:20

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