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I've got a Peltier element rated at 12V / 2.5A (30W), but I'd like some clarification on how I should power it. I'm familiar with building power supplies with LM78xx linear regulators, and I've had a little experience with the LM317, but I've never tried to power anything beyond about 500mW. I'll also be powering two small 12V fans to cool the hot side of the device.

I found the LM350, which seems like a good bet - 3A rated, voltage up to 33V.

I'd be powering this off a UK mains socket (230V AC), so I'd probably set it up the same way I would an LM78xx regulator - run the mains into a transformer's primary, run the secondary into a diode bridge to half-rectify, use a 1mF electrolytic cap to smooth it, then run that source into the regulator.

I have a few questions about this, though.

  1. What voltage should I aim for on the transformer's secondary coil? Somewhere around 15V (e.g. this) seems about right to me, but I'm not entirely sure. As far as I can tell from Farnell and RS, there don't seem to be many transformers around that have low voltage secondaries.
  2. Do I need to use a heatsink on the regulator? If so, are there any resources that would help me select an appropriate one?
  3. Can/should I run the two fans off the same regulator as the element, or should I use a second regulator to power them? I figure an inductive load in parallel with the element might complicate things.
  4. Would I need to do this on a PCB due to higher currents, or could I get away with using stripboard?
  5. Are there any special safety considerations I should take into consideration for this build, aside from the usual ones when playing with mains electricity?

Thanks.

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  • \$\begingroup\$ Do you even need the regulator? Why not just connect the Peltier straight to the bridge rectifier? \$\endgroup\$ – Rocketmagnet Apr 1 '12 at 0:25
  • \$\begingroup\$ maxim-ic.com/app-notes/index.mvp/id/3318 This is an application note by Maxim "How to Control and Compensate a Thermoelectric Cooler (TEC)" \$\endgroup\$ – Nick Alexeev Apr 1 '12 at 0:45
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Use full wave rectification, not half wave. HW uses transformer poorly, may not be good or TEC, has no obvious advantages except the cost of 3 diodes.

If you want to operate it at full power with no control of cooling level then 12V is fine. LM350 regulator needs about 3V headroom. So 12V out from regulator = 15VDC in min.

Full wave rectified transformer will give ABOUT Vmax DC ~= 1.4 x AC voltage.
Or VAC_min ~= (Vdc + dropout) / 1.4 So 12V + 3V = 15V
VACmin ~~~~= (12 + 3) / 1.4 =~ 11 VAC if ~= NO ripple voltage ie 12 VAC transformer will give 12 VDC after regulator if well smoothed.
More is better, so maybe 14 VAC - 15VAC will allow regulator headroom plus some ripple allowance.

If all you want is to run it at full power then a transformer, bridge rectifier, smoothing capacitor and series resistor are all that is needed. Resistor drops excess voltage. A 10 VAC transformer will be about enough (1-VAC x 1.4 = 14 V with smoothing and ripple) and 12 VAC will definitely be enough. Resistor dissipates 2.5 Watts per volt of drop. A length of Nichrome wire adjusted to provide correct voltage to Peltier is one option or 0.4 Ohms per volt of drop select-on-test.

Regulator WILL need heat sink - how much depends on transformer. At say 3V regulator drop - about the minimum you should figure on, the dissipation V x I = 3V x 2.5A = 7.5W. Say allow 10 Watts. More if transformer is of excessive Voltage.

Heatsink can be selected using degree C (or K) per Watt for commercial heatsinks. For a 10C rise at 10 Watts you need 1 C/W heatsink which is "very large indeed".
If you want heatsink at almost cool enough to touch (almost) say 60 C the if ambient = 30 C worst case heasting delta T = 60-30 = 30C so heatsink = 30C/10W = 3 C/W
Even that is largish. Going other way, 10C/W is common so 10 W = 10W x 10 C/w rise = 100C./ Add ambient + Tsink = 100C rise + 30C = 130 C.
You rally don't want 130 C heatsinks.
so somewhere between 3 C/w and 10 C/W leaning towards 3 C/W end.

Fans and Peltier together are OK. Fan load is small compared to Peltier load.
Fans could run from smoothed DC before regulator - maybe with a dropping resistor of their own suited to VFan an Vdc.

Strip board construction OK but keep wires short and heavy. If running higher currents along a piece of stripboard you can solder wire to the strip for longer high current leads or use a wire link from points to be joined.

Main mains caution is DON'T PLAY WITH MAINS whn not needed. eg here all the circuitry is LV apart from mains feed to transformer primary. Do the primary side wiring well. Insulate as required. Then leave it alone.

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  • \$\begingroup\$ I actually meant to say full rectified over a diode bridge, no idea why I said half. Sorry 'bout that. Anyway, I think I'll take your first bit of advice and run the Peltier directly from the smoothing cap, to save me a lot of hassle trying to regulate it and heatsink the reg. I'll run an LM350 off there to power the fans, since I'd like to be able to control the fan speed by adjusting R2. Is there a reason for using the nichrome wire, instead of an actual resistor? \$\endgroup\$ – Polynomial Apr 1 '12 at 11:06
  • \$\begingroup\$ @Polynomial - A length of Nichrome wire with more resistance than you need allows you to adjust the resistance easily until you get the right voltage on the Peltier. You can get a semi adjustable resistor of usefully high wattage without much effort. At 2.5A you get 2.5W per volt dropped. Use a too high transformer and you get 10+ Watts with ease. Nicrome is Watts rated per length so if it works OK at 2.5A you just need more for more R with no concern about dissipation./ \$\endgroup\$ – Russell McMahon Apr 1 '12 at 12:46
  • \$\begingroup\$ I was actually just looking at a 10W resistor, which seems to be simple enough. Reckon it'll do the job without needing extra sinking or other messing around? uk.farnell.com/welwyn/wh5-1r5-ji/resistor-10w-5-1r5/dp/9507809 \$\endgroup\$ – Polynomial Apr 1 '12 at 12:48
  • \$\begingroup\$ @Polynomial - A 10 Watt resistor would be fine as long as it dissipated no more than 10 Watts continually, and preferably less than 5 Watts continually for best longevity. One point re using Nichrome was that it can have resistance adjusted as required but a fixed resistor needs to have the correct value chosen. If you can determine the correct value then a resistor is fine. | The resistor you suggest depends on having good heatsinking. \$\endgroup\$ – Russell McMahon Apr 1 '12 at 23:47
  • \$\begingroup\$ @Polynomial - ... If you are happy to heat sink it (and fan blowing helps muchly) it would do BUT cheap ceramic bodied air cooled resistors run at about 50% of Wattage ratings work as well and cost less. They too can be informally fan cooled for even better reliability. \$\endgroup\$ – Russell McMahon Apr 1 '12 at 23:47

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