How-to for building a simple TEC (peltier) power supply

Back in the old days, processors like the Intel Celeron could be cooled quite well using 60-80W TECs (TEC = Thermo Electric Cooler, also named Peltier element). These TEC's could deliver 70% of their maximum cooling power when powered from the 12V line of an old AT style power supply.

Those days are gone. To cool hot-running processors like AMD's Athlon and Duron, or Intel's Pentium 4, a much larger TEC (or multiple TEC's) in the range 150-200W is needed. Because of the immense power requirements of such TEC's, powering these TEC's from a standard AT or ATX power supply is not that simple anymore. Multiple 400W+ supplies must be connected together to provide the right amount of current and voltage. This is a big, bulky, noisy and expensive solution.

One of the solutions for the problem is to use a 230V to 12..18V transformer, a bridge rectifier and a bunch of capacitors to provide the power for the TEC. The basic schematic diagram of a simple unregulated transformer, bridge rectifier and capacitor power supply is shown below.

The basic schematic for an unregulated TEC power supply

The fuse in the circuit is used for protection. It's exact value is provided by the manufacturer of the transformer, and you MUST use exact this fuse to prevent fire when something goes wrong.
The transformer is used to bring the 110V/230V mains voltage back to a useable voltage.
The secondary output of the transformer is connected to a silicon bridge rectifier, which converts the AC voltage into a pulsating DC voltage. Since the TEC doesn't like this pulsating voltage, it is flattened out by the electrolytic capacitor(s). The end result is a DC voltage suitable for powering your TEC, waterpump, fans, whatever. And since no fans are involved in the supply, this power supply is completely silent.

Dimensioning the transformer

First of all, you need to know the maximum voltage of the TEC used, and it's maximum current draw. A suitable transformer can be calculated as following:

V_{out_transformer, required} = (V_{TEC, max} + 1.4) / 1.414.

Now, round this number to the first lower stock value available at your local retailer. Thus, if the formula says you need a transformer for 13.3 Volts, round it down to 12 Volts.
The voltage applied to the TEC with this stock transformer will be:

V_TEC = (1.4 * V_{out_transformer}) - 2

Due to the limited range of output voltage of stock transformers this is probably lower than the V_max of the TEC. If you can live with this (for our application of a TEC it doesn't hardly matter whether you run the TEC at full power or 80% - 95% of it's rated V_max/I_max. More power into the TEC means an increased temperature difference over the TEC, but also a higher coolant temperature.), you do not need to modify the transformer, and this will be your final V_TEC. If this voltage is too low in your opinion (Hey, I paid for 80 Watts, not 60!), you can modify toroid type transformers so they will generate an increased voltage. If you plan to do this, continue all calculations with the planned V_TEC This modification will be explained later.

The next question is: what should be the minimal power rating of the transformer? This can be calculated with the following formula:

P_{out_transformer} = 1.4 * V_TEC² / (V_{TEC, max}/I_{TEC, max})

where:
V_TEC is the voltage calculated above, or the desired voltage if you plan to modify the transformer.
V_{TEC, max} is the maximal rated voltage of the TEC.
I_{TEC, max} is the maximal rated current of the TEC.

To make it easier for you, I have put a few common combinations in a table for you:

transformers used for common V_TEC/I_TEC combinations

V_out (Volts) I_{out, max} (Amps) Transformer output voltage (Volts) Minimal required transformer power (Watts) Sample use

11V 7A 9V 110W 72W 12V @ 7A TEC

15V 8A 12V 170W 80W 16V @ 8A TEC

15V 16A 12V 340W 156W 16V @ 16A TEC

23V 11A 18V 375W 172W 24V @ 11A TEC

23V 22A 18V 750W 2 172W 24V @ 11A TEC's in parallel

Modifying toroid type transformers

Since a transformer is nothing more than a few pieces of copper wire wound around an iron core, it is possible to get a higher voltage out of a transformer than the rated voltage by adding some turns to it. With block-type transformers this is difficult, but with toroid-type transformers this is pretty easy. All you have to do is to wind a piece of thick, solid, insulated wire around the transformer, and connect it in series with the existing secondary output. At least 2.5mm² wire should be used, and the turns should be evenly distributed over the transformer. The amount of turns depends on the transformer, so you have to experiment a bit here. Only one way of connecting the extra coil will raise the voltage. When connected the other way around, it will lower the voltage. This is also dependant on the transformer, so you have to just connect it, and measure the output voltage of the transformer with a voltmeter.
Although this method can also be used to lower the output voltage of a transformer, this is not recommended.

Dimensioning the bridge rectifier

The bridge recifier should be able to withstand the maximum peak output voltage of the transformer, which is about 1.5 * V_{out_transformer}.
The current rating of the bridge rectifier should be greater than two times the current flowing through the TEC when using a toroid type transformer, or 1.5x the current flowing through the TEC when using a block type transformer.

Since the extra cost of a heavy duty rectifier compared to a lighter one is so low, just use a 35Amps/200Volts or 35Amps/400Volts type when the peltier current stays below 16-20Amps. When it rises above 16-20Amps, use four separate silicon diodes and build your own rectifier.

Dimensioning the capacitor

To smooth the output of the bridge rectifier a large capacitor is required. This part should also be able to withstand the the maximum peak output voltage of the transformer (1.5 * V_{out_transformer}).
To keep the ripple on the output within 15%, 4000 microfarads of capacitance per Amp of current drawn by the TEC should be used. Thus, for a 24V @ 11A TEC, a 25V, 45000uF capacitor is required. Larger is always better, since a larger capacitor reduces the amount of ripple on the output.

When the required value is not available, you can connect multiple capacitors in parallel to obtain the desired value.

Footnotes

The formulas presented here are not equal to the ones you can find in general electronics books. This is because the formulas presented here are derived using an electronics simulation program called SPICE. But even these formulas are only a guideline; actual results can differ from the calculated results.