I am doing a custom project, where I have to heat up a big resistor controlled by a PID and a thermistor.

Normally, I use a 12V 30A power supply, but I would like to design my own power supply, to learn how to do it.

Well, I am now choosing a simple transformer, and I find that they are pretty expensive. Maybe I am choosing the wrong transformer or the wrong provider (I am looking in Digikey).

So, I would like to ask:

For a European primary voltage and 12V secondary voltage, if I need 30A, I would need a 360VA transformer right? I see that many cheap power suppliers are cheaper than the transformer. Ok, those are optimized, but... cheaper than everything?

Which technology is best for this? I see many different types of power transformers, but I don't get the differences.

So, how would you look for a transformer for your own power supply?

  • 1
    \$\begingroup\$ Many of the cheap supplies you see use switching regulators which don't use a large step down transformer. \$\endgroup\$ Aug 7 '13 at 17:22
  • \$\begingroup\$ From your question it is not clear if you are ready to do it yourself or just seek an advice to buy a transformer? It's pretty easy to make transformer to your specs from some suitable old parts (provided you make calculations right). \$\endgroup\$
    – Roman Susi
    Aug 7 '13 at 18:14
  • \$\begingroup\$ well, i want to do a first prototype, but let the design open for a small batch made on a factory. \$\endgroup\$ Aug 9 '13 at 1:07

For ANY primary voltage (except 12V!) and 12V secondary, 30A would require a 360W transformer, and for a resistive load that means 360VA.

At 50Hz or 60Hz, that is a substantial quantity of copper and iron, so the material costs alone impose a lower limit on the possible price.

Switching power supplies eliminate this lower limit by driving much smaller and lighter transformers much faster, and thus eliminating the basic need for large quantities of raw materials.

All else is detail; but it is certainly feasible to make a big SMPS much lower cost than a big 50Hz transformer.

For a one-off project, a transformer is a simple solution, and you may be able to find a "surplus" bargain if the "proper channels" are too expensive.

Alternatively you may find a "gamer" PC power supply with 30A or more available from a 12V DC rail; the commodity nature of PCs helps to keep prices low.

  • \$\begingroup\$ For a resistive load VA is the same as Watt, but you should have said ..."360VA transformer, for a resistive load this means 360W". i.e. 360W is not the same as 360VA for a non resistive load. \$\endgroup\$
    – Jason M
    Aug 7 '13 at 21:48

An ACDC converter (aka SMPS) would work, but for your application is not necessary (unless you can find a cheap one, e.g. one intended for a PC computer).

The three main problems with a transformer are:

  • No regulation
  • Physical weight
  • Losses due to incomplete magnetic coupling and saturation of the core.

I don't think you need to be worried about any of these as your PID will from a feedback loop, controlling the temperature regardless of the variaion in input voltage and variation of the output due to losses in the transformer. (Assuming the load is not so large as to damage or overload the transformer you select)


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