AC/DC Converter Incorrect Output Voltage


I am building several continuous culture devices, which consists of an arduino that controls many downstream circuits, including pumps, lights, peltiers, etc. which require different voltages. To power the components, we use an AC/DC converter that takes wall power and converts to 5V and 12V. For this we have used the 102-2525-ND part (which is now obsolete) and more recently the 1866-3985-ND converter. The both have the following issue lately:

The voltages output by the converters are too high, (8-9V for the 5V output and 18-19V for the 12V output) which causes problems with the powered components. The parts have a tuneable knob for the output voltages, but this only tunes about 1V, it is not enough to compensate for these large voltages. This is happening with both models of converter, older ones and new out of the box converters. I’ve measured the input AC voltage as ~200V which is within the range of the converters. I’m at a bit of a loss, does anyone know what might be causing this problem?

Thanks in advance!

Hi DianeS,

Both of the power supplies you mention have a minimum load spec of 0.3A on both the 5V and 12V outputs. Below this, the voltage is likely to rise. Therefore, you’ll need to draw at least that much current to keep it within spec.

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Thanks for the help, David! I measured the current of my circuit and this seems like it’s the problem! Forgive the naive follow up, but does that mean that another converter like 941-1697-ND which lists a max output amperage but no minimum, would be likely to work?

When there is no minimum load spec then, yes the load regulation specs apply from zero to full load. However it is very rare in modern high efficiency switch mode power supplies. No minimum load is more common in older low efficiency linear supplies. FYI - it is usually more energy efficient overall to add a minimum dummy load to a high efficiency switch mode supply than to use an old low efficiency linear supply when nominal current ratings are > 0.5A.

On the data sheet for 941-1697-ND see the following specs:

  • Load Regulation, Iout=Min. to Max., Dual/ Triple Output Models, ±2.5% Typ, ±5.0% Max
  • Minimum Load 10 %Inom

So for 941-1697-ND where Inom = 1.5A @ 5V & 0.625A @ 12V the minimum loads are 0.15A @ 5V & 0.0625A @ 12V to hold within the load regulation specs.


Paul is right on about that. I wouldn’t count on a switching power supply properly regulating down to zero load unless stated explicitly. If not defined in the datasheet, it is more than likely an oversight rather than an indication that it regulates down to zero load current. And, as he shows, the 941-1697-ND needs a 10% load to regulate properly. Datasheets can be tricky – you have to read the fine print.

Regarding your options, there are a few to consider. As Paul suggested, a very simple solution is to place a minimum load (dummy load) in parallel with your standard load. It will consume some power, but not a huge amount. So, for the RD-50A that you have, you could place a resistor of less than 16.7 Ohms across the 5V output (5V / 0.3A = 16.67 Ohms) in parallel with your load. A 15 Ohm 3W resistor, such as the A129418CT-ND would do the job. It would draw about 0.33A and consume about 1.67W. Going this route, you may be able to get by without placing another load on the 12V output – it all depends on the internal architecture of the supply. Often times, when one output is loaded properly, it will regulate the other output as well. If not, you would need another dummy load of ≤ 40 Ohms and rated for at least 4W. The PPC5W39.0CT-ND would be suitable.

Another possibility would be to go with a single output 12V supply and place a DC-DC converter on it’s output to get your 5V supply. Single supply AC-DC converters are much more likely to regulate properly down to no load, and most DC-DC converters can regulate at no load.

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Great, thank you both so much for the help! I’ll try adding a dummy load!