Small AC-DC converter

One of my goals recently has been to make a small (no transformers) application-specific AC-DC converter. Here are the specs I need to meet:

  • Iout > 15 mA (DC)
  • 3.0 V < Vout < 32.0 V (DC)

The reason I was planning for these specs is because I am testing my converter with a solid state relay (SSR) that is driven by a DC input of 3 V, 15 mA minimum, and connects an AC load of up to 240 V, 10 A maximum. I am trying to keep the power consumption of my circuit down, so that means I need to try to keep both the voltage and current as low as possible. Here are some of the paths I may try to take:

  1. Use resistors as a voltage divider. This will only be effective if the load resistance will always be the same.
  2. Use some Zener diodes to drop the voltage down after rectification. Then almost any voltage can be obtained, and a resistor can be used to limit the current if no load is present.

I have tried #1 above, and it works fine for my testing because I planned the circuit for a specific load resistance. I think option #2 will be a better option, and I am going to start building and testing it. I will use some medium-voltage (around 20 V) Zener diodes to reduce the voltage to what I want, and then use a capacitor to get rid of any ripple voltage and a very high value resistor (1 MΩ) in parallel with the capacitor to drain the capacitor when the circuit is disconnected from a power source. Any load would then go in parallel with the resistor.

The reason I would use several medium-voltage Zener diodes instead of one high-voltage Zener diode is because if the load resistance (that is in parallel with the several MΩ resistor) is relatively small, then the current through the Zener diodes could be very large, which means I would need high wattage Zener diodes, which means I would have to spend a lot more money on them. If I try to stick with 20 V, 1 W Zener diodes, then I can go up to a maximum of (1 W) / (20 V) = 50 mA of current. For now, that’s more current than I will be using. If I need more current than that, then I can use more lower-voltage Zeners. This may not be the best solution, but it is a good learning experience for me and it is fun! All of this is preliminary research to some larger projects that I have planned (which I will post here when I start them.)

I’ll post some pictures and schematics as I continue this project.

2 thoughts on “Small AC-DC converter”

  1. Very cool experiment. I was wondering just the same, if I should use resistors to bring the voltage down or if I should use zeners on the dc side to control the voltage.
    But I was wondering, wouldn’t there be a lot of heat on low-voltage high-amperage devices? You were talking about having 20 volts and 50 mA, but how much more current could a simple device handle at that voltage until you get too much heat?

    Thanks in advance,

    1. Yes, it would create a lot of heat. 20V at 50mA per zener diode ends up being 1 Watt that is dissipated as heat for each of the zener diodes. It doesn’t make much sense to drop down the voltage this much with resistors or zener diodes. It makes much more sense to to just use a transformer. Or you can use a switching regulator, which are very efficient (i.e., almost no wasted energy) and will therefore generate very little heat – but the problem here is that you probably will not be able to find a switching regulator that will convert 120V input down to just a few volts output. Something that is a compromise of these would work best – use a transformer to bring down the voltage to around 10 – 12V, and the use the switching DC-DC voltage regulator to drop that down to 2.5V, 3.3V, 5V, or whatever output voltage you’re looking for. Using the switcher will also help to create a nice, smooth output voltage level.

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