Electronic – How does this transformerless 5V power supply work

power supply

I recently took apart a crock-pot cooker that stopped working correctly.

It had two boards:

  • One that dealt with the high-voltage for controlling output to the heating element
  • The other had a 4-bit microcontroller and a few buttons/LEDs.

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I was surprised to see no transformer anywhere, and very few parts, considering there was a signal labeled +5V for powering the microcontroller.

I reverse-engineered the schematic:


simulate this circuit – Schematic created using CircuitLab

Is it me or is this a really crappy design?

I was very surprised to see that the DC GND was connected directly to the 120VAC L!

Is the 5V DC basically just a rectifier with a voltage divider?

Best Answer

  1. It's you.

  2. The 1.5uF film capacitor acts as a reactive (lossless) dropping element. D21 shunts negative current away and D22 conducts positive current to the filter capacitor C23 and it is limited by the 5.1V Zener diode D23. The 1M bleeder resistor (across the 1.5uF) prevents a shock from touching the plug pins after it's pulled. R21 limits the peak current to about 7A if plugged in at an AC peak.

    This kind of circuit requires all elements that could come in contact with a human to be isolated from the mains for safety, including any kind of display, switches etc. Any breach in that insulation (including you opening up) could lead to a potentially fatal shock.

As in the comments, your heater should go directly to the N side of the mains. There's probably an overtemperature fuse cutout in there somewhere too, maybe buried near the heater, and probably an overcurrent fuse somewhere.