QW-MS305D Power Conversion
22 January 2023
Need to convert a power supply from 120 volts to 240 volts, or the other way around? Here's a surprisingly simple procedure.
While I was mapping the circuitry, I ran across this oddity. Right at the line power input there are a pair of thermistors (marked in red). Thermistors are normally used to detect temperature changes. That makes sense for a power supply, they could be used to execute a safe shutdown if it was overheating. But these thermistors aren't attached to the main heat-sink, so they don't make sense.
Here's the schematic.
It turns out to be really clever. When current passes through a thermistor, they naturally heat up, thus affecting their measurement. Normally this is a bug, but the thermistors here (NTC 10D-11) are using that as a feature. In this case the thermistors normally have a high resistance, then as they heat up the resistance quickly lowers. The effect is that they provide protection against a power surge when the device is first switched on. I hadn't run into this trick before.
This was not my first time tinkering with this power supply. Three years ago I had to replace a blown power MOSFET.