Diode bridge over heating without a load.

Thread Starter


Joined Jun 4, 2021

I soldered together my design for a power supply for a heat sensor module, a pc cooling fan and warning LEDs. I was going to assemble it to measure it better before I finish it but as soon as I plug it in to a 12V transformer, the diode bridge over heats even though there is no load on the circuit, except for the 10k bleeder resistor for the capacitors. My ampere clamp shows 6.8A from the transformer to the circuit and I don't know what's wrong.

On the picture I put red X over the resistors for the LEDs because I'm waiting for them and I don't know if they are white with tinted class or colored LEDs. The crossed out resistor on the bottom for the relays is to take the voltage down to 5V but I don't know yet from how high so there is no resistor there at the moment.

The three triangles that are crossed out are from the bottom up:
1: the output from the heat sensor that I'm waiting for (the input to the sensor is the top blue square, P4) and it's suppose to cut off the switch (see below) when a certain heat threshold is reached.
2: a foot switch to trigger a control board that gets power from outside of this schematic.
3: the input for the foot switch for that control board.

These squares are just pares of pads that wires will be soldered to.

It's literally the first thing from the input that starts to over heat and the only thing I can think of is maybe the bleeder resistor, but it doesn't heat up when the diode bridge do. And this diode bridge is way overkill for this circuit. It's just what I had so I used it instead of buying a new one. And since these are 20.000uF capacitors I thought I needed a bleeder resistor for safety reasons.

But if it's not the resistor, what is it then. And if it is, what value should I put instead?



Joined Nov 6, 2012
Your Bridge is either wired wrong, or
has a shorted Diode caused by previous usage when it was probably over-loaded/over-heated.
Take it out of Circuit and measure it with a Meter.

D-8 needs to be parallel to R-3, not across the Choke.

D-9 is not necessary when You only have 100nf on the Output of the Regulator.

If You are just running LEDs with the Voltage Regulator, get a 5V Voltage-Regulator instead of 12V,
or set-up the 12V Regulator as a Current-Source for the LEDs.

Add a ~2-Ohm, 5-Watt, Resistor right after the Bridge
to limit inrush Current to that crazy 40,000uf worth of Bulk-Capacitors You have.