Resistor Heatsink?

Hi Guys,
I built a power supply (half wave rectifier) for a project. It simply rectifies normal 120VAC and gives me a pulsed 120VDC output. The resistors are rated for 20 Watts (I calculated that I only needed 4.32 Watts), but they still seem to get extremely hot and I think they are beginning to break down. Is there any way I could put a heat sink on them?


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orksecurity6 years ago
If 20W resistors are really "breaking down", you're asking them to dissipate more than 20W. Check your circuit design. But simply getting hot may be normal. Remember, 20W will provide enough heat and light to run a Lava Lamp, or to run a compact fluorescent bulb capable of lighting a room; it's not a small amount of energy.

If you can strap them against a piece of metal, or affix a piece of metal to them, that will act as a heat sink... Back in the days when homebrewing meant single transistors, some folks made their own heatsinks by taking a rectangle of sheet metal and bending one end of it into a tube which fit fairly tightly over the component they were trying to protect. Of course that metal had to be isolated from the leads coming into the component and from anything else, to keep it from causing a short. You might be able to revive that concept.

transistorguy (author)  orksecurity6 years ago
Actually, I was working on the circuit and the smell was the circuit board burning and not the resistors breaking down, but thanks. I will probably add a heatsink anyway and follow your advice on how to make them because they do get really hot and I can't work around them until they cool down.

Wendymoon4 years ago
I like that answer by orksecurity, the one about attaching a piece of tin to a resistor to make a heat radiator!

  For my project (lighting an LED from 115V AC -- it's a garage door indicator light, sans transformer), for that project I used 3K ohms, a capacitor, and the switch, all in series. But the resistor got too hot to touch.  So I spread the heat dissipation over three 1K resistors to replace the 3K resistor.  But those three are still quite hot.  Running this thing for three days produced no smell, so probably all is copacetic.  It's all on a circuit board in a small plastic box from Radio Shack (c.a. 1980 :) with drilled ventilation holes.  The box is just warm enough to be detectable by touch.  But I worry.  I'm going to try that thar wonderful tip of bending a small square of metal from a coke can -- in a sort of J-shape and attaching it, blackened with felt tip, to each of those three resistors with a dab of epoxy.  Thanks, orksecurity! 
lemonie6 years ago

Your project is a bit dodgy: running off mains and throttling current with resistors.
You are losing energy and input voltage across the resistors, an isolating transformer would be better.

transistorguy (author)  lemonie6 years ago
I tried an isolating transformer.... It blew up. I'm guessing I need some sort of resistor on it.

Oooh... I "blew up a transformer" once: it was the wrong device for the job. Resistors are dropping your voltage & power.