I have attached a picture of a PCB that I have which is burning from the heat generated by the resistors. Is there any way of protecting the PCB from burning.

I was thinking about putting aluminium foil under the resistors but needed conformation and then that also comes with a set of its own problems.

 

Hello,

You must try to keep a larger distance between the PCB and the resistors.
There are ceramic spacers for the resistors available at farnell:
http://cpc.farnell.com/1/1/92175-spacer-ceramic-stand-pk50-cb3-multicomp.html
They can be stacked to create a larger space between the resistor and the PCB.

Bertus

 

You can also buy resistors that don't run so hot per watt, or just buy over rated resistors like 5W rated in a 3W position. I recognize that particular shape as a resistor that can survive higher temperatures...but the circuit board can't!

 

Hello,

I think that #12 is pointing at the ceramic wire wound resistors as in the following PDF:
http://www.ohmite.com/cat/res_tuw_tum.pdf

Bertus

 

Yes. There are several ways to get bulkier resistors, some round, some square. What I wanted to emphasize is the ends being larger than the resistor body and that light grey background color indicating the, "run hot" resistors.

 

Resistors should always be sized at 2 to 3 times the required wattage and then keep them off the board by 1/8" or more.
That should be more than sufficient.

 

if thbere are no pc runs under the resistors, drill some holes in the fiberglass for ventilation. also, if you have replacement resistors, mount them on longer leads to space them above the board.

 

All good advice but as the circuit is just powering leds there should never be that much heat generated resistor wise to burn the board in that way if the resistor value/wattage has been properly worked out with reference to the supply voltage available. ....are these ran direct from mains ? What is the mystery supply voltage used ??? You could fit slightly higher wattage resistors and space them away from the board a little.....

 

If you look carefully at the picture the annular rings for the resistors also show discoloration. This is due to the leads themselves conducting heat out of the resistor into the board. Spacing the body of the part will only help a little with that issue.

The pads should be greatly increased in size to large copper masses to aid in heat dissipation. These pads should be of the "thermal via" type that isolated the pad a bit from the mass so you can still solder to it.

 

I think the issues are very obvious:

1. PCB is charred
2. Resistors not rated for proper power dissipation
3. Solder joints will deteriorate

You are powering the LEDs from AC mains, correct?

That is a violation of the Terms of Service (TOS) of All About Circuits and further discussion along those lines will result in this thread being closed.

 

I doubt that it is powered from mains as the resistors appear to be 100 Ohms.

Bob

 

There's diodes there too(1N4007 ?)......still shouldn't be running hot as they are only driving leds....so....what is the supply voltage used???...