I had a 10 watt 50 ohm non-inductive resistor strapped to a heat sink and it appears to have burned itself out. For a long time it was hooked up to a 10 watt transmitter and working. The entire heat sink would get too hot to touch, but the transmitter remained cool and the SWR was 1.2. However, a few days ago it took a poop. The SWR shot up and the transmitter got hot.
Are these resistors not intended for long term use? Should I have used thermal grease or a fan?
Resistors are rated for their power at a certain temprature - usually 25C. Above that you have to derate them.
If your heatsink was too hot too touch (generally if you have to pull back in 3 seconds, 60C. 2 seconds, 70C and one second, 80C.), your resistor was being overpowered.
Rule of thumb with dummy loads, even on heatsinks is 5 x the power of your TX. So, a 50W resistor for a 10W TX. A fan on the heatsink can drop this rating to 30W for a 10W TX.
