A request from a non member

[Refugee]

I would just fit an aluminum resistor to the inside of the case so that the heat will spread out over the outside of the case where there is plenty of ventilation and it will run a whole lot cooler.

[richard296]

Ok refugee will do that its gonna save me a lot of work. I will order the alloy cased 25 watt resistor. thank you .all

[Skywave]

Richard - just to be quite sure that you fully understand this matter of 'wattage' when it comes to resistors: here's an analogy.

Consider a car engine. It is designed to have a maximum speed of, say, 10,000 r.p.m. Now you can drive that car at any speed you like - just so long as the engine speed doesn't exceed 10,000 r.p.m. The stated wattage rating of a resistor is equivalent to the maximum r.p.m. of that car engine. The actual power the resistor will dissipate is the equivalent to the speed the engine actually runs at. So the objective is to always keep the engine r.p.m. less than the maximum r.p.m.; the power actually dissipated in the resistor must always be less than the maximum power the resistor is designed to dissipate - this is the stated wattage rating of that resistor.

Like lots of things in life, there is a safe limit as to how hard you can 'push' something: the stated wattage tells you that. But if you exceed that figure - trouble!

Al.

[richard296]

thank you for that sky wave. richie

[richard296]

Just a note to thank you all for your assistance. I fitted a alloy shrouded resistor 25 watt 150 ohms. Still got hot coudn't touch after 30 mins or so. I decided to cut computor processor heatsink in half to fit. I bolted resistor to heatsink and bolted heatsink to alloy wall of power supply case. does the job doesn't even get warm. I will post pics for your comments. Cheers richie

[Yorkie]

Well done on the restoration Richard.

Just a small point to note - as you've discovered, metal-clad resistors only have the capacity to dissipate their stated Wattage if affixed to an effective heatsink. I restored a Portadyne Princess a while ago which had a burnt out 2 section dropper resistor. I fitted two 50W metal clad resistors in its place, mounted on a thick aluminium angle heatsink, which only gets warm to the touch, like yours does. Below is a pic of the original dropper and one of the new resistors on the heat sink. (Nice little ' minimalist' TRF set with a low component count, that works quite well, all things considered).

Have fun!