Hot resistors and semiconductors
Posted: September 29th, 2013, 7:15 amWhy do so many PSU designers let resistors and semiconductors get hot enough to discolour PCBs and, worse, place them near electrolytic capacitors which therefore fail prematurely? It's not like any common electrolytic capacitor can withstand >100°C for any decent time.
The documentation for Stackpole "CF" series (a standard carbon film resistor) says they can withstand an element temperature of 155°C (similar to the junction temperature limits of power semiconductors) and can dissipate their nominal power rating at 70°C ambient (with an unspecified lead length). I can certainly see them, mounted on a less-than-thermally-conductive PCB, getting far, far above the air temperature. Run them near their rating and any surrounding electrolytics are finished.
Most rectifier diodes rated for 1A are about the size of a quarter-watt resistor, and they definitely don't have only a quarter-volt drop. You can work out the rest yourself.
The documentation for Stackpole "CF" series (a standard carbon film resistor) says they can withstand an element temperature of 155°C (similar to the junction temperature limits of power semiconductors) and can dissipate their nominal power rating at 70°C ambient (with an unspecified lead length). I can certainly see them, mounted on a less-than-thermally-conductive PCB, getting far, far above the air temperature. Run them near their rating and any surrounding electrolytics are finished.
Most rectifier diodes rated for 1A are about the size of a quarter-watt resistor, and they definitely don't have only a quarter-volt drop. You can work out the rest yourself.