Originally Posted by
RomanLini
Assuming the PSU is actually attached to something (like stepper motor drivers!) it is probably sourcing 2A or 3A into the drivers, so that 50mF cap will be discharged very quickly after power down.
Normally a bleed resistor like that is for safety situations where the PSU may be on test etc with no load and you can use any reasonable value resistor. Normally manufacturers will pick a physical size resistor they wish to use (say 1W) and spec the bleed at a safe continuous heat dissipation in a warm environment (say 0.2W) which in your case would be; 80v @ 0.2W = 2.5mA which is about 33k 1W resistor.
Of course if you have specific reasons why you need to quickly discharge that PSU when it has no load then there are options for relays, SCR crowbar circuits etc.