Fault-finding a linear supply
Symptom: the 5 V supply hums and its output dips under load. Work from the output back to the mains: check the output, then the regulator’s input, then the reservoir capacitor’s ripple, then the transformer. The first stage that is wrong points to the fault.
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How it works
A good 2200 µF reservoir gives about 398 mV of ripple at 0.1 A. Ripple of several volts means the capacitor has lost most of its capacitance (here it behaves like about 110 µF).
The 7805 needs its input above about 7 V even at the bottom of each ripple dip; below that it drops out and the dip comes through to the output.
Try it on the bench
- After the repair, set R_load to 10 Ω: the good capacitor still holds the regulator out of dropout.
Parts on the bench
In the course
Electronics Technician, module “Power supplies”.