Capacitors in Parallel with diodes in a Bridge rectifier - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

G8HQP Dave · 8th Mar 2019, 3:59 pm

Modulation hum due to the rectifier is more commonly seen (and fixed by the parallel cap) in AC/DC sets, because in AC sets the mains transformer won't pass much RF anyway.

Craig Sawyers · 8th Mar 2019, 5:51 pm

Indeed; but the schematic posted was for an AC/DC set.

I agree that a mains Tx won't pass RF, but it can still inductively couple into the circuits via the mains wiring inside the set (via the fuse and to and from the voltage selector and mains switch)

Maarten · 10th Mar 2019, 2:37 pm

Quote:

Originally Posted by GMB

When silicon diodes first became readily available I am sure I read somewhere that the idea of putting capacitors in parallel was to protect the diode from voltage spikes on the mains. This being when the PIV ratings available were only just good enough.

That may not have been the only reason, at least Philips and ITT used capacitors with more modern diodes such as BY127 and BY133 as well.

8th Mar 2019, 3:59 pm	#21
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Capacitors in Parallel with diodes in a Bridge rectifier Modulation hum due to the rectifier is more commonly seen (and fixed by the parallel cap) in AC/DC sets, because in AC sets the mains transformer won't pass much RF anyway.

8th Mar 2019, 5:51 pm	#22
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,984	Re: Capacitors in Parallel with diodes in a Bridge rectifier Indeed; but the schematic posted was for an AC/DC set. I agree that a mains Tx won't pass RF, but it can still inductively couple into the circuits via the mains wiring inside the set (via the fuse and to and from the voltage selector and mains switch)