03-09-2014, 09:49 AM
When several power diodes are connected in series for rectifying a high a.c. source - such as you refer to - it is necessary to ensure that the reverse voltage across each diode is the same - or very nearly. If the reverse-biased resistance of those diodes could be guaranteed to be equal - then that equality would be assured. But that reverse-biased resistance will not be equal - so high ohmic-value resistors are wired in parallel with each diode to equalise that reverse voltage. The usual value of those resistors is in the range 220 kΩ → 470 kΩ.
The purpose of then connecting low-value capacitors in parallel with each diode/resistor combination is to enable short spikes of voltage (usually from the incoming mains supply) to by-pass the diodes. Such voltage spikes could 'punch through' the P-N junction of a diode bringing about its destruction. The usual value for those capacitors is in the range 1,000 pF → 0.01 uF; 1,000v working is usually adequate.
When the voltage source to such a silicon rectifier arrangement is derived from the H.V. winding of a transformer, as an alternative to those capacitors, sometimes an R/C series network is wired across the secondary winding: the purpose of that is the same: to present a low-impedance path to high-voltage very short duration spikes.
When only one (or pair, in a bi-phase rectification scheme) of diodes is used, that R/C network is then still required for same reason.
Al. / Skywave.
The purpose of then connecting low-value capacitors in parallel with each diode/resistor combination is to enable short spikes of voltage (usually from the incoming mains supply) to by-pass the diodes. Such voltage spikes could 'punch through' the P-N junction of a diode bringing about its destruction. The usual value for those capacitors is in the range 1,000 pF → 0.01 uF; 1,000v working is usually adequate.
When the voltage source to such a silicon rectifier arrangement is derived from the H.V. winding of a transformer, as an alternative to those capacitors, sometimes an R/C series network is wired across the secondary winding: the purpose of that is the same: to present a low-impedance path to high-voltage very short duration spikes.
When only one (or pair, in a bi-phase rectification scheme) of diodes is used, that R/C network is then still required for same reason.
Al. / Skywave.
