30-11-2016, 01:12 PM
This comes up from time to time, and the question of balancing resistors usually comes up. Same with capacitors in series.
In most cases (of diodes and capacitors), resistors aren't needed.
It is just like putting two zener diodes in series. The voltages add up. Hence yes, the PIV doubles.
If the voltage across one reaches the breakdown voltage, then nothing happens beyond the voltage across it staying reasonably constant. The other diode is still an open-circuit, passing nothing but a tiny leakage current. As that current really is tiny (perhaps 100uA at worst for a 1N400x, and more like 0.1uA at room temperature), no power to speak of is dissipated in the diode that is acting like a Zener, so nothing bad can happen to it. Obviously, when the total voltage is such that both diodes can break down, then that gets "interesting", but that would be the same for a single diode of double the voltage rating.
Unlike Zener diodes, the breakdown voltage is not tightly specified. Generally, it is very conservatively specified for the 1N400x types that I've measured in the past. Most of the lower-voltage parts are "binned" after manufacturer, so it's quite feasible to find a 1N4001 that meets the 1N4004 spec.
The point about transient behaviour is an interesting one. Very much "it depends" - careful testing would be required. At 50Hz, in a low-energy situation, it's probably not a big deal.
Think back to those early metal rectifiers - the type that were full of stacked discs. Did you ever see balancing resistors in there?
http://www.vintage-radio.net/forum/showp...p?p=510861&postcount=17
In most cases (of diodes and capacitors), resistors aren't needed.
It is just like putting two zener diodes in series. The voltages add up. Hence yes, the PIV doubles.
If the voltage across one reaches the breakdown voltage, then nothing happens beyond the voltage across it staying reasonably constant. The other diode is still an open-circuit, passing nothing but a tiny leakage current. As that current really is tiny (perhaps 100uA at worst for a 1N400x, and more like 0.1uA at room temperature), no power to speak of is dissipated in the diode that is acting like a Zener, so nothing bad can happen to it. Obviously, when the total voltage is such that both diodes can break down, then that gets "interesting", but that would be the same for a single diode of double the voltage rating.
Unlike Zener diodes, the breakdown voltage is not tightly specified. Generally, it is very conservatively specified for the 1N400x types that I've measured in the past. Most of the lower-voltage parts are "binned" after manufacturer, so it's quite feasible to find a 1N4001 that meets the 1N4004 spec.
The point about transient behaviour is an interesting one. Very much "it depends" - careful testing would be required. At 50Hz, in a low-energy situation, it's probably not a big deal.
Think back to those early metal rectifiers - the type that were full of stacked discs. Did you ever see balancing resistors in there?
http://www.vintage-radio.net/forum/showp...p?p=510861&postcount=17
