>
>Rich says:
>
>>Not a good wager. Without equalization, neither diode's piv limit is
>>exceeded and reverse current is virtually zero. With equalization the
>>101v diode is toast.
>
>Sorry Rich - NO. Let me repeat:
>
>> > If the PIVs are that different, the capacities are likely to be so
>>
>Now, when two capacitors are placed in series ACROSS DC, let alone AC, the
>voltage divides inversely proportional to the capacitance.
True enough. However, you are trying to fly on an assumption. .
>So the smaller
>capacitance gets the most voltage. As the smaller capacitance tends to go
>with the higher diode reverse voltage for a given leakage, that means that
>the higher voltage diode gets more of the volts, and the lower voltage diode
>gets less.
>
>Next factor is the cause of the leakage. It could be straight electric field
>breakdown across the barrier, or it could be surface leakage, or it could be
>a crystalline defect in the silicon - defects need not cause a catastrophic
>problem in large geometry devices, depending on exactly what they are. All
>these leakage causes have different V/I characteristics, so once the
>voltage exceeds an amount that is the 'reverse voltage measured at some
>specified current' we can get even more rapid differential voltage
>distribution.
>It might be such as to improve matters. However, 'Murphy rules'. So it
>probably won't improve matters, but will screw up the voltage distribution
>between diodes even more.
>
>The bottom line there is that if the surge is to be 250 volts, you need 250
>volts worth of diodes under all conditions,
agreed. 101piv plus 190piv = 291v which exceeds what we need by 41v.
.The ''equalizer'' resistors act to divide the 250v surge equally. Approx
125 v causes the 101v diode to short, whereupon the 190v diode's reverse
withstanding ability is exceeded and it also fails.
> and assuming equal voltage
>distribution - preferably 300 volts or even more. .
>Keeping surges within
>bounds is also useful, but that's why we have varistors and AC rated
>capacitors.
>
>73
>
later, Peter
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