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[AMPS] Diodes

To: <amps@contesting.com>
Subject: [AMPS] Diodes
From: 2@vc.net (measures)
Date: Tue, 22 Feb 2000 11:33:38 -0700
>
>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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