[Amps] Equalising resistors with HV diodes

[R.Measures]

On Sep 20, 2004, at 7:14 AM, DF3KV wrote:

>
> ----- Original Message -----
> From: <peter.chadwick at Zarlink.Com>
> Subject: Re: [Amps] Equalising resistors with HV diodes
>
>
>>> Please explain why equalizing resistors have merit.
>>
>> Because if the reverse leakage differs between diodes, the reverse
> voltage
>> distribution also differs. The one with the least leakage ends up
> with the
>> highest voltage across it, which could exceed the breakdown volts.
> With
>> resistors, the voltage distribution across the diodes is fixed by
> the
>> resistor ratios.
>>
>> If the capacitances aren't matched, that will also affect the
> voltage
>> distribution. That's one reason why it is not wise to mix diodes of
> the
>> same nominal type from different manufacturers without some
> equalisation.
>> Another is the reverse recovery time.
>>
>> Experience also suggests that for best reliability, buy diodes from
>> reputable manufacturers - some of the far Eastern ones are
> distinctly dodgy
>> froma reliability viewpoint.
>
> Hi Peter,
>
> If that theory is solid, all fullwave bridge rectifiers should have
> equalising resistors connected across the diodes, but no manufacture
> suggests that.

-  Around 30-years ago, manufacturers of HV rectifier stacks stopped 
using "equalizing resistors". Apparently, someone realized that the 
currents in all parts of a series circuit are already exactly equal.  
Adding parallel resistors can quite possibly un-equalize these 
currents.


> I always used triple safety on Vrrm when I used diodes in HV rectifier
> stacks and never experienced failed diodes (most BY255)
>
> 73
> Peter
>
>
>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org