[Amps] MOV - Varistor Reccomendations

[Ian White, G3SEK]

David Pruett wrote:
>At 10:44 AM 1/29/2005, aborg wrote:
>>Can anyone suggest what MOV to use for my 1N5408
>>rectifier board. I an using 32 diodes , 8 per leg. My
>>transformer is a 4800 Vac pole pig. I want to use the
>>MOV's instead of the resistor/cap combo on each diode.
>>

>Mike,
>
>While this is not the question you asked, I'd point out that in a 
>bridge rectifier, each diode (or string) has to stand off the peak AC 
>input voltage during the half-cycle that it's not conducting, and this 
>is not referenced to zero, but to the DC output voltage when used with 
>a capacitor filter.
>
>The 4800 volt AC input is around 6700 volts peak.  The filter capacitor 
>is charged to this value, meanwhile on negative swings the rectifier 
>has to stand off an equivalent negative peak.  You need about 13.4 KV 
>worth of diode rating and you've got 8 KV.
>
>I've run the power supply for my 40M 4-1000A amp off a 4800V pole pig 
>with four K2AW 14-1 (14 KV rated) diode blocks.  Indicated plate 
>voltage no-load is about 6600V.  My understanding is that the K2AW 
>blocks are fourteen 1N5408s in series.  The above analysis shows that I 
>could probably use some design margin in this application, but it's run 
>for more than a decade with no problems.
>
>73,
>
>Dave/K8CC

Agreed. In terms of reliability, the best value for money seems to come 
from very long strings of well-matched diodes, totaling at least 2x the 
normal calculated PIV requirement.

In other words, spend your money on more diodes, not on parallel 
"equalizing" components. When you look at the prices of Rs and Cs rated 
for >>1kV with all the other ratings required, you will find that adding 
some more series diodes works out *much* cheaper than any other option.

They don't have to be one of K2AWs potted modules, or one of the modules 
sold for microwave oven spares. As Rich has pointed out, the 
encapsulation increases the chip temperature, so bare diodes on 
perf-board will be both cheaper and better cooled.

The only remaining problem is to find well-matched diodes. The military 
select their diodes by High Temperature Reverse Breakdown testing at 
high temperature, and maybe K2AW does that too, though I'm sure the 
microwave oven manufacturers don't.

Unless you have your own HTRB test rig (and time on your hands) the best 
practical solution is to buy named-brand 1N5408s from a reputable 
supplier like Digi-Key or Mouser. You're taking them off the bandolier 
in the same order as they came off the original production line, and it 
will be hard to find diodes better-matched than that.

If in doubt, simply add more diodes! Also remember that your diodes on 
perf-board will run cooler than diodes that are potted in thick epoxy, 
so they will be less likely to break down anyway.



-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek