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In Reply to: RE: safety margin posted by Lew on December 03, 2016 at 11:38:10
I added Hexfreds to a buddies SLI-300b. Thought one 1200V fred would be adequate. Lasted about 6 months before failure. Went to two in series with 4.7M voltage equalizing resistors (not sure if they are absolutely necessary) and it's been good for the last several years. I used DSE-160 (TO247) FREDs and mounted them back-to-back with a mica spacer, the two leads on one side were clipped, bent and soldered together. The resistors were soldered across the leads (parallel connection) and wire leads soldered to the two remaining leads. If you use TO220 devices be sure to use nylon hardware if you decide to mount back-to-back.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Follow Ups:
Were the Hexfred diodes installed with snubber caps (usually ceramic) across them?
Yes indeed! They were 1600V 0.01uF polypropylene.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
It was fairly common that ham radio operators that built electronics used diodes rated 2.8x the secondary transformer voltage. It was written in ARRL Ham Radio books as a minimum safe design. Dennis Had of Cary is a ham radio operator.
How do they fail?
My experience too with other diodes. It's a hobby so I never cared to track it down and quantify but I suspect there are some really high voltage spikes that just break down the SS diodes over a relatively short period of time. Once that happens they just become shorts.
The safety margin needs to be much higher than simple calculations would predict. I was blowing 1200 V PIV parts on a ~ 450 DC B+ supply.
Mike
Short. Main PS fuse goes "poof" on turn-on.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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