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Old 24th April 2009, 06:42 PM   #1
bigwill is offline bigwill  United Kingdom
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Default HT crowbar

Thinking of adding an HT crowbar to an amp to protect the output transformers in the event of output valve runaway. The crowbar circuit I'm thinking of would be the usual thyristor with a 220 ohm high pulse power resistor (to stop insane current) in the anode after the HT fuse.

Any feedback on this? I'm not sure what resistor type would survive - I've seen ceramic composition ones that might work - can a relatively small resistor handle a large power spike long enough to blow a fuse?

Any suggestions would be welcome
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Old 24th April 2009, 07:52 PM   #2
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Hmm,

So, You're using a Valve-rectifier?--That wont like it!

Remember the crow-bar cct used in Philips/Thorn and other colour sets that was rigged up just as you describe, that triggered for no apparent reason to destroy the PSU's....??

I do....

Hideous things IMO......

Nothing better than a good old Fuse for the job, or if you are That worried, monitor the current via the O/P bottle cathode and use a relay to cut the supply, and not summit to short-out the PSU!!

Each to his own I suppose, Go with it if you like, but its summit I would never even consider--even for a micro-second!

IF you are gonna do it, Use what Philips used. A 10ohm wire-wound. The resistor never failed but most of the silicon in the PSU did!

Valve 'run-away' shouldnt be an issue in a well designed set-up as long as you stay well within Max dissipation and dont exceed the grid resistor values quoted in data-sheets...
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Old 24th April 2009, 08:08 PM   #3
bigwill is offline bigwill  United Kingdom
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Quote:
Originally posted by Alastair E
Hmm,



Nothing better than a good old Fuse for the job, or if you are That worried, monitor the current via the O/P bottle cathode and use a relay to cut the supply, and not summit to short-out the PSU!!

Actually I'd much rather use a relay to chop off the HT instead, but I can't seem to find any suitable for the voltages involved
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Old 24th April 2009, 08:13 PM   #4
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How much volts are you thinking...?
Your average relay should easily handle up to 400V....

Then again, IF you are using say, a kilovolt or up, why not cut the Primary voltage to the mains TX and power the protection cct via a separate small Tx...?
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Old 24th April 2009, 08:24 PM   #5
bigwill is offline bigwill  United Kingdom
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HT is around 380V. Logically I would've thought they could handle it, but a lot of the ratings say 230Vac, but then something like 25Vdc

I'm guessing this is related to the inductance issues you get with interrupting DC current

Maybe this wont matter if I do the usual diode / cap bypass across it?
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Old 24th April 2009, 08:49 PM   #6
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Havenít heard anyone mention those horrible crowbar circuits for years. A correctly rated fast acting fuse in each HT line is all Iíve ever used.

How about put a low value resistor in series in the HT line then monitor the voltage across it with an op-amp/comparator and use this to control a relay? Iíve thought about developing such a circuit but never got around to it.

If you are worried about using a relay to switch the HT put it before the rectifier so you are switching a.c. [self extinguishing arc] Iíve been using the same relay to switch 500V a.c. in a soft start for a couple of years with no problem. I've connected two sets of contacts in series with a snubber. Disadvantage is the stored energy in the reservoir and smoothing caps supplying a fault current until they discharge.

Just an idea to throw in the pot.
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