>
>Wilbert Knol wrote:
>>
>>> This keeps on coming around. OE5JFL now has an English-language version
>>> of his thyristor crowbar on his own site, with readable schematics:
>>>
>>> http://www.qsl.net/oe5jfl/flashover.htm
>>>
>>> That solution is better than a glitch resistor, but it also requires a
>>> lot more effort.
>>
>>
>>I had a look at the OE5JFL crowbar. It looks like a good idea. But I
>>would still use the glitch resistor.
>>
>The 10 ohm resistor in the negative return has many of the functions of
>the glitch resistor, along with the inductor in the B+ lead. Those two
>components limit the peak surge current, and are also where the stored
>energy from the smoothing caps ends up.
>
With the glitch R in the negative return, the cathode circuitry may be
driven to several kV by a B+ to chassis arc.
A typical HV-RFC has less than one ohm.
end
>>The cascaded thyristor network looks a bit dubious though. The ones
>>slowest to fire could fail, due to having to block excessive voltage.
>>
>>I would put one thyristor across each electrolytic/bleeder. That way,
>>each cap gets discharged individually, and there is no risk of zapping
>>the thyristors.
>>
>That isn't actually necessary.
>
>All the thyristors in the cascade are paralleled by high-value
>equalizing resistors, so that takes care of the "off" situation.
>
>When triggered, each thyristor has its own gate drive through an opto-
>coupler, but still they can never all fire at exactly the same moment.
>However, that's no a problem because thyristors will trigger quite
>happily due to a voltage step. What happens is that the dV/dt step at
>the anode is coupled to the gate through the device's internal anode-
>gate capacitance, and that produces gate current. The device then fires
>in the normal way. (Usually this is an unwanted mode, because it can
>cause premature firing due to voltage spikes in AC applications. The
>detailed data sheets often specify the maximum dV/dt that the device can
>stand without self-triggering.)
>
>In this cascade circuit, if the later thyristors are not triggered by
>their own gate drive, they will fire due to the dV/dt steps caused by
>the earlier ones in the cascade. If the current is limited (which it is
>in this circuit) that mode of firing is *not* a "failure" mode, and the
>thyristor recovers in the normal way when the voltage is removed.
>
>Many versions of OE5JFL's circuit have been built, and it does work.
>
>He doesn't mention the ratings of the thyristors, but in the original
>they were 1200V/8A (that's a continuous current rating - this circuit
>uses the one-time surge rating which is always much higher).
>International Rectifier now do a range of 1600V 30A plastic-package
>thyristors for about $6, which should be just right for this
>application.
>
>>Thanks for mentioning the URL.
>>
>
>73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.com/g3sek
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/amps
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>
>
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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