2 wrote:
>
>>
>>Ian, is there any preference for the type of resistor used for the glitch
>>resistor?
>>I have some 25 ohm/25 watt Dale RER type metal body resistors, but I am
>>concerned that they might be prone to flash over due to the relatively small
>>gap between the leads and the metal case.
>
>// True
>
Yup, they do. The wattage rating only applies when they are bolted to a
large piece of metal, and the rated maximum voltage between the resistor
and the body is only about 1000V AC. (I have used them, sometimes two in
series with the bodies floating, but am aware that I'm pushing my luck.)
>> Presumeably a ceramic body wire
>>wound resistor is the way to go.
>
>// During a glitch, the ceramic case tends to explode and throw
>schrapnel around one's abode.
>
That's certainly true of the square type which has a square ceramic body
over a fibreglass core. These are no good as glitch resistors are only
available up to 17W and they can't stand a high end-to-end voltage - for
a few moments they may see the full B+, remember - and there's a very
high risk of local flashover between turns.
You'll never find a transient end-to-end voltage rating for ordinary
commercial wirewound resistors, so we just have to go for the big, long
tubular ceramic ones - 50W size or larger. The longer and thinner, the
better.
>>Should I be concerned about inductance
>>(L*di/dt kick) -
>
>// No
>
The professionals use carborundum 'glo-bar' resistors because they have
a continuous resistive film which is much less likely to flash over
locally. These happen to be substantially non-inductive, but there is
actually no problem about inductive kick with ordinary wirewounds - on
the contrary, the L will help to slow down the rise time of the current
surge.
A while ago I did some modeling on this problem, and found there's no
way you'll get any inductive kick or ringing until the inductance gets
up to at least 10 millihenries or so (based on typical values for
amateur amps). With large wirewounds you're still in the few-hundred-
microhenries region so the C-L-R circuit is well overdamped.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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