2 Wrote:
>>Has anyone had any experience using Vishay Type RH metal-cased wire-wound
>>resistors (for example Mouser p. 205) as glitch resistors? There is no
>>voltage rating given, and I'm skeptical about flashover between the
>>windings and the metal case at 2500V and up. What about the Vishay HL
>>silicone wirewounds ( see Mouser p. 204)? They seem like a much more
>>conventional choice. Price is right for both, as is range of values
>>available.
>>
>>73, Pete N4ZR
The metal clad power resistors are not suitable for glitch-resistor
service. The resistor-to-case voltage rating isn't high enough, and the
metal case greatly increases the risk of end-to-end arcing - which is
exactly what we need to avoid. Also, without very good cooling (who
wants a heatsink floating at B+?) the resistor element will burn up the
insulation, which is only epoxy-fibreglass. Someone here recently
mentioned degradation of voltage breakdown in use. A definite No.
Don't know about the HL silicone wirewounds, but I agree with Rich:
>We buy Ohmite glass-coated/vitreous resistors for glitch service. They
>are not as tough as high-energy discharge type resistors, but they cost
>Much less $$. A 10-ohm, 10Wer will discharge a 3kV SB-220, TL-922 ps
>without damage. We used to use cheaper resistors - but they arced
>between turns, blew apart, and threw schrapnel about during a glitch. My
>guess is that glass-coating improves the insulating ability from turn to
>turn during a major fault. For more than 3kV, two or more such resistors
>in series will do the job. For a higher amount of joules [C >30uF],
>additional units are needed.
The vitreous enameled types are by far the best value for money. The
glass coating does work, and they are also long and thin, which is just
the right shape to avoid end-to-end flashovers. At the price, I'd go for
two 10-ohm 10W as a minimum, and three for larger supplies.
The Welwyn 14W resistors available in the UK are even better.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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