[Hallicrafters] SX-42 meter - use a ZENER or DIODES

[Rodney Bunt]

Use a zener diode across the meter movement. Check out the maximum voltage across the meter when
full scale, add a bit and put a Zener across the meter terminals. any excess voltage/current will
go through the zener, should the zened be destroyed, it will go short circuit ( and protect the
meter anyway) then you can blow your fuse at leisure.....

If the voltage is not greater than the smallest zener diode ( I think it is 3.3 volts not shure on
this) use ordinary 1 Amp diodes in a string, each will develop .7 volts across them, so three will
be 2.1 volts etc. same deal with this configuration, when the voltage is too high the diodes will
conduct ( protect the meter) and then your fuse can blow...

Rodney
VK2KTZ



--- "Ray V." <w2ec at attglobal.net> wrote:
> Depends on the fuse characteristics. Besides current, voltage and
> size/configuration, fuses are also rated as Time Delay, Standard and
> Fast. Standard is the typical fuse used in most ham gear although you'll
> see the occasional Time Delay (often referred to as "Slo-Blo"). Fast
> fuses are specifically for protecting test equipment like meters etc. We
> used them in the Navy and I'm sure the other services used the same
> nomenclature. Under the military nomenclature you could tell the
> characteristics from the ID, for example a commonly used fuse for meter
> protection circuits was a F02GR010C or F02GR002C. F02 translates to the
> common 3AG or AGC sized commercial fuse of today. The G indicates a 250V
> gap voltage and R010 = 1/100th of an amp. The C indicates it is a fast
> action fuse. A chart comes with the fuse so you can check the
> time/current graph to determine what you need for a particular circuit.
> You'd choose which one you needed based on the allowable current flow
> thru the meter and how fast you needed it to open. Proper selection
> would have the Fast action fuse blow before the meter pegged hard or
> burned out. You can get commercial fast acting fuses too but you need to
> have the catalog to determine the time characteristics, it's not marked
> on the new fuses as part of the info like the military versions.
> 
> Ray W2EC
> 
> Rick Smedberg wrote:
> 
>   > About the fuse to protect the meter the answer is "no". A fuse
>   > would never blow fast enough to protect a meter. If it was added
>   > it ment there was something else wrong with the sx 42. The best
>   > meter protection is two diodes in parallel in opposite directions
>   > across the meter terminals. Some of my test meters have fuses
>   > but they are really small values. I suspect they were trying to
>   > stop the meter from banging against the end on strong signals.
>   > Proper meter adjustment would cure that problem.
>   > rick
>   >
> 
> 
> 
> 
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> dfischer at usol.com
> ----
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> ----
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