Re: Fuse(d) Note

["Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

I would add that putting one of those vacuum spark gaps surge supressors
(like the ones from Victoreen mentioned on the list recently, or similar
products from Siemens) across the meter is a good idea. Particularly if you
are using a series string of resistors for voltage metering.  If the meter
goes open, the voltage on the terminals will rise spectacularly unless
something clamps it lower (like a spark gap).

A zener diode pair would also work to clamp the max voltage, but you have to
watch the peak current.

Another trick, although I wouldn't trust it for HV protection, but, it will
save the meter movement, is to put a pair of diodes in parallel, back to
back, across the meter terminals.  The voltage across the meter coil will
typically be very low, well below the diode forward voltage drop, so they
will have no effect, in normal operation.  However, if you overload the
meter, the diodes will conduct and save the movement (you hope!).  You still
need the sparkgap, though, because the diodes might open under a transient
(of course, a small 1n914 has the leads close enough together inside the
package that it probably works as a small sparkgap, much like the fuse
problem.

Fusing HV is a real tricky operation, by the way, because a relatively low
voltage can draw a very long arc, and fusing a piece of wire is a good way
to draw an arc.  Commercial HV fuses either provide a means to spread the
arc out and cool it (magnets, packing it in silica granules (sand), etc.) or
to have an ablating coating that literally blows the arc out with the gas
produced. More at http://home.earthlink-dot-net/~jimlux/hv/fuses.htm

-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Tuesday, June 29, 1999 5:23 PM
Subject: Fuse(d) Note


>Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
>
>Hi All,
>
>> Original Poster: Tedd Payne <teddp-at-earthlink-dot-net>
>> MA AC METER
>> 0-50 mA AC, measure neon transformer current directly, metal
>>enclosure with binding posts on top, fuse, good cond $15.00
>
> Snip
>
>Just a thought for measuring freaks:
>
>If you measure the short circuit current of a (limited) hv xformer
>DO NOT use a fuse in the meter circuit!! While this may sound
>silly at first (and might get some eyebrow raising), there is
>reasoning behind this thought. A 50mA meter will be able to
>handle around 150-200mA without being damaged. While the
>needle will peg to the endstops, the coil and the mechanical
>mechanism won´t be damaged by this. A 50mA fuse, on the
>other hand (meter fuses are fast or SURE-BLO type), will
>NOT be able to handle a 3-4x rating. If the meter fuse is your
>typical 5x20mm (D) or 6x32mm (U.S.) kind, the open distance
>will NOT stop the arc from a 12-15kV xformer. This WILL
>fry your meter instantly, as the meter (coil) doesn´t have the
>proper insulation to handle the hv (now) present.
>
>The same goes for a DVM. ALWAYS measure the current
>on the 10 or 20A range. This range usually isn´t fused,
>(although some expensive units do have one) and the shunt
>is of very low ohmage (0.01ohm or thereabouts). This will
>act as a "full" short for a current limited xformer. The DVM
>will take no damage because the voltage drops to very low
>values (way under the rated 750-1000Vac of the meter).
>
>Just to be safe, you shouldn´t touch the meter case. It IS
>better to fry your meter (shouldn´t happen) than it is to fry
>yourself. NEVER switch the DVM range while the xformer
>is under power. This will ruin the DVM, as most DVMs use
>a "break before make" type of rotary switch to change
>ranges. This kind of switch will allow the voltage level to
>rise up to very dangerous levels (during movement of the
>contacts) that will overvolt your meter and destroy (it WILL
>definately destroy the precision AC->DC rectifier) it. But
>then, you shouldn´t be touching the meter case anyway....
>............. ;o)
>
>
>Coiler greets from germany,
>Reinhard
>
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