Re: Fuse(d) Note

[Terry Fritz <twftesla-at-uswest-dot-net>]

to: Ross

Never use a digital meter on a HV circuit.  If the connection or ground is
broken for any reason it will usually blow all the IC's along with the fuse.
A few good cheap analog meters from any surplus house are worth their weight
in gold when it comes to doing HV testing.

Regards,

Dr.Resonance


-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Wednesday, June 30, 1999 4:26 AM
Subject: Re: Fuse(d) Note


Original Poster: Ross <ross-o-at-mindspring-dot-com>

Beautiful Advice!  Why couldn't you have posted it 6 months ago right
before the fuse blew in my Fluke 77!  The fuseholder ends make a pretty
good sealed spark gap, athough the arcs cheats a bit as it wanders
around on the components on the board.

I killed the meter while trying to figure out how I could take two NSTs
that each measured 60ma by themselves,  parallel them, and only get
about 100-110ma.  Yes, they were phased correctly.  Any ideas as to why
this happened?  It could have been bad measurements on my part, but I
repeated it 2 or 3 times with the same result.

I wanted to somehow blame it on additional L or R, but  both of these
quantities were reduced with the primaries in parallel.  What am I
missing?

Thanks,
Ross Overstreet
Huntington Beach, CA

Tesla List wrote:

> 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



--
Ross Overstreet
Huntington Beach, CA
ICQ 20762411