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Re: Fuse(d) Note
----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
Sent: Wednesday, June 30, 1999 6:22 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.
Yes, I remember you posted that. I just somehow forgot to answer
it then and there.. ;o(.
>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.
If each NST measures 60mA on its own and around 100-110mA in
parallel, quite obviously you have them phased correctly, but are they
truely of equal voltage? I mean, did you measure it (not the faceplate
values)? Since the difference is between 10 and 20mA, I would
suspect that one has a lower voltage output and thus it draws current
from the other one, resulting in heat and a combined lower output than
you would expect to see. Are they from different manufacturers,
perhaps? If so, they might be built completely different (wire gauge,
number of turns and turns ratio). But even if they are from the same
manufacturer, you just might have "lucked out" in a combo, where one
NST is on the low and the other one on the high side of the
manufacturer´s specs. One thing you could try:
1.) Parallel both xformers.
2.) Short the secondary side.
3.) Place a (really 2) ampmeter IN a leg of EACH NST.
Both ampmeters should now show you 60mA (i.e. their full
capability). If so, then your problem is really a difference in
voltage between the two. I wouldn´t worry about this too much,
because you have no idea what the transformers really do at
a particular load. I.e.: in real coiling usage you have more than
one load condition. You have an open circuit during non conduction
of the gap and an almost full short when the gap conducts. It is
almost impossible to match transformers so that they will share
the current equally for every load from 0 to 100%. However, the
NSTs will take no damage from this slightly unequal loading. You
will just have to accept the slightly lower total output (you CAN
always add a third one in parallel ..;o}). This is why I have said
quite a few times, don´t depend on the faceplate values (for cap
sizing, etc), but rather measure your psu setup´s output.
Coiler greets from germany,
P.S.: What´s up in HB? (I used to live in Westminster, which
is (well was...) kind of next-door to you ;o})