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Re: Measuring Output Current from a NST
Hello Ross, all
You wrote:
>Original Poster: "Ross W. Overstreet" <ross-o-at-mindspring-dot-com>
>
>The first several times I tried to measure the output current of my NST
>bank everything went great. Last night things took a turn for the worse
>when the fuse blew in my Fluke 77 series multimeter and the arc started
>"looking around" inside the DMM for somewhere to go. I managed to burn
>a hole in the PC board before I realized that the funny buzzing sound
>was comming from the meter. I haven't examined it too carefully yet, but
>it seems to be OK. No components exploded and the trace on the card
>isn't damaged too bad. Guess this is a bad idea, huh? Especially when
>the fuse somehow blows and suddenly the two terminals inside the meter
>become a sealed spark gap, hehehe
>From what you wrote I am guessing you took a measurement using the mA scale.
You should always use the 10 or 20A scale (depends on your meter). The mA
range has a too high an internal resistance (meaning a high voltage
drop across the measuring "shunt") and this may cause the arc over you
experienced. The 20A scale has very low resistance and will act as a full
short for your NST (meaning almost zero volts across the meter terminals).
Although the physical damage may be small in your DMM, you might have shot
the precision rectifier inside (AC->DC) or other electronics. I would be
very careful about using this DMM again (esp. for higher voltages i.e.:
>40V).
You should also take the input selector switch apart and check for carbon
tracking. DMMs I have blown (3 or 4) usally are NOT repairable.
Good luck on new measurements and stay away from those mA scales......
Coiler greets from germany,
Reinhard