Re: High Voltage but Low Current fuses...

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>

I'm still very confused to why you are worrying about providing high voltage
fuses for the differential probes.  The first stage of your differential
probe should be
a very high impedance divider on the order of 1Meg up to 100Meg (assuming
pure resistance now) depending on the voltage being used.  You should not be
providing
any type of fusing or transient suppression until after this divider.
Therefore, there is no need for a high voltage fuse etc...  If you look at
any run-of-the-mill differential probes made
by Tektronix or similar, they are all built this way.  Remember, you could
have a 1000:1 divider, so a 10,000V transient spike would only look like 10V
at the output of this stage.  If say 5V was
the limit going into your differential amplifiers etc..., then a clamp /
suppression network at 5V would suffice here.

Dan



 > The problem is with high voltage you can draw really long arcs.  Like 6
 > inches off a MOT and stuff.  So "real " high voltage fuses are made to
 > break high voltages at really high (5000 amps!) sustained currents.  Thus,
 > they are very big and long as they provide a very long quenched distance
to
 > break the arc.  "Mid-voltage" fuses may be say 15kV at 5 amps but they are
 > like 17 inches long!
 >
 > For our need.  We may see a sudden very high current pulse that the MOVs
 > can handle.  Then the standard 1/4 x 1-1/4 low current style fuse will
have
 > a pretty good chance of at least providing a nice arc area to dump
 > power.  Once the arc goes out, the voltage standoff is very high.  For the
 > purpose of our differential probe.  Two MOVs and two say 0.25 amp pigtail
 > fuses provide fair protection against "accidents".  Far far better than
 > nothing.
 >
 > If they could take a primary cap discharge or a MOT, and give the scope a
 > reasonable chance of avoiding damage that would be great!  Without
 > anything, scope destruction is assured.  We are just "trying" to do
 > something just in case.  Of course, it could not hold a direct connection
 > to a power line or a "real" lightning strike, but for our need it should
do
 > well.  There is even a very good chance the probe itself would only need
 > new fuses for most mishaps.
 >
 > Of course, operation over 1000 volts will void the warranty ;o))  But with
 > just a few more parts we can add a great deal of protection in case
someone
 > does goof.
 >
 > Cheers,
 >
 >          Terry
 >
 >
 >
 >
 >