Re: High Voltage but Low Current fuses...

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

Original poster: "Crow Leader by way of Terry Fritz <teslalist-at-qwest-dot-net>" <tesla-at-lists.symmetric-dot-net>

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Friday, July 18, 2003 1:32 PM
Subject: Re: High Voltage but Low Current fuses...


 > Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>"
<jimlux-at-earthlink-dot-net>
 >
 > Commenting on many previous posts, all of which I've snipped..
 >
 > Unlike a usual HV probe where you have a 1000:1 divider, this is a
 > differential probe designed to measure small voltages with a large common
 > mode voltage.

I guess my point was, things you connect to high energy circuits are big.
The idea of some small plastic box that can take a ligtning strike, and
still be safe to use and not damage you scope sound a bit more complex than
some $40 project it's being made out to be. Intrinsic safety should be at
the top of the list.

 >
 > In the usual HV probe case (e.g. the dividers made by Ross, etc.), you're
 > not trying to measure small voltages on a higher voltage, rather, you're
 > interested in measuring the high voltage itself, and for that a 1000:1
 > divider with 1% accuracy feeding a 1% or 5% accurate measuring device is
 > just the ticket.
 >
 > However, here, you're really looking for millivolt changes in a volt scale
 > signal on top of, say, 1000 Volts.  If you used a 1000:1 divider, then the
 > volt signals are now millivolt signals, and you're looking to measure
 > microvolt changes on a several volt background, which is quite
challenging..
 >
 > In both cases, you're faced with needing a good common mode rejection
ratio
 > (60 dB+, I would think), but that needs to be balanced against the noise
in
 > the amplifier and with the bandwidth.  Instrumentation amps give great
CMRR
 > (particularly if you use monolithic IAs rather than discrete combinations
 > of 3 op amps), and can have a handy output to drive the guard/shield
 > terminal at the common mode voltage, so that cable impedance effects can
be
 > nicely balanced (and cancelled).
 >
 > The other problem with the "big robust" divider approach is the bandwidth
 > needed vs the physical size.  Getting 10's of MHz class bandwidth to look
 > at switching transients in a PWM circuit from a big divider is quite a
 > challenge.

I question if $40 will be the safe answer to this problem. I can blow up a
radio shack project box with just two electrodes in it, connected to a large
cap, you don't even need parts inside to catch a chunk of plastic in the
eye.

KEN

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