Re: High Voltage but Low Current fuses...

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

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.

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.