Re: HV Probe and DMM Input Impedance

John Gord

I don't think your "add 1M to the probe" will work. The 11Meg VTVMs had 1Meg in the probe and 10Meg in the instrument, creating a 10/11 divider in normal operation. The instrument scaling compensated for that. Adding 1Meg to the HV probe will actually make the situation slightly worse. (The voltage at the hot end of the 1Meg will be 1/100th of the probe tip voltage, but the DVM is looking at the cold end of the 1Meg resistor.)
--John Gord

On Mon, Oct 28, 2019 at 09:06 AM, David M wrote:

Barry, the Eico (and other manufacturers as well) made those probes so that
the resistor could be changed to accommodate the particular VTVM or multimeter
you needed to use them with. In the case of the 1090 Meg resistor, it was to
be used with a meter having 11 Meg input resistance. That would calculate to
a 1000:1 divider.
So, to use your HV probe with a meter having a 10 Meg input, you would need to
add an additional 1 Meg resistor into the probe (or wherever you could make it
fit) so that the probe works into 11 Meg.

Dave M

Join to automatically receive all group messages.