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Re: High Voltage Probe Instability

Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi Dave, all,

Yep, the divider resistor heating was what I was starting to
think as the culprit myself. I would assume that the initial volt-
age measurement would be the most accurate one and  not
the "heated" divider measurement? Thanks for the input.

David Rieben

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Monday, February 07, 2005 9:01 PM
Subject: Re: High Voltage Probe Instability


Original poster: Sparktron01@xxxxxxxxxxx
Hi David

My bet is you're seeing a +TCR effect (heating) of the divider resistors. As an example, 1.0 Megohm 1W Yageo metal-oxide resistors at Digikey are listed as 300ppm (0.03%) per deg C. If you have a 100 deg C rise on the divider string (a lot I realize) will equal ~ 3% TCR. If the TCR stacking is + on one side
of the divider and - on the other, a 5% delta would be relatively easy to achieve.

On my HV monitoring system, the divider resistors are mounted on a PCB immersed in oil as an insulating (and heat sink cooling) medium. Worse case error term seen (even with 50kV thermal soak) was << 1% (at 80% rated power dissapation).

Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA