Re: strange vttc

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

On 31 Mar 2004, at 17:44, Tesla list wrote:

 > Original poster: Mddeming-at-aol-dot-com
 >
 > In a message dated 3/31/04 10:03:39 AM Eastern Standard Time,
 > tesla-at-pupman-dot-com writes:
 > Instead of a HV probe, couldn't one just wire several resistors in
 > series, read the drop across one of them and just apply Ohm's/
 > Kirchhoff's law? Maybe those 10M resistors used for draining the
 > primary caps? Three resistors will give 1/3 of the applied voltage
 > across any single resistor; 4 resistors-1/4.  If you think the 4000V
 > label might be correct, go with 5 resistors, a 1000V meter could do
 > that one.  I don't know why I haven't done this myself, I have been
 > putting 120VAC into the HV windings and measuring the output of the
 > low voltage side, and figure my ratios.  I'm not sure how accurate
 > this is, but it seems to be pretty close.
 >
 > Hi Randy,
 > Putting a voltmeter across one of the resistors will give good
 > readings IF and ONLY IF the internal resistance of the meter >>
 > resistance of the resistor. Let's say you have 5 10Meg resistors in
 > series across  a 5 kV source. The voltage drop across each will be 1
 > kV and the current will be 100uA. If you place a meter with an
 > internal resistance of 10Meg across one of the resistors, the
 > effective resistance of the string is now 45Meg and the current
 > through the string and meter combination is 111uA, half through the
 > resistor. The drop across the resistor while being measured is thus
 > only 555V. Error >44%.  With a 100Meg input meter, the effective
 > resistance of the string is now 49.1Meg, the total current is ~102uA
 > of which 90% flows through the resistor making the voltage drop ~918V
 > or about 8% error. Meters with an input Z of about a Gig are sort of
 > pricey. Possible solutions: 1) Always calculate the change caused by
 > the meter 2) Many of the old Tripplet analog multimeters have a
 > 5000VAC or 6000VAC scale and cost ~$20-$30 on eBay. This eliminates
 > the need for the voltage divider.
 >
 > Hope this helps,
 > Matt D.

Another solution is to put a sensitive current meter in series with
the string and use the current to calculate the drop across the
resistors. The series resistance of a current meter will be orders of
magnitude below that of the string. This is my preferred method for
measuring high voltages. The limits on measuring high impedance high
voltage sources is defined by the current drawn and hence the
sensitivity of the meter together with the amount of series
resistance added by the meter (this can be compensated for by
reducing the value of the resistance chain).

Malcolm