Re: HV Measurement - The Divider Problem (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 23 Sep 2004 16:44:58 +1200
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: HV Measurement - The Divider Problem (fwd)

On 22 Sep 2004, at 22:36, High Voltage list wrote:

> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Wed, 22 Sep 2004 16:53:00 -0700
> From: Ed Phillips <evp@xxxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Re: HV Measurement - The Divider Problem (fwd)
> 
> "Just built one myself.  It works perfectly to give 1/100 reading AC
> or DC.  Only 2 cheap resistors needed.  On a $30.00 DMM, I've measured
> up to 15 kV so far and regularly 2 to 8 kV with MOT voltage
> double/tripler experiments without incident.   Less than $5.00
> invested.
> 
> 
> 1 Ohmite SLIMOX 100Meg thick film 2 Watt 15 kV (Mouser $4.20) + 1
> single 1.2 Meg resistor of any type (1/4Watt is fine as current will
> be in micro amperes) - can also take several smaller resistors in
> series to give 1.2 Meg and treat them as a single resistor.
> 
> Solder both resistors in series.  Connect high end of circuit to the
> free end of the 100 Meg Ohmite and the opposite end to ground.  Place
> DMM leads  - one between the the two resistors and on the low (ground)
> end.
> 
> Reading on DMM is multiplied by 100 to give exact voltage.
> 
> 
> 
> 
> Carl Litton"
> 
>  I'd be inclined to add a small, low-leakage capacitor (0.01ufd
> polystyrene?) across the meter "just because".
> 
> Ed

That approach will give errors if you are measuring voltages at 
frequencies more than a few Hz. Proper divider design demands 
compensation to avoid the thing behaving like a LPF. For that reason, 
two resistors instead of a string of, say, ten or twenty could make 
building compensation in interesting. NOte that the meter input shunt 
capacitance also has to be taken into account when compensating.
 
Malcolm