Message 25515 in TECH.WW
From: G8MNY
Date: Wed, 08 Jul 26 08:24:00 Z
Newsgroups: TECH.WW
Subject: Kelvin Varley Voltage Divider
Message-ID: <64430_GB7CIP>
Path: N2MH4|VE3KPG|VE3CGR|LU9DCE|LU1DBQ|VK6ZRT|GB7CIP
R:260708/0832Z 33413@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
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R:260708/0830Z 17259@LU9DCE.TOR.BA.ARG.SOAM LinBPQ6.0.25
R:260708/0830Z @:LU1DBQ.#MOR.BA.ARG.SOAM #:37382 [MORON] $:64430_GB7CIP
R:260708/0828Z @:VK6ZRT.#BUN.#WA.AUS.OC #:40732 [Boyanup] $:64430_GB7CIP
R:260708/0824Z @:GB7CIP.#32.GBR.EURO #:64430 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Nov 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I came across this intriguing circuit in an old Fluke Differential Valve
Voltmeter. It divides the reference 500V into 50,000 x 10mV steps, with just 5
2 pole 10 way switches & 49 Rs.
10's 1s 0.1s
40K | 8K | 1K6 | 320 | 64 |
40K | 8K | 1K6 | 320 | 64 10mV Output
HOW IT WORKS
The last 0.01V decade divider is conventional, with 10x 64ohm (11 steps). This
is then fed from 2 taps apart of the 0.1s (11 320ohm Rs) switch giving 10
steps. Each step has a 100mV, as the 0.01 chain load is 10x 64 = 640, across 2x
320 = 640 making the 2 taps apart look like 320 to the other 9 320 in that
chain keep the 10:1 divide ratio perfect. This only works for 11 resistor
chains with 5x the previous value. The other divide banks do the same thing.
All resistors were special 0.1% stable types. In practice the 6 high value 40K
Rs are all made with a calibration 1k preset + 39K5 for good accuracy.
Note the output Z is quite variable from 64 ohms for 10mV to 50k for 255V. So
the dialled up voltage is only with no load like a valve volt meter.
MAKING YOUR OWN
Of course standard E12 values could be used & a correction R put across each
stack, to calibrate the load to be correct to the next stack...
33Kx6 0.5W, 2M2+6K8x11, 150K+1K5x11, 33K+330x11, 22K+68x10 gives 0.2% chains.
10's 1s 0.1s
The input voltage can be what you like e.g. 200V to 1KV, just use the
appropriate number of 33K Rs (V/100 +1) to keep the system calibrated.
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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