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G8MNY  > TECHNI   14.10.21 10:20l 92 Lines 4690 Bytes #999 (0) @ WW
BID : 50046_GB7CIP
Subj: Kelvin Varley Voltage Divider
Path: PI8DRE<PI8CDR<DB0RES<DB0ERF<DK0WUE<GB7CIP
Sent: 211014/0815Z @:GB7CIP.#32.GBR.EURO #:50046 [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
             o    o    o
                8k         1k6        320        0.01s
                 o        o        o    o 10
                8k         1k6        320        64
                 o        o        o        o 9
                8k         1k6        320        64
                 o        o        o        o 8
     100's      8k         1k6        320        64
                 o        o        o        o 7
+500V           8k         1k6        320        64
 o        o        o        o        o 6
     40k        8k         1k6        320        64
      o        o        o        o        o 5
     40k        8k         1k6        320        64
      o        o        o        o        o 4
     40k        8k         1k6        320        64
      o        o        o        o        o 3
     40k SW5a   8k  SW4a   1k6 SW3a   320 SW2a   64
      o<    o<    o<    o<    o<
     40k    |    8k     |    1k6    |    320    |    64          |
      o 100V    o  10V    o  1V     o 100mV   o 1    Selected
     40k    |    8k     |    1k6    |    320    |    64 10mV   Output
0Vo<o<o<o<o 0    Voltage
         SW5b       SW4b        SW3b        SW2b        SW1     |
     ___________________________________________________________0V

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
             o    o    o
                6k8        1k5        330        0.01s
                 o        o        o    o 10
                6k8        1k5        330        68
                 o        o        o        o 9
                6k8        1k5        330        68
                 o        o        o        o 8
     100's      6k8        1k5        330        68
                 o        o        o        o 7
+500V           6k8        1k5        330        68
 o        o        o        o        o 6
     33k        6k8        1k5        330        68
      o        o        o        o        o 5
     33k        6k8        1k5        330        68
      o        o        o        o        o 4
     33k        6k8        1k5        330        68
      o        o        o        o        o 3
     33k SW5a   6k8 SW4a   1k5 SW3a   330 SW2a   68
      o<Ĵ    o<Ĵ    o<Ĵ    o<Ĵ    o<
     33k        6k8        1k5        330        68          |
      o   2M2   o   150k  o   33k   o   22k   o 1    Selected
     33k        6k8        1k5        330        68 10mV   Output
0Vo<o<o<o<o 0      |
        SW5b        SW4b         SW3b       SW2b        SW1     |
     ___________________________________________________________0V

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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