# EHT Indicator images (continued) - <<<Under repair>>>

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For 1 to 1 correspondence of input kV to meter uA in balanced mode:
Rcb=Rm*(Rd1+Rd2)/(10^9-Rm-Rd1-Rd2)

Where:
Rcb=Shunt resistor for balanced mode
Rm=meter movement resistance
Rd1 and Rd2 = dropping resistances (which are each ≈400M in HP's circuit)

So for unbalanced modes it follows that:

Rcn=Rcb*Rd2*Rm/(-Rd2*Rm-Rcb*(-10^9+Rd2+Rm))
Rcp=Rcb*Rd1*Rm/(-Rd1*Rm-Rcb*(-10^9+Rd1+Rm))

Now just for example I can plug in values from schematic:

Rcb=750Ω*(400MΩ+400MΩ)/10^9-750Ω-400MΩ-400MΩ)≈3kΩ
So
Rcn=3kΩ*400MΩ*750Ω/(-400MΩ*750Ω-3kΩ*(-10^9+400MΩ+750Ω)) ≈ 600Ω
Rcp
=3kΩ*400MΩ*750Ω/(-400MΩ*750Ω-3kΩ*(-10^9+400MΩ+750Ω)) ≈ 600Ω

So my math jibes with HP's but plz remember that in real life RD1 and RD2 won't be exactly equal so neither will Rcn and Rcp

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Hypatia's Protege
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