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When I opened the supply board to the detector (Geiger counter,) I got the above circuit.

The detector uses a gas-filled tube with a central wire at high voltage to collect the ionization (charge) produced by incident radiation.

What is the use of the capacitor-1 (C1) and 2 (C2?)

The detector has only one connection. Both high voltage input and the ionization charge (output) are through this connection.

How is it possible? I marked the splitting point as X in the picture.

The amplifier is a charge sensitive preamplifier.

  • \$\begingroup\$ I'm voting to close this question as off-topic because it's really about how geiger tubes work and there is no evidence that the OP has bothered to google this extensively covered subject. \$\endgroup\$
    – Andy aka
    Feb 17, 2020 at 13:42
  • 1
    \$\begingroup\$ the question is not about the working of the Geiger counter. The diagram isn't even Geiger. The question is about the two parallel ceramic capacitors, those are part of high voltage system. !! \$\endgroup\$ Feb 17, 2020 at 13:46
  • \$\begingroup\$ So if it isn't a geiger system why did you say this: When I opened the supply board to the detector (Geiger Counter), I got the above circuit? And I didn't say geiger counter I said geiger tube. \$\endgroup\$
    – Andy aka
    Feb 17, 2020 at 13:49
  • \$\begingroup\$ R2 is "how is it possible". C1, C2 and R1 are just HV supply decoupling. \$\endgroup\$
    – user16324
    Feb 17, 2020 at 15:47

2 Answers 2


Very often. the high DC voltage from the power supply is boosted from a much lower DC voltage (perhaps a battery). A DC-to-AC-to-DC converter circuit is used.
Some pulses inside this converter may appear superimposed on its high-voltage DC output. Since a Geiger tube is sensitive to variations of its high-voltage input, C1 helps to smooth any variations caused by the AC portion of the DC-to-AC-to-DC supply. C2 helps as well.


The Wikipedia article describes why the input voltage must be isolated from the HV supply. When a particle ionizes the gas inside the Geiger tube, you must allow the voltage to momentarily dip to quench the ionization. C1 just filters the HV, C2 and the two resistors are the quench circuit.

The amplifier detects the dip in voltage.


BTW, my father somehow scrounged a Geiger tube and built what you diagrammed about 50 years ago, before I knew much about electronics. Now I am curious to see what he did. I think he still has it.


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