HV Test & Adjust
This is going to sound more complicated than it is, but this page will show you how to measure, test and adjust the HV section of the
circuit.
Measuring the HV:
It's a bit tricky to measure the high voltage. The GM tube needs a lot of voltage but only a tiny amount of current. So
the HV circuit only needs to provide a very small current, and that's what it does. This is good because
the battery will last a long time, but it makes measuring the high voltage a bit more complicated.
When measuring voltage, a decent DVM puts a load on the circuit it's measuring of about 10MΩ (1MΩ for a cheap
DVM). This load is far too much for the tiny amount of current available, and the DVM will read much lower than the
actual voltage. I've read that you need 1 gig-ohm (1000MΩ) of input impedance to get accurate values of the HV for
Geiger circuits.
One way to increase the input impedance of your DVM is to put large resistors in series with the probe and multiply
the reading on your DVM. For example adding 9 10MΩ resistors adds 90MΩ. For a 10MΩ DVM you multiply the
reading by 10. For a cheap 1MΩ DVM you multiply the reading by 91. If you want to a full gig-ohm of
input impedance it's best to just buy a high value resistor - (example). There are good explanations of the subject
with build instructions here or here.
I wanted all of the HV readings that I refer to on this site to be easy to duplicate without the expense of a 1GΩ probe.
So for the HV readings mentioned here, I'm using a cheap 1MΩ DVM with a 90MΩ probe and multiplying by 91. (If
you're using a 10MΩ DVM you should get better readings.) I'd be interested in hearing the readings people get from
very high impedance DVMs.
The high voltage is measured from the cathode (band side) of D2 (HV TP) and the negative side of the GM
tube.
Using the 90M probe I get 570V with the SMB-20 operating well, and the battery drain at a good point.
Testing the HV:
After building out the HV section of the board, it's a good idea to see if things are working up to that point. First read
through the next section to get a sense of the final picture. Then, using the 90M ohm probe described above, or even
without it, connect the battery, and measure the voltage between the cathode (band side) of D2 (HV TP) and the
negative side of the GM tube. If you get something like 200V without the probe - congratulations! The HV circuit is
working.
I can't recommend this (for obvious reasons), but I found that putting two fingers (same hand) across the wires that
go to the GM tube will simulate an event on the tube pretty well. The current is so low, I can't feel it. I use this as an
easy way to test the circuit without the tube. Again, I can't recommend this, however.
Adjusting the HV:
R5 controls the high voltage. (see Circuit Description on web site). It has been preset for about 18Ω. This should give
you about 570V at the TP with the 90MΩ probe described above. The idea is to adjust the HV so it's working voltage
is in the middle of the plateau as shown below. Note that the circuit will work just fine with R18 set to 18Ω.
Using some kind of active source, lower the HV (CCW) until you get no response. Note R5 is a 10 turn pot. Now
increase the HV (CW) until you are getting a good response from the source. This point is the threshold in the graph
above. Note the position of the pot or the HV at this point. Continue to increase the HV along the plateau. At some
point the HV will drop to near zero. Note the position or voltage and set the pot CCW so it's between that point, and
the threshold. (You'll probably wind up with the pot at 18 ohms again!) Adjusting the HV is probably more important
when you use tubes other than the SBM-20.
These are the results I got in my testing (1 MΩ DVM with 90M probe):
R5 = 25R - 390V - detection just starts - threshold
R5 = 23R - 440V - stable readings start- beginning of plateau
R5 = 20R - 525V - (sweet spot) - about center of plateau
R5 = 16R - 650V close to breakdown, battery current jumps up
Powering the Geiger
The Geiger kit will accept voltages between 4-9VDC. When applying voltage in the range of 5.5-9VDC (a 9V battery for instance) the
voltage regulator must be installed. This will supply a working voltage of 5V to the circuit. Remember that the working voltage can not
exceed 5.5V with ATmega328 installed.
When powering the Geiger kit with a voltage in the range of 4-5.5VDC (3 AA or AAA
ZnC batteries or 4 NiMH batteries) I recommend that the voltage regulator is not installed
and is bypassed. This is easily done by soldering a straight piece of wire between the IN
and OUT pads on the regulator like the picture on the right shows.
Since the HV circuit is self regulating, there is no increase in its output when higher voltages are used. For the best battery life, I'd suggest
running without the voltage regulator, using 4 NiMH batteries, or 3 AAA ZnC batteries if case size is an issue.