support LND712 GM tube

[ThomasWaldmann]

From another GM kit (offline only, without WiFi, without LoRa) I have a LND712 (alpha, beta, gamma) which could be put online using multigeiger maybe?

Tube specs: https://www.lndinc.com/products/geiger-mueller-tubes/712/

LND712: 500V (recommended. 450V-650V overall), 10M Anode resistor.

Si22G: 400V (recommended. 360-440V overall).

So, just change the Z-diodes to a sum of 500V?

What I wondered: is putting Z-diodes in series and adding the voltage a valid method?

[ThomasWaldmann]

answering my z-diodes in series question:

https://toshiba.semicon-storage.com/us/design-support/faq/diode/is-it-ok-to-connect-multiple-zener-diodes-in-series.html

[ThomasWaldmann]

Idea:

Currently, 2x 200V Z-Diodes are used to define a fixed 400V "full" voltage.

Use 3 places on the PCB (future PCB revision) for Z-diodes.

• for 400V use 2x 200V Z-diodes and a piece of wire
• for 500V use 2x 200V and 1x 100V Z-diodes

With the current PCB revision, this can also be done, just a little less pretty.

[ThomasWaldmann]

Hmm, anode resistors also need to adapt.

[rexfue]

I asked Jürgen: We think, the LND712 could be used.

[ThomasWaldmann]

I tried searching some Z-Ds on mouser.com, but the huge amount of choices is confusing...

What are the design criteria to select for? Guess <= 500mW, zener current = ?, tolerance = ?, price, through hole, ...?

[rexfue]

Z-Dioden z.B. BZX85C200, die kleine mit 3V3 ist ziemlich beliebig.

[rexfue]

Yes, sounds like a good idea. There should be space left for one additional diode.

[ThomasWaldmann]

See also #86 - it seems the LND712 somehow works (although I am not sure about whether the CPMs seen are as they should be - it's hard to compare with the bigger / different tubes and also I do not have a conversion factor yet for the LND712).

[ThomasWaldmann]

After a long idle time for the LND712, I pulled that out again, flashed current firmware and it works!

It uses the standard PCB and parts, just with 1 addtl. 100V Z diode in series with the 2 on the board, so the tube gets ~500V as required.

It's ticking happily at:

• ~25 CPM (natural background) and ¨1..2 charge pulses per 10s
• ~500 CPM (with some radiation source) and ~7 charge pulses per 10s

OK, so I guess the only stuff that is missing is:

• a reasonably calibrated entry in the "tubes" array + #define for the index
• a spec how to send this to sensor.community / madavi so it does not get mixed up with the Si22G

[rexfue]

sensor.community will not be a problem, we just have to tell Rajko to take in the LND712