SparkFun Forums

[os6]

Hello,

i bought the SparkFun Geiger Counter and i try to understand the circuit. I think i nearly got it, but in die schematic there are 2 SMB-Zener-Diodes that are labeled U$6 and U$7.
Schematic: http://cdn.sparkfun.com/datasheets/Sens ... ter_v4.pdf

On the Diodes on the board is written:

B31
951B

I'm wondering which Zener-Diode is taken. i guess they're placed to get rid of over-voltage, but i can't find anything about the ratings.

Thanks a lot for your help and excuse my bad English.

Best regards
Ole

[jremington]

Those diodes determine the output voltage of the high voltage generator. If the voltage rises past a certain level, they conduct and turn on Q4, which stops the oscillator.

[os6]

Hi jremington,

thank you for your reply. Your description of the functionality of the diodes is the same i expected. I was just wondering, because i measured ~3,3 Volts over the diode, and was some how excepting something like (650V-5V)/2 = 322,5V per diode to bring the Voltage back to logic-lvl. That's why i was interested in the rating of the diodes, because i only found this datasheet (https://www.onsemi.com/pub/Collateral/1SMB5913BT3-D.PDF) for the diodes on my board, but it could be every diode in the range of 3.3 to 200V.

So i'm still wondering :'(

Best regards
Ole

[exeng]

Ole, the sum of the zener's nominal voltages Vz would determine the total zener voltage. In the spec you referenced the 951B nominal voltage appears to 120V. If you could figure out the Vz for the B31 then you could calculate the total Vz. It's my "limited" understanding that this is how one achieves a specific zener voltage when there is no single zener that would provide the desired Vz.