čtvrtek 26. března 2015

Miniature GM detector, part I

First thing first. I tested the GM tubes from yesterday, and the results are more or less as expected. The huge tube, SI-22G, is really sensitive to the background, and it was nice to have it sitting on my desk and clicking. The STS-5, that one in the middle, was more or less on par with the SBM-20 I am using for my detector. The tiny one, SBM-21 does not seem to care about background much. I did not have any active source at hand, but I believe the results will be different when actually measuring something hot. The tiny tube is simply too small, compared to the monstrous SI-22G, and the chances that a particle will hit such a small target are simply... small.
I started to work on the miniature version of the GM detector, and I cloned the HV supply from my very first design, using the same parts, and duplicating the capacitors to achieve similar 
capacity as the original design (original caps are 18nF, these small blue ones are 10nF only). The results are positive - without load, the HV supply draws less than 0.5mA from 9V battery, and 0.25mA from 4.2LiPol 18650 accumulator. I am not sure what was the trick, though. The basic schematic comes from the Geiger Counter Circuits, the second circuit. Picture above is the component side, the picture on the left is the bottom side. I used SMD parts for pretty much all I could. Transistors are SMBT3904, 3906 and SMBTA42. I believe the transistor selection is vital, the SMBT39xx have relatively high hfe. Inductor uses cca 100-200 turns on ferrite core (hand wound). I don't remember what is exact inductivity, but it was pretty high when I measured it before, around or over 50mH. My impression is that bigger is better here. The ferrite core bears marks "SEM TM12". It's either coming from Pollin (part of the mixed inductor sale), or from local shop. Pollin is more likely.
The power supply seems to be relatively 'hard' - measuring the voltage with common 10MOhm multimeter shows 270V, while my previous constructions usually did not get over 150V. I still did not add the GM tube and clicker, that's left for tomorrow, or more likely after my vacation.

Žádné komentáře: