biemster
The net file pwmHV.asc can be found in the FILES section of the project.
The circuit seems to reach the operating voltage of the tube of ~400V after about 15 ms. I used a PWM duty of 66% at V2 in this run. The plateau voltage is however highly dependent on the resistor at R2, which I took to be the anode resistor.
My main purpose for this simulation was to check if the lower input voltage of 3.3V and the lower inductance of 4.7 mH could still generate enough HV for the tube, and it seems that it can. So next step is to put it on a breadboard, and start searching for the Geiger plateau.
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Hello. Did you ever make (or find) a spice model for a GM tube?
Thanks,
John
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I know cost is a concern, but since the tubes require so little current, you can get away with a lower-rated transistor followed by a diode/capacitor voltage multiplier. Even a doubler (2 caps + 2 diodes) reduces the required transistor Vce rating by 2:
https://en.wikipedia.org/wiki/Voltage_doubler
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2N5550 is only rated for 140V between Collector and Emittor, so it is not the right part for a 400V supply.
https://www.fairchildsemi.com/datasheets/2N/2N5550.pdf page 1
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Yes, true. It's my first LTspice sim, and it had the highest rating of the transistors that were predefined by the software. I did not bother searching for a better match, on my breadboard I will use a KSP44.
Do you think it would matter for the simulation?
Are you sure? yes | no
From the help file for BJT modelling:
"It is possible to annotate a model with device ratings. This information is
displayed in the schematic capture GUI to assist in selecting a device but does not directly impact the electrical behavior in simulation. The following
parameters may be specified."
VCEO rating is one such parameter that is not used in the simulation.
KSP44 is only rated for 400V. You might want something higher than your output voltage. e.g. 500V? 600V?
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The ratings of KSP44 is equal to those of MPSA44, which is shown to work in the original circuit. But indeed, ideally a higher rated transistor should be used.
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