Compact Mains G-M Tube HT PSU Module

Proud Mary, Sat Feb 23 2008, 11:24PM





A compact mains PSU was required to supply >600V DC to a pair of Centronic B12N glass G-M tubes in a coincidence counter now under construction.

T1 and T2 are identical 230V/24V 3VA PCB transformers wired back-to-back, to act as a single mains isolation transformer

C1 and C2 are 100nF/63V box polyester capacitors by-passing RF supply line noise on the 24V links to earth.

C3 and C4 are 220nF/1kV and form the capacitors of a full wave voltage doubler together with D1 and D2

D1 and D2 are 1N4007.

R1 is 68M VR25 HV resistor acting as safety bleeder.

R2 is 156K 3W (in fact 3 x 270K 1W) which together with C5 (220nF 1kV) form a passive filter with a cut-off of 5Hz and 6dB roll-off per octave. R2 also acts as an output current limiter, allowing a maximum short circuit current of a few mA. C5 also acts as a reservoir capacitor.

Construction: The two PCB transformers and the three large capacitors were lightly sanded, de-greased with an alcohol swab, and bonded together with epoxy.

Results: This module can comfortably supply two G-M tubes with 625V HT without the need of additional voltage stabilisation.

The transformer T1 case has a temperature rise of 25 deg C above ambient after 24 hours operation. There is no obvious warming of T2.

No significant supply line noise or 50Hz hum could be detected when an amplifier was connected to a cathode-coupled G-M tube.

I(out) is of the order of 75uA before the voltage begins to fall significantly - perfectly adequate for the task in hand.

In a word - it works! Cost: About £5/EUR 6.63