[sdiy] Opinions on Switchmode PSUs for analog synths?

KA4HJH ka4hjh at gmail.com
Sat Nov 9 20:48:15 CET 2019

> On Nov 8, 2019, at 4:28 PM, Ben Barwise <clackjunk at gmail.com> wrote:
> Apart from ripple, a small problem I have found with DC/DC converters is that there is a maximum capacitive load. Correct me if I am wrong but I think what tends to happen is it trips the short circuit protection when the module is powering up because of the inrush current. I have seen them get stuck in a hiccup like loop (this was in a non synth project with a lot of caps!). 

My favorite example is a cold cathode device such as a helium-neon laser. Ionized gases have negative resistance: the lower the voltage, the lower the resistance. IOW, as the power supply reduces the output voltage the DC resistance of the load decreases and you have the perfect conditions for an oscillating-under-load power supply.

Longer version...

With He-Ne lasers the currents are low, typically 3.5mA to 7.5mA, and the voltages are high. I have everything from 900VDC to 3800VDC (the latter is for a 25" head) in my collection. At these voltages the parasitic capacitance on the +HV wire, which may be as much as 12" or even 24" long, is relatively high. Since electrons move from negative to positive a large ballast resistor has to be placed as close to the anode on the tube as physically possible, typically ≤3". This limits the inrush current into the "capacitor" to an amount the power supply can deliver at power-up otherwise the entire circuit will oscillate. Tube gets ionized, electrons rush into the anode wire/lousy HV capacitor, excess current shuts down PS, wait so many milliseconds and then start all over again. Thank you negative resistance..

HeNe tubes also have a large cathode but it's at ground potential so as long as you don't accidentally reverse the polarity it's not a problem. It'll work...for a while. 8D

Terry Bowman, KA4HJH
"The Mac Doctor”

(The notorious thread hijacker)

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