Problem with MC34063 180V power supply (huge voltage drop) - Solved


I tried to build a Nixie power supply. I got the schematic from this site nixie-power-supply.

I attach the schematic.

With no load I get 178V but if a put a small load (1mA) the voltage drop to ~110V.

I changed some of the components:

The input capacitor is 470uF instead of 220uF,
The output capacitor is 10uf (400V Electrolytic) instead of 2.2uF.
I use the following 220uH inductor: Inductor
BC327 PNP transistor instead of MPSA55.

What do you think is wrong?



I suspect that the Inductor is saturating when under load, and the current limiting resistor (R1) is causing the MC34063 to go into current limiting, thus pulling the output voltage down.
Try reducing the value of C2 to 1500 pf or even 1000 pf and see if the output regulation improves.
This will increase the switch frequency.

Thanks Mauried!

That solved the problem. I replaced C2 with 1000pf and tried with the same load, no voltage drop at all.


Glad it works.
Boost converters are tricky things, and with a high voltage conversion ratio even more so.
The circuit wont be able to source much current, so as long as the load doesnt need much it will be OK.