Transformer output voltage not rising with input voltage

Question

I recently stripped a microwave of its transformer and I was looking forward to playing around with it. I didn't want to plug it directly into the wallsocket so I decided to design a circuit that can run on my DC power supply. Here is my circuit:

^{simulate this circuit – Schematic created using CircuitLab}

I created a monostable oscillator circuit for my clock like the one shown here: and I am using it to create a square wave across the transformer. In the schematic I have a 10K ohm resistor at the end of the transformer but in real life I have a spark gap (couldn't find one in the builder). Now as I raise the input voltage (at the peak of the square wave) I don't see anything until the current rises past around 0.03A and which point I can get sparks! The output voltage is somewhere around 30V unloaded which will drop after a load but I don't really care about this as I am using a spark gap instead of a real load. The problem is, when I raise the input voltage to around 30V with extra resistors behind it so that the current is below the 0.1A mark at which point my transistor explodes, I still only get a voltage difference of 30V at the output. Why does this happen? Do I need more current to get a higher output voltage?

Answer 1

A transformer and an inductor are often confused. A transformer for a microwave oven is a combination of an inductor and a transformer.

Your transistor effectively has an inductor in series with its collector (imagine exchanging the positions of the supply and transformer in the circuit -- they are in series and so this has no effect) -- then when the transistor switches off, the inductor acts to keep the current flowing -- which it will -- so the collector voltage will rise to the breakdown of the transistor and ultimately damage it.

Secondly, a 2N3904 is not capable of much power dissipation (or current) -- it may just be overheating.

You don't explain precisely how you drive the base, so it's hard to be more definitive.

You may need a more sophisticated base drive (and a more robust transistor). Try using a power FET (2N3055 NPN), or a TIP3055 (MOSFET). Using a MOSFET is easier. Put a resistor (1k) in series with the gate, and add a ~ 50 V zener diode + a1N4148 in series between the drain and gate. That will protect it.

Answer 2

First you need to protect the transistors from blowing up by putting in signal diodes in series with each base. The rating for Vbe is -5V abs max. !

Then change the R ratio from base to collector from 100:1 to say 10:1 for more current drive, which changes f, so use a Darlington in each or cascade 2 NPN’s. You possibly also need reverse diodes across Vbe .