Transformer troubleshooting

[johnsonjp34]

Greetings All,

I was wondering if anyone could tell me why the attached circuit will not work. I can't seem to get anything off of the output of the transformer when I wire it up this way.

 

[johnsonjp34]

Opps Here is the attached diagram

 

[Reloadron]

Here is the IRF630 data sheet. Do you have pulses on the Gate? What is their amplitude?

Ron

 

[johnsonjp34]

My multimeter is swinging back and forth rapidly when I use a .47 uF capacitor. When I use the 221 it stays in place and the multimeter is pegged out at 250 mA. That was at the output of the 630. The gate is receiving pulses at about 1 mA. With the 221 capacitor the 630 gets really hot, so I have been using a bigger capacitor (.47uF)

 

[crutschow]

What's a 221 capacitor?

The 630 will get hot if the frequency is too low and the transformer saturates. Then there is nothing to limit the current except the primary resistance of the transformer.

 

[johnsonjp34]

221 is a 220 pf ceramic disk. So if I use a capacitor with lower capacitance the frequency will increase?

 

[crutschow]

Yes, frequency is inversely proportional to capacitance.

You still haven't said what the voltage is that you applying to the MOSFET? The device has a gate-source threshold voltage of 2-4V and requires 10V to fully turn on. And you are using it as a source follower so the source (transformer) voltage will be more than 2-4V below the gate voltage.

 

[johnsonjp34]

The oscillator is feeding the gate 4 v. The Mosfet is receiving 12 v instead of 9 v as indicated in the schematic. I'm attempting to use the transformer as a stepup. Am I doing this correctly?

 

[crutschow]

4V is barely enough to start turning on the MOSFET.

The circuit is not correct. The MOSFET needs a larger gate voltage to properly operate so you would need to add some gain.

The easiest is just to use a bipolar transistor in place of the MOSFET with the emitter to ground, a resistor from the base to you oscillator output, and a collector resistor to the +12V. You cannot apply a unipolar signal to a transformer since that creates net DC current which will saturate the core. So you must connect the collector output to the transformer input through a large capacitor (to block the DC).

How much power do you need out of the transformer? If you want significant power then you would need to use a complementary transistor driver instead of the one transistor with a collector resistor.

 

[johnsonjp34]

I'm trying to build a high voltage power supply for a geiger tube. The working voltage is about 400 V. So I am going to take the output from the transformer and send it through a voltage multiplier using some diodes and capacitors. This is what I need:

Working Current 0.015-0.02 mA
Working*Voltage 380-460V

So I guess from your post I would be able to use something like a 2N2222. Just how big of a capacitor would I need to connect to the input of the transformer?

Thanks so much for your help so far.

Josh

 

[ronsimpson]

Now we know what you want to build.
Search for "Trigger coil transformer Flashtube flash tube xenon". The audio transformer might work but use the real transformer, "flash tube transformer". There are many circuits posted for 12 volts to 400 volts.

 

[audioguru]

You need 460V at 20mA which is 9.2W. If you use a push-pull driver for the transformer then it will be about 80% efficient so will use 11.5W.
11.5W from 12VDC is a current which is too high for two 2N2222 transistors. So you need two power transistors or two Mosfets connected properly.