Flyback not working, any ideas?

[andynerd]

So I'm using the design from re(updated) Simple flyback driver Even so it isn't oscillating at all. The current just flows through the flyback coil, to the base, and then power drains along the primary. There is no high voltage and their is no electric humming noise. I've tested the 3055 and it is working. The only thing I can think of is that the one resistor that is supposed to be 5w is only a 1w in my circuit but they aren't getting hot and they still show proper resistance values on the multimeter. I know I had an ignition coil thread a few days before but when using an actual 3055 the sparks where minimal and I assumed the coil to be dead (it was at least 40 years old).

 

[gerty]

What are you using for your 12v dc supply? This circuit draws several amps.
Post your schematic so we can see exactly what you have.

 

[BrownOut]

These circuits tend to blow any IC's or regulated power supplies because they put large spikes on the primary power rails. I'd check for that.

 

[andynerd]

**broken link removed**

Alright well I'm using 12 volts at 18 amps. And I tested the power supply by booting up an old 98 "testing" computer in which everything worked normally and the voltage readings stayed consistent.

 

[crutschow]

Are you certain the polarity of correct on the feedback winding? It's not shown on the schematic. You might try reversing to see if that makes a difference.

 

[andynerd]

I have also tried that. I was wondering would the gauge matter? I don't remember gauges and such but the primary is close to or is 1 mm and the secondary is a 1/2 mm.

 

[crutschow]

Wire gauge does matter as far as power dissipation and voltage drop, but it shouldn't keep the circuit from operating.

How are you applying the voltage to the circuit? If the voltage comes up slowly it may keep the circuit from working. If you haven't done this, try connecting the circuit to the power supply output with a switch after the power supply is on.

Incidentally, the primary and feedback winding polarity should be with the dots adjacent to each other (bottom of feedback winding and top of primary winding)

 

[BrownOut]

Might need more windings. Try other flyback t-formers if you have them. They are not all the same.

 

[andynerd]

Alright well I have to run some errands but when I get back I'll rewind the primary more. It might also be a problem that my primary is actually above the feedback. And for the power I just turn the power supply on so it should jump to 12v but with the capacitors and such in the psu it might take a moment I think I will add a switch.

 

[BrownOut]

I meant more windings in the feedback coil. If your primary is wound on top of the feedback, that could be a problem.

It's "hit and miss" with these kind of circuits. It might help if you had a scope. I would recommend disconnecting the feedback coil from the transistor and driving the transistor with a signal source while 'scoping the output from the feedback coil. Then add loops until you get several mV and try again. That is IF you had a scope.

 

[crutschow]

It's "hit and miss" with these kind of circuits. It might help if you had a scope. I would recommend disconnecting the feedback coil from the transistor and driving the transistor with a signal source while 'scoping the output from the feedback coil. Then add loops until you get several mV and try again.

Don't you need more than a few mV to drive the base of the transistor? I would think you want at least a volt or so, if you are switching the transistor fully on and off during the test.

 

[BrownOut]

Don't you need more than a few mV to drive the base of the transistor? I would think you want at least a volt or so, if you are switching the transistor fully on and off during the test.

I'm not sure exactly how much, but the feedback is regenerative, so if the feedback loop is closed, a few millivolts will get it started, and it will grow until the transistor is fully switching.

 

[crutschow]

I'm not sure exactly how much, but the feedback is regenerative, so if the feedback loop is closed, a few millivolts will get it started, and it will grow until the transistor is fully switching.

So you are suggesting doing the test by not fully switching the transistor. I would think that it would be better to fully switch the transistor on and off during the test, in which case I would expect the feedback signal to be at least a volt, if not several volts.

 

[BrownOut]

NO, I"m suggesting that he disconnect the FB and drive the transistor with a sig-gen ( which will fully swtich the transistor ) and measure the output of the FB coil. The FB coil is typically just a few turns; just enough to get the process started, and the transistor's regeneration does the rest. But it might require more than a few mV, it all depends on the voltage gain of the transistor. Maybe even a small trasistor to drive the 3055 would help alot. That would increase the overall gain and make up for the low amount of FB being injected.

 

[andynerd]

Ya I defiantly do not own a scope, maybe rig up something with a speaker because it shouldn't be above hearing frequency. And I have a leftover 555 circuit from the ignition coil thing that will generate a frequency. Add a little amp circuit and some such and I should be in business. So let me see if i get this I will disconnect the feedback ENTIRELY, connect it to my "scope", Then use the signal generator connect it to the base of the 3055 (maybe with an amp circuit or not) to drive the transistor. This way I can see if anything is wrong with any of the coils.

 

[andynerd]

Well I found the problem. I checked all the leads on the Flyback to see if they had any resistance to the positive HV cable. Well all of them read a nice flat error. Now some of them had little resistance between each other (probably the internal windings or something) While others had no connection to anything. So I've decided I probably burnt the wire in the coil while desoldering it. I guess I'll have to look for another Flyback.

 

[crutschow]

NO, I"m suggesting that he disconnect the FB and drive the transistor with a sig-gen ( which will fully swtich the transistor ) and measure the output of the FB coil. The FB coil is typically just a few turns; just enough to get the process started, and the transistor's regeneration does the rest. But it might require more than a few mV, it all depends on the voltage gain of the transistor. Maybe even a small trasistor to drive the 3055 would help alot. That would increase the overall gain and make up for the low amount of FB being injected.

If you drive the transistor so it fully switches with the FB disconnected then you need at least a volt from the feedback to drive the transistor. Regeneration does not add any voltage to what you get when fully switching the transistor. That is the maximum regeneration feedback voltage, which equals the supply voltage times the turns ratio between the primary and feedback windings.

 

[BrownOut]

If you drive the transistor so it fully switches with the FB disconnected then you need at least a volt from the feedback to drive the transistor. Regeneration does not add any voltage to what you get when fully switching the transistor. That is the maximum regeneration feedback voltage, which equals the supply voltage times the turns ratio between the primary and feedback windings.

That's true, basically the condition for sustained oscillations is the loop gain must be greater than unity. So, the feedback factor multiplied by the transistor gain must be greater than 1. Then the required feedback will be the reciprocal of the transistor gain, which can be measured by applying the test signal from the signal generator and measuring collector voltage. I should have said that from the beginning.

In order to get an accurate measurement, the test signal frequency must be the same as the eventual operating frequency. That would probably require a frequency sweep to find the maximum gain and feedback. I think that would be a good place to start. 'Course without a scope, it will not be possible.

 

[BrownOut]

PS: Here is a flash that just came to me: if you load the feedback circuit with the same load as the amplifier input, then measure the same voltage or higher as being injected into the amp by the signal gen, then the conditions for sustained oscillations well be satisfied. Also, the two voltages need to be in phase, of course.

 

[andynerd]

I'll keep that in mind while I look for a new Flyback. The one I have doesn't seem to have a ground to the HV positive lead.