Explain the output of the old TV flyback transformers.

Some color TVs had a potted flyback-rectifier thing.

Any idea what the duty cycle of the input was on the TV flybacks? Mikek

For CCIR type G analog raster (625 line, 50 half-frames / second), the trace time was about 53 us and retrace 11 us, for a sum of 64 us.

I have a couple of defective High frequency switching type neon sign power supplies. Neither work and seem to have open secondaries. I removed the switching pcb and found defective IRF 630 FETs, I replaced them and got it switching, but no HV. So, of course I stuck the transformer in the barbecue and melted off all the tar, and a little kerosene and it was free.

But, I was surprised to see a pcb encased in the tar, that connects the two secondary windings at the CT. It has a 555 a couple of SCRs and caps a diodes to make a power supply for the 555. I'm not sure of the purpose of this pcb, it would seem like current limiting, but this runs at 21kHz, I would think secondary impedance would be enough to limit current. Any thought as to what the pcb connecting the two secondary windings together would be doing? Mikek

What sort of HV are you after and what are the current requirements?

A Neon sign transformer will give you milliamps at the rated output voltage which could be deadly if misapplied...

The HV from a TV won't kill you, it just gives you one heck of a jolt!

You may want to just use an old B&W or colour TV that you pick up at a thrift shop and use that as your HV source.

It all depends on your application.

John :-#)#

Please see "Developing High Voltage pulses" June 5. I have a functioning Pulser, but need a 12kV power source that will provide 2ma of current. btw, do still see old CRT TVs at thrift stores? Thanks, Mikek

100w is unlikely. If true it's lethal. If untrue, the voltage is also unlikely to be true. Output is either ac or rectified dc. Roll your own cap if you want nice dc, or buy an old doorknob cap that's tested good.

I sent a question about the output to the seller, they forwarded it on to the manufacturer.

I have a pile of monitors here and flybacks as we service old arcade game monitors. You need 200ma at 120VDC if the flyback is lossless. Might be able to do that. You can use a B&W flyback as they tend to be running around 12KV and the primary side is pulsed 73VDC or so. 19" colour monitors flybacks run around 15 to 20KV at a B+ of 120VDC pulsed. Lower B+ would give lower output voltage of course.

A common flyback is the GO7 Electrohome which runs at 15,xxx Khz and has built in HV diodes. I sell these here:

You could use a picture tube as a smoothing filter cap I imagine...

John :-#)#

... or alternating sheets of an insulator such as glass or teflon sheet and aluminium foil. John

totally pointless

What is pointless? Asking for more info, or expecting to get it? Mikek

or pick up a scrap crt tv Cap can use glass sheets or jars, or wax soaked card. Test it to at least double run voltage - or that of course you need a cap.

I drew the schematic of the tar potted pcb that is connected between the two halves of the secondary. I'd like opinions on what the function of the pcb is. It has a 555 driving two SCRs. The transformer already has a driver circuit on the primary. What is the function of this circuit on the secondary.

Picture of pcb, Thanks, Mikek

Has this Center Tap circuit stumped everyone? Mikek

More likely they can't be bothered redrawing it in a way that makes it easy to make sense of what it is doing. Since there's a 555 at the heart of it it's either old or designed by somebody who didn't notice when the 555 went obsolete - like John Fields. The fact that the winding direction of the two secondaries isn't specified - no dots - is bad start.

No turns ratios and no specification of the primary drive makes it difficult to work out what might be going on. I'm not temped to try.

Are you sure those "SCRs" aren't triacs? Ed

Not absolutely, but it looks like they are, the label is, Motorola, 642 CR 106-6 Here's what I found,

Thanks, Mikek

I'm assuming this is in response the the circuit at the center tap of the secondary. So far my experience with 60 Hz neon sign transformers is that the inductance of the secondary limits the current to 30ma, and they have no other regulation. With this switching style running at 20kHz, would lead me to believe that impedance limiting is even more likely to happen.The series secondaries measure 10H, which is 1.257MΩ at 20kHz.

7.4kV / 1.257MΩ equals 6ma. That seems low in my somewhat limited experience, but I have 8 or 10 neon sign transformers and all have 30ma impedance limited output current. So, that 6ma is a bit confusing. Thanks, Mikek