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Identifying flyback pins for a 2n3055 based driver (other than HV/primary)

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aonomus

Thu Jun 26 2008, 05:43AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

I've been toying with a flyback transformer today and I figured out which pins are the HV/ground and primary pins from a few guides/posts. The question I have is which pin do I use for feedback when building a 2n3055 or similar based driver? Most of the pins all read 0-1ohm across each other so I can't seem to figure out which is which, can I use a LCR meter (whose batteries need replacing as I write this thread)?

This is going to be my first foray into HV (I don't think 300V + coilgun counts?), and after reading this guide I'm going to substitute the 2n3055 for a beefier transistor, possibly expanding to a push-pull just for kicks

The 42v brackets indicate the peak reading I got when sending 9v across those pins, forgive the poor drawing.

Also I looked at the pinout found in this thread which seems to somewhat match my flyback (picture below).

1145644628 103 FT0 Pinout2

Note: the guide here describes a better way of determining flyback HV secondary by passing 20-24v through various pins until the voltage reads low (due to resistance of the secondary). This seems better than letting the HV arc to the pins, maybe it should be added to the wiki?

[Edit: Image size]

Coronafix

Thu Jun 26 2008, 09:42AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Wind some turns (insulated wire) on the actual core for the feedback.

Shaun

Thu Jun 26 2008, 05:32PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

You can use the original primary coil potted in the flyback, but it was meant for like 150V I think, and therefore has far too many turns. You will only get like 1mm sparks with the 2n3055 driver using the original primary.

Coronafix is right, use the exposed core for your windings. You have no idea how long it took me to figure this out; nearly every site on flybacks says either "DC flybacks are useless" or suggests an insanely complex driver that doesn't do much at all.

rp181

Thu Jun 26 2008, 06:26PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I think his is like mine, theres barrely any space between the core and potting, only thin wire can be used.

aonomus

Thu Jun 26 2008, 08:54PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

This flyback has a comfortable amount of space between the casing and the core, enough to make my own primary windings with decent sized wire. The core is held together by a metal spring, and epoxy. There is a very small gap between the core halves, will this affect making the windings, and if so which side would I want to put them on?

Dr. Dark Current

Thu Jun 26 2008, 09:15PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Put the windings anywhere on the core. The gap is needed if you operate it with single-ended driver (single switch). If you make your own windings they have two advantages:
1) if they burn you can rewind them
2) primary winding opposite secondary is better as it limits output short-circuit current more and makes your transistor run more happy.

aonomus

Thu Jun 26 2008, 09:32PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Just another question too, if I connect the circuit to my benchtop power supply (a commercial one, brand new too) is there any risk of back-emf frying it? Should I put big beefy protection diodes on the 0v and + connections?

Arcstarter

Thu Jun 26 2008, 10:35PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

aonomus wrote ...

Just another question too, if I connect the circuit to my benchtop power supply (a commercial one, brand new too) is there any risk of back-emf frying it? Should I put big beefy protection diodes on the 0v and + connections?

Yes, high voltage could get into the power supply and ruin the rectifier or the capacitor if there is one.

Dr. Dark Current

Fri Jun 27 2008, 10:37AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Put a big (several 1000s of uF) filter capacitor across your PSU in parallel with a low-ESR film or foil type cap, in the 0.1-1uF range. This should get rid of all and any voltage spikes.

aonomus

Fri Jun 27 2008, 08:57PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Wait, a several 1000 uF lytic + a low-ESR/foil type cap?

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