Winding 30W Amp Transformers

[katopan]

Thanks Chad. One of the reasons I wanted to post all this was that it is really hard to find info about the practicalities of winding your own transformers. It took me quite some time to piece it all together for the first set I did. This being only my 2nd go at it I am far from an expert, but hoped it would be of some interest.

[Chad]

but hoped it would be of some interest.

Interested I definitely am as I am sure many others.Keep up the great work, Chad

[Chad]

Visited your web page Kato, So many cool projects.So much info on low watt amps and other interesting amps.I'll be checking out some of those in the future.Thanks,Chad

[xtian]

This odyssey reminds me of the chap who built his own toaster from scratch, including mining and refining the minerals:

http://www.youtube.com/watch?v=5ODzO7Lz_pw

[katopan]

Well I'm not enamelling my own winding wire at least. But yes this has turned out to be a bit of a saga. I'm still enjoying it though.

[katopan]

And now begins the fun with the output transformer.

OT_1: Start of the winding, with the teflon sleeved jumper wire which will connect from here to the very last layer, connecting the two sections of primary 1.

OT_2: First layer of the primary. Got the room for some winding margin at the edges on this one, which I'll maintain throughout.

OT_3: First layer is then varnished in clear nail polish and the greaseproof paper put on while still wet. All layers in the OT will be done in this fashion, with mylar added in between each primary and secondary section.

[katopan]

OT_4: First section of primary is finished, layer of mylar and the beginning of the 4 ohm section of secondary.

OT_5: 4 ohm section finished.

Couple of things I'm finding here.... I'm still having real trouble keeping the end windings from 'falling off' into the winding margin. I suspect if I was using thicker paper that was pre-laquered like the traditional way this wouldn't be so much of a problem. The baking paper is thinner, which is good for fitting more on the bobbin but bad for this problem. Have decided because I have bobbin cheeks as an insulating layer between the windings and the iron core that I'm going to wind the primary hard up against the top cheek and the secondary down near the lower cheek. That will give plenty of margin between primary and secondary, but allow me to wind the thin primary wire up against a surface to keep it from slipping.

Also I just wanted to check the heat resistance of clear nail polish against the polyurethane varnish I used on my first set of transformers. Painted each on cardboard and once dry hit it with the heat gun. Varnish started turning brown and being effected well before the nail polish. Just wanted to check this in case there was a good reason I should be using the varnish instead. I'll stick with the nail polish.

[katopan]

OT_6: Start of the 8 ohm section.

OT_7: 8 ohm section finished.

OT_8: 16 ohm section finished. At first I wound this in the 1mm of the 4 & 8 ohm sections before realising I was going to use 0.8mm wire to allow it to fit keeping the winding margin. That's better - plenty of margin left with 0.8mm.

[katopan]

Getting close now - I've finished winding!

OT_9: Primary 2 link wire connection soldered to the wire for the start of the outer section of Primary 2.

OT_10: Last layer of the OT. Hand winding isn't perfect, especially when you get to this stage where the surface of all the previous layers isn't very flat. Still this is an improvement on what the last layer looked like in my previous attempt at winding an OT. I'm pretty happy with this being much neater overall than last time.

OT_11: End of the wire of the very end soldered to the Primary 1 link wire connection that goes back to the very beginning.

In sizing the winding window fit I knew this would be tight, but I wanted to get a good number of turns in. Well, there's a couple of high spots overhanging the bobbin that unfortunately will need some encouragement to be able to fit the laminations. It's not far out so I won't be squeezing the hell out of it or anything. But still ideally not what you want to have to do.

Still need to solder on the flying leads for the OT. I also need to go pick up my laser printer cartridge that is getting refilled so I can print some labels. Both PT & OT need a final layer of mylar, then the labels, and then I can assemble the cores and test.

[sagitt]

hello kato!I was looking at this varnishing jar on your page...I am interested to build one.when you power on compressor does it sokeup all the varnish from jar?can you explain this please...

[katopan]

The tube is connected to the top of the jar so it doesn't suck the liquid directly. But I was worried about droplets being drawn up and into the compressor. I had about 5 metres of clear tubing between the jar and the compressor and I was watching it carefully. After pulling a vacuum and releasing it a number of times on both windings I did have some varnish up the tubing but only up about 2 metres of it. So it didn't get close to the compressor suction inlet.

If the tubing wasn't enough I was going to add another jar inbetween setup as a knockout pot, but it wasn't needed.

[katopan]

So I thought I'd have some dry labels ready for applying this morning but I used nail polish on them. The solvent smeared the print toner. That didn't happen last time with the polyurethane varnish so I've done another set using that instead. All this way and I'm waiting on labels to dry before I can assemble these babies! Almost there - gotta be patient.

[katopan]

Labels are on and laminations are all stacked. Because I made two transformers out of one donor I'm short a set of bolts and mounting brackets, so a trip to the hardware store is in order and I'm probably not going to get there today. I'm really happy with how these are looking and will be over the moon if they test out OK. I'm not expecting any problems with that as I know exactly what's inside.

So no sneak peak pics this time. Next post will be once they're completely finished.

[katopan]

Still haven't got the second set of bolts and brackets to finish these off, but I did get to clamp up the cores and test them out.

Power transformer normalised to 240V (mains actually measured 232V through a light bulb limiter) but unloaded I get:
230V primary tap = 230V
300V secondaries = 316V each side (98.4% of design)
275V secondaries = 292V each side (99.2% of design)
225V secondaries = 236V each side (99.9% of design)
3.15V secondaries = 3.41V each side (101.2% of design)

I'm pretty happy with that!

Also put mains onto the primary of my output transformer to test the ratios:
242.5V across the whole primary
Primary CT = 120.4V (close enough to half)
4 ohm = 5.97V -> corresponds to 6600 ohm reflected for a 4 ohm secondary load
8 ohm = 8.46V -> corresponds to 6573 ohm for 8 ohm load
16 ohm = 11.89V -> corresponds to 6655 ohm for 16 ohm load

Also the whole primary of the OT measures about 18H of inductance (with my multimeter so accuracy is not great) giving a roll off at 58Hz for 6.6K. Shorting the secondary gives about 55mH of leakage inductance which is a bit high but still OK.

Happy with that too!

I'll post the final photos once I get the final bits and bolt them together.

[VacuumVoodoo]

Numbers look decent. For a more realistic test of the OT repeat with a resistive dummy load on the secondary side. You'll probably find impedance ratio a couple % below design target. Not that it matters sound wise.