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Registered Member #561
Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230
I needed a 10V 10A transformer to run the filament on a GU-48 tube so I found a reasonable large toroidal transformer in my junk box. Not sure why it was in there but I suspected it was burnt out (somehow). Tested the primary winding with my multimeter and got 28 Ohms back and it didn't smell like burned plastic so I assumed the primary was good.
I pulled the old secondaries off and counted how many turns the old and very thin 10V secondary was (50 turns) for when I rewound it with heavier wire. I rewound it with 2mm magnet wire and checked the primary again with the multimeter just in case I broke it. Everything checked out but just in case the primary was burnt out I built a rather ratty isolation/ballast transformer using two little identical transformers rated at <20W each (I think they came from some old radios) and ran them secondary to secondary.
I hooked the output up to the 240V primary of the toroidal transformer, stripped off the enamel from the new secondary ends and hooked it up to the multimeter. I turned it on and nothing; no voltage out the secondary and no burning (thankfully). I checked each stage of the isolation transformer and got the correct voltages, got 240V into the primary but nothing out.
I've never built a transformer before and I'm hesitant to hook it straight up to the mains given the primary might be burnt out. The only thing I could think off was that the <20W limiting was simply too small to get anything in the secondary. I also figured that unless the primary was completely wrecked, given I got 28Ohms on the primary, I should ahve at least gotten some voltage on the secondary.
Any suggestions? Should I hook it straight up to the mains?
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
You should get some voltage out of the secondary, even when powering it from the weak isolation transformer. There must be an open circuit somewhere, either in the primary of the transformer (try measuring it again, something could have happened to it), between the isolation transformer and your transformer, or in the secondary you wound.
Registered Member #561
Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230
Well I found the problem, the copper wire which was clearly marked enamel wire is actually bare copper wire :| Just had to touch 2 parts of the winding with my multimeter probes to get less than 1 Ohm.
Thanks Jaycar.
I thought it was a bit odd that I didn't get black smoke when I heated the ends of the wire to strip the insualtion.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Make sure you use a suitable fuse when you do connect it to the mains. Use a 3 amp fuse in the plug, but I'd suggest using an additional, smaller fuse as well.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
One trick that I use whenever dealing with mains operated transformers or motors is powering them through a bank of globes. The one in my shop has 20 sockets and switches and when I am satisfied that the inductance has correctly dimmed the lamps, the bank can be shunted and full voltage applied.
I built that switchboard years ago when instruments were as scant as money was.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
It's sometimes a good idea to test transformers (xformers) by feeding them with a low voltage (from a mains driven xformer). If the xformer you are testing should turn 240 volt mains into 10 volts then it should also turn 24 volts from an isolating xformer into 1 volt. This way, if things go wrong (like a shorted winding) the currents flowing will be 10 times less.
It's an even more useful trick when you are testing a step-up xformer.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Download wrote ...
Well I found the problem, the copper wire which was clearly marked enamel wire is actually bare copper wire :| Just had to touch 2 parts of the winding with my multimeter probes to get less than 1 Ohm.
Thanks Jaycar.
I thought it was a bit odd that I didn't get black smoke when I heated the ends of the wire to strip the insualtion.
Yeah, I've stopped buying enamel wire from Jaycar. Its insanely expensive for a tiny spool containing barely enough wire for a small-value inductor. I've also had the same problem with bare wire marked as enamel, it's annoying that the enamel insulation is crystal clear and looks identical to bare copper. It's much cheaper to get a big spool from Element 14 or Digikey, plus you get a choice of colour. Jaycar is good for small bits like switches and terminals, especially if you need a specific resistor or cap to finish a circuit and don't want to wait for it to arrive.
Registered Member #561
Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230
GrantX wrote ...
Download wrote ...
Well I found the problem, the copper wire which was clearly marked enamel wire is actually bare copper wire :| Just had to touch 2 parts of the winding with my multimeter probes to get less than 1 Ohm.
Thanks Jaycar.
I thought it was a bit odd that I didn't get black smoke when I heated the ends of the wire to strip the insualtion.
Yeah, I've stopped buying enamel wire from Jaycar. Its insanely expensive for a tiny spool containing barely enough wire for a small-value inductor. I've also had the same problem with bare wire marked as enamel, it's annoying that the enamel insulation is crystal clear and looks identical to bare copper. It's much cheaper to get a big spool from Element 14 or Digikey, plus you get a choice of colour. Jaycar is good for small bits like switches and terminals, especially if you need a specific resistor or cap to finish a circuit and don't want to wait for it to arrive.
It is a pain. I ended up buying a new roll of wire and rewinding it. Got 11V out from 240V. Just got to back off a few turns for my filament transformer.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Don't unwind too much before measuring your secondary voltage under load. A good load would be your actual tube filament, after it's warmed up. Then you can find out if your transformer gets too hot, before you spend time mounting and terminating it.
Do big-tube enthusiasts like to limit the current to cold filaments? Do they like to avoid running over (or under) the nominal voltage?
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Trouble with rewound toroidal transformer, never rewound a transformer before
