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In Reply to: Hammond filament transformers - primary voltages posted by Kiwi Brian on March 29, 2007 at 02:18:02:
After you plug it in and your lights all go out, you won't be so concerned about regulation...The primary has JUST enough turns to support 117V (actually about 125-130) before the flux in the core reaches the saturation point. Higher voltage (or lower frequency, which allows longer for the flux to build) will result in greatly increased current as inductance is dropping. 100V at 50 Hz would be OK... or 230V at 120 Hz, though core loses would be up, and insulation would be marginal.
Hammond 266M2 has dual 2.5V secondaries which will give you 5VCT or dual 2.5V windings. If you want a center tap for 2.5V, use a pair of resistors to split it - or a hum pot.
Follow Ups:
> The primary has JUST enough turns to support 117V (actually about 125-130) before the flux in the core reaches the saturation point.I think you're being generous. Hammond transformers run very hot at typical line voltages (120-125 VAC), even with no load. I think 115-117 VAC is the real upper limit for reasonable heating. Someone told me once that this is the result of designing for the Canadian market, where line voltage is lower. Whatever the reason, older transformers from companies like Triad and Stancor don't exhibit this weakness.
You can add some power resistor in series with the primary side to bring ac voltage down a little. Only works when secondary current load is constant, like with a tube heater. Also helps during turn-on, in that a cold start doesn't have quite the same inrush current.One of my projects used a flashlight bulb (5V, I think) instead of the resistor. This actually has a couple of benefits. One, it drops the line voltage to where the Hammond is more happy. Two, you can make it part of an RC type low pass filter to remove some line hash. Three, it makes for a great power indicator. And four, it works as a fuse! See article below for more info.
For 230V operation, you can run TWO Hammond transformers with the primaries in series. Tie the secondaries in series, and you get a free center tap.
jh
I've had this problem far more with Hammond's high voltage transformers than with the filament transformers. I currently have a bunch of Hammond filament transformers in my big monster and they work just fine at 122V 60Hz, the HV one on the other hand does not. It buzzed like mad and got quite hot, I had to go with a different transformer.
That makes sense. Just need to find a stockist now, or convince PCX to get some 266M5's in
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