Dumb, yet important, Power Transformer wiring question.

[Gtr0]

Hi.

How should the following 100 watt Dagnall power tranny specs be interpreted?

0V Black
175V Yellow
350V 300mA Brown

Is the brown the CT in this case? Or the black? and while I'm at it - or the yellow?

Better to be safe than sorry :-)

Thanks!

[Helmholtz]

The CT should be yellow as it measures 175VAC to either black or brown. Yellow is in the middle of the winding so CT .

[Gtr0]

Awesome - thank you!

And now that you say it, it makes sense!!!!

[eschertron]

How should the following 100 watt Dagnall power tranny specs be interpreted?

0V Black
175V Yellow
350V 300mA Brown

The CT should be yellow as it measures 175VAC to either black or brown. Yellow is in the middle of the winding so CT .

I agree, based on the info provided. I might suggest measuring resistance (no power applied to primary, of course) between each of the three combinations of two wires, and then measuring AC voltage (when the PT is safely powered up). Both tests should agree on the following points:
1. one pair of wires has the largest magnitude of resistance/voltage,
2. and the remaining wire is equidistant from those two.
of course, I'm a belt-and-suspenders kind of guy.

[mikepukmel]

Not too far an aside, but how do you all do voltage measurements on an PT that isn't hooked up yet, assuming you know what the primaries are? I mean, do you clamp the transformer down to the bench, and wire up the secondaries to a terminal strip, so wires are connected solid? The voltages are so high, a small mistake could result awful. I would not trust myself to hook up alligator clips and power a PT up.

[HaroldBrooks]

I have plastic clamps for just such work, I got them at a cheap tool store. They are spring loaded with a seriously powerful clamping force, so you can as you say clamp the wires away from each other onto a wooden tale top and there is no risk of short. The transformer itself is heavy, so just place it well away from the edge of the table and to the center, and no women or children in the room, seriously !

High voltage is only dangerous if you are loose in your precautions to insulate HT leads and yourself, as that type of voltage will never jump accross really big spans of distance, or jump through insulated surfaces. You just have to secure everthing once and properly, make your test, and then power everthing down 100% before you start to disconnect anything. If you have a switched outlet for the AC at the primaries, all the better.

You probably know this already, but the voltages are much higher with no load on the transformer, but I believe the widing ohms are the same more or less as installed.

No question is dumb, really, it's how you learn and other here (Me for one !) learn when others ask the fundamental questions. I have swallowed a lot of crow, showing everyone what I still don't know.

[Helmholtz]

It's much safer to connect to an isolating mains transformer. I use my isolating variac which also features a current meter and an automatic circuit breaker. But alternatively you can power the PT from a 6.3V heater transformer connected to its heater winding (instead of connecting to mains). In any case you should measure both input and output voltages.

[eschertron]

Not too far an aside, but how do you all do voltage measurements on an PT that isn't hooked up yet, assuming you know what the primaries are? I mean, do you clamp the transformer down to the bench, and wire up the secondaries to a terminal strip, so wires are connected solid? The voltages are so high, a small mistake could result awful. I would not trust myself to hook up alligator clips and power a PT up.

I use a test jig I constructed for my first build, still useful! An IEC connector and fuse block for the primaries on one end of a wooden plank, a term strip with space for all secondary wires on the other end of the plank. IIRC I have a couple 1N4007 diodes installed on the term strip at the ready. Simple to store, and fast when I want to work with any PT.

[The Dude]

It's much safer to connect to an isolating mains transformer. I use my isolating variac which also features a current meter and an automatic circuit breaker......

That's my method, also. I confess, I do just use alligator clips, though. Just make the connections with power off for safety.