Are primaries and secondaries relative?

[jeff]

  Are primaries and secondaries relative to how they are being used? I have a transformer that has two 120V primaries that are wires in parallel for 120V operation or series for 240V can you wire one of the 120V to the wall and get 120V out of the other?

I want to run a main amp and a secondary amp using the other 120V as an isolation transformer for a amp that used straight 120V wall voltage.

If this is possible, how can I figure the new current ratings. Using only one intsead of both I imagine would cut the current in half but now I'm also drawing more current from the other 120V.

[RicharD]

I would be inclined to say yes you can do that as long as the 2 120V windings are completely independent, ie, 4 terminals not 3 (center tapped).  You really need to watch your current.  You'll notice that the 120V parallel current rating is 2x the 240V series current rating.  Now that you are using only 1/2 the primary, you must adhere to the 240V current rating which means you now have 1/2 the available V/A, but remember this V/A must not be exceeded by the combination of all the secondary V/A's.

Dummy up the transformer feeding it through a lamp limiter and measure your unloaded secondary voltages.

[jjasilli]

Maybe Buttery is getting this, but I'm not.  1. On the primary side we have 2 independent windings. You can use one; the other; or both in series or in parallel.  OK so far.  2.  What is on the secondary side?  3.  How exactly do you propose to wire the iso tranny function?

[RicharD]

Speaking very generally, a transformer is a hunk of iron with 2 or more coils of wire.  How you wire it up is entirely up to you.  Turns ratio = voltage ratio.  Current capabilities depends upon wire size.  Breakdown voltage is all about insulation.  We've talked about wiring wall warts backwards to create HT.  What Jeff wants to do isn't really all that different.

[PRR]

> can you wire one of the 120V to the wall and get 120V out of the other?

Yes. BUT!

Perhaps only half the total power. (Generally a bit more than half if the other windings are un-loaded.)

You are not SURE the two primaries are rated for LARGE voltage differential. You know the insulation is good for 120V and probably 240V. However ISOLATION transformers are usually tested to take 500V differences between wall-power and user. If Buttery has wired your house and there is no major mistake in the utility company, you never get such voltages, but stuff happens.

Yes, I have a published plan using this scheme. I think the risk of insulation breakdown is small. Nobody has risen from their grave to say otherwise.

[Willabe]

Nobody has risen from their grave to say otherwise.

               


                  Brad             

[jjasilli]

OK, so with dual primary windingas you can input into one primary, and get output form the other primary.  Live & Learn.

[RicharD]

>OK, so with dual primary windings you can input into one primary, and get output form the other primary.

Yes, as long as the primaries are completely separate.  If it's a center tapped primary being called a "dual" primary (which it's not) then it won't work because the "so called" 2 primaries are electrically connected on 1 end.  There is no transformer isolation.  What you call a winding, primary or secondary, are just names for how the supplier intended the transformer to be used.  Ultimately, they're all coils of insulated wire wrapped around a hunk of iron.

[jeff]

 I believe it would have to be in order to wire series or parallel for 120V or 240V. There are four wires two for each 120. If they weren't seperate you'd be out of phase either when you wired them series or in parallel. I know the kind you're thinking that has a tap for 120 or 240 but this one has two seperate primaries, four wires. I'll double check with a meter to be sure there's no connection between the two.

[jeff]

You are not SURE the two primaries are rated for LARGE voltage differential. You know the insulation is good for 120V and probably 240V. However ISOLATION transformers are usually tested to take 500V differences between wall-power and user. If Buttery has wired your house and there is no major mistake in the utility company, you never get such voltages, but stuff happens. 

I don't understand the 500V difference you're talking about. Basically I was thinking I'd have 120V out instead of 120V in on one of the primaries.
 
 Maybe I have my terms wrong. Is there any difference between a 120V:120V transformer and an isolation transformer? I've been using the terms interchangably. I need a 120V tap. 

[HotBluePlates]

I don't understand the 500V difference you're talking about. Basically I was thinking I'd have 120V out instead of 120V in on one of the primaries.
 
 Maybe I have my terms wrong. Is there any difference between a 120V:120V transformer and an isolation transformer?

There is a difference. An isolation transformer is used to only to provide a 1:1 voltage ratio (which doesn't seem very useful by itself), but also to provide electrical isolation between the primary and secondary. As such, the transformer is usually built and rated to withstand several hundred volts difference between each winding.

It's not just about the a.c. voltage present when you energize the primary; you may have a large d.c. voltage component as well, especially if you use the transformer the way a lot of o'scope and power supply manufacturers do, where the secondary is floated sevral hundred volts above ground, so that a the a.c. voltage output is added to the existing d.c. voltage present (the "float voltage").

In pratical terms, a true iso transformer is about keeping a piece of gear from being directly connected to wall power, so major faults don't do major damage to the item under test, the wall wiring, or you.

[jjasilli]

This thread turned out to be very helpful.  A friend just brought-in a 1970's vacuum tube Swan Cygnet 270B short wave radio for repair.  The filaments were dead.  The filaments are 12VAC-0 powered off the primary side of the PT, from dual taps near the center of the winding.  This would have really thrown me without this thread.   

(Anyway, the problem was a bad intermittent mechanical connection in the filament supply; easily fixed.)