#### Rick A

Joined Jul 30, 2010
37
Hello,
I have an Advance #71A6572 transformer I want to use for isolation but I'm uncertain if the two windings are actually separate:

The four inputs are 1220-277 and output 120, but looking at the connection between the windings I wonder if it means the 120v input goes straight through to output and would not really provide isolation. Any idea?

Thank you,
Rick A

Last edited by a moderator:

#### LowQCab

Joined Nov 6, 2012
2,508
That is a "Ballast-Transformer" for an Arc-Lamp, ( Metal-Halide, Mercury-Vapor, etc. ),
they are NOT suitable for anything other than operating their specified Lamp-Type.

It is not, strictly speaking, a "Transformer", it is a "Multi-Tap-Inductor",
which must be used with its matching Capacitor.
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#### DickCappels

Joined Aug 21, 2008
9,331
From the schematic they are not isolated.

#### MisterBill2

Joined Jan 23, 2018
13,173
An ohm meter check would quickly verify that the circuit is as shown. And indeed, the construction of a ballast type transformer is intended to limit the power to some level, and so without some serious modifications they have a very limited application. An arc is a very nonlinear load.

Joined Jul 18, 2013
25,998
I have an Advance #71A6572 transformer I want to use for isolation but I'm uncertain if the two windings are actually separate:
The four inputs are 1220-277 and output 120, but looking at the connection between the windings I wonder if it means the 120v input goes straight through to output and would not really provide isolation. Any idea?
AKA Auto transformer, i.e. non-isolated.

#### MisterBill2

Joined Jan 23, 2018
13,173
The bad news is that even if you were able to break into the windings and isolate the secondary, it would still not be suitable for use as a standard isolation transformer because it is set up to work as a power limiting ballast transformer for a gas-discharge lamp, (arc lamp).

#### Rick A

Joined Jul 30, 2010
37
MIsterBill2, thank you for your reply, even though it wasn't what I was hoping to hear! Would you think it realistic for me to cut out the windings and replace them with a simple 1:1 set of windings? If so, how many turns would it take? It only needs to pass 120v.

Thank you,
Rick A

Joined Jul 18, 2013
25,998
The typical turns/volt for a mains IE transformer is 5turns/volt.

#### LowQCab

Joined Nov 6, 2012
2,508
You'd be better-off starting with a MOT, ( Microwave Oven Transformer ),
but their Cores are not very good either,
and they don't have enough Copper in the Primary-Winding.

You can purchase a Transformer with 2-separate Primary-Windings designed for
240 / 120 connection, and simply use one Winding as a Secondary.
But keep in mind the designed-in maximum Current per Winding will probably be fairly low,
and the output of the second Primary-Winding will probably be
~5 to ~10-Volts lower than the Input-Voltage,
and that, the Windings are on the same Bobbin, so the "Isolation" factor is not that great.

By the time you spend all that calculation time, and money on
enough Enameled-Magnet-Wire to wind your own Secondary,
You could have simply purchased one used, (or even a new one ).

A proper 2-Amp, 120/120-Volt, 250VA, Isolation Transformer is just ~$62.oo from DigiKey. Now calculate how much Magnet-Wire you'll need for just a Secondary-Winding to end up with a questionable hack-job Transformer .......... Probably around ~300-feet of ~18-gauge Magnet-Wire, it will probably be in the range of ~$30.oo,
plus many hours of labor.
And it might have disappointing performance.
And wont be guaranteed to be safe.
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Joined Jul 18, 2013
25,998
For me, I prefer to modify a toroidal, about 2turns/volt. and much easier to wind.
If you do try a MOT, it pays to drive the magnetic shunt out if possible and if one is fitted.

#### MisterBill2

Joined Jan 23, 2018
13,173
The labor in winding an E-I core transformer is not "many hours", and the hard part is keeping track of the turns. Re-working a ballast transformer into a plain isolation transformer would not be so very bad if the line winding can be saved, and if the magnetic shunt part can be removed easily. Microwave oven transformers all seem to have the lamanations welded and that makes rewinding into an exercise in misery.If one that does not have them welded is found then itcould much more easily be rewound. But still, they are made for short operation times.

Joined Jul 18, 2013
25,998
I found the opposite, even if not welded laminations, to remove the centre bobbin and remove turns and rewind is not trivial and very time-intensive.
I did it in my younger days, but now stick to the toroidal type, just because I can!
One of my very first projects was a valve/tube tester, it required a winding for all the various cathode heater element voltages! .

#### MisterBill2

Joined Jan 23, 2018
13,173
How did you handle threading all of those turns through the core? Winding a transformer bobbin I have the wire on the spool, and just make sure that the lathe chuck holds the mandrel for the coil very stable. LOW SPEED is the only way to go, 15 RPM tops. And got to have the brake/stop parts working properly before starting. Also a stop clamp for the wire..
And it has occurred to me that I have access to a 250 watt light fixture that was replaced by LED lighting so perhaps I will examine that transformer in detail. But ballast transformers all seem to have the similar requirements, just different sizes.

Joined Jul 18, 2013
25,998
How did you handle threading all of those turns through the core?.
I didn't.
I used a transformer that I could remove all EI laminations, rewound it and rebuilt!
The EI had bolts through them, not weld.

#### MisterBill2

Joined Jan 23, 2018
13,173
I didn't.
I used a transformer that I could remove all EI laminations, rewound it and rebuilt!
The EI had bolts through them, not weld.
I was asking about the rewinding the toroid as you described in post #10. E-I laminations are simple if they are not welded. I have done a few of them. But not much can n\make it simpler than a metal lathe with good back gearing and a 4-jaw chuck.

Joined Jul 18, 2013
25,998
The toroids were mainly secondary winding's, either an increase in existing or a completely new secondary winding.
So not that many turns.
Using versions with existing 120v/240v primaries.

#### Rick A

Joined Jul 30, 2010
37
I want to put about 15 or so amps through it so I wouldn't think I could get enough turns in a MOT core for that, assuming maybe 14 ga wire. The core I have might accommodate that much wire. Does that sound realistic?

Rick A

#### MisterBill2

Joined Jan 23, 2018
13,173
You are correct, A microwave oven transformer core is simply not big enough to deliver the power.
Look at the size of transformers rated for the power you want in the on-line catalog of a company like Heavi-duty. Those are adequately rated transformers intended for industrial use. That size information will tell you a lot. Avoid Amazon and the like because they know nothing about what they sell.

#### LowQCab

Joined Nov 6, 2012
2,508
15-Amps X 120-Volts is 1800-Watts,
so the Transformer-Core that you will need will be roughly 1800-VA,
( "VA" is a slightly different measurement method for Watts, but very similar results ).

The average generic MOT-Core is good for around half of that under continuous service.

Transformer-Core-VA rating is directly related to the Weight and Volume of the Core.
A Toroidal-Transformer-Core will be roughly ~20 to ~30% smaller / lighter
than a standard design "E-I" style Iron-Core.

Do you really "need" 1800-Watts ????
Or are you just using that "15-Amp" figure thinking that that way you'll have
the full capacity of a standard 120-Volt Branch-Circuit ?

Do some calculations and come up with some realistic expectations, or just get out your wallet.

That's a huge ~10+ pounds chunk of Iron and Copper,
which will probably consume ~15 to ~20-Watts of Power just sitting there with no Load attached.
( that's for an efficient commercial unit, a DIY unit using a MOT-Core, or Ballast-Core,
will consume ~2 to ~3 times that much Power as Wasted-Heat-Dissipation ).
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