I'm trying to build an isolation transformer. I bought two identical transformers at the Re-Store. A photo of one transformer is attached. The transformers originally had a direct connection between the primary and secondary coils. I disconnected that so that the coils would be isolated from each other. I tested the transformers independently and they perform identically (i.e., same voltage applied to primary - same voltage out on secondary). So, when I connect the two secondaries together and apply a voltage to the primary of one transformer, I should get close to the same voltage out at the primary of the other transformer. However, when I apply 30V I get 20V out. When I apply 120V I get 90V out. I know there would be some loss across the transformers, but that seems excessive.

Any ideas would be appreciated.

 

That is designed as a ballast, not a normal transformer, so it may have characteristics not suitable for normal transformer use.

 

That is designed as a ballast, not a normal transformer, so it may have characteristics not suitable for normal transformer use.

Yes, I think they saturate (magnetically) easily. Not a nice sine wave out of the first one. The second one will then have poor efficiency converting back to the input voltage.

 

The second transformer will draw a large magnetizing current. This current times the combined resistances of the two connected transformer windings caused a significant voltage drop in the windings that causes the output voltage to be lower than it would be with ideal transformers.

 

Bottom line is, to make an isolation transformer from two transformers, they need to be standard power types.
There will still be some voltage loss from input to output, but it should be acceptable if you operate the transformers at no more than about 50% of their rating.

 

Thank you all for your helpful replies. A good learning experience.

 

Those work just fine for making custom transformers provided you take the metal shunts out that were between the original primary and secondary windings.

I've used them a number of times over the years without problems that way..

 

Those work just fine for making custom transformers provided you take the metal shunts out that were between the original primary and secondary windings.

I've used them a number of times over the years without problems that way..

I cut the wire connecting the primary and secondary windings. The transformers seem to operate like normal transformers. You mentioned metal shunts. Is there something that needs to be removed in addition to the wire that I cut?

 

Lighting ballasts have a set of metal shunts on the iron core that are between the primary coil and the secondary coils that divert some of the magnetic flux away from the secondary to make their secondary coils output more of a constant wattage which is what an HID type arc lamp needs to operate correctly..

They need to be removed to get the full magnetic flux going to the secondary coil otherwise you will end up with a secondary coil that has very poor voltage stability and turns ratio matching from the primary to secondary.

Usually they can be knocked out with a punch unless they are made as a solid part of the core. In that case the core needs to be disassembled into its two or three primary pieces and these shunts need to be ground off.

 

Lighting ballasts have a set of metal shunts on the iron core that are between the primary coil and the secondary coils that divert some of the magnetic flux away from the secondary to make their outputs more of a constant wattage.

They need to be removed to get the full magnetic flux going to the secondary coil otherwise you will end up with a secondary coil that has very poor voltage stability and turns ratio matching from the primary to secondary.

Thanks, tcmtech. I saw the metal shunts, but I wanted to be sure that was what you were describing before I removed them. Also, I didn't understand the purpose of the shunts. Thanks for your explanation.

 

I knocked out the metal shunts and hooked up the two transformers as an isolation transformer. This time I get ~115V out with ~122V in with no load. This seems workable. I'll test it and see how it works with various loads.

 

Sounds much better!


I don't recall exactly what the crossover taps output was on the primary side of a HPS ballast but if it's one of those that is only 10 - 15 volts up or down from either the common end of the winding or the 120 volt tap you can move the common or hot connection for the primary over to that to gain a bit more secondary voltage.

Either that or take some turns off the primary unless you ever intend to use it as a boosting transformer and need the extra 277 and 347 volt taps for outputs.

 

Sounds much better!


I don't recall exactly what the crossover taps output was on the primary side of a HPS ballast but if it's one of those that is only 10 - 15 volts up or down from either the common end of the winding or the 120 volt tap you can move the common or hot connection for the primary over to that to gain a bit more secondary voltage.

Either that or take some turns off the primary unless you ever intend to use it as a boosting transformer and need the extra 277 and 347 volt taps for outputs.

I tried various wiring combinations including the crossover tap. I seem to get the best result (115V out) with the input between the 120V tap and the 347V tap. Some of the other combinations give a similar output (113V -114V).

On the wiring diagram on the transformer, the tap that's marked COM looks like it's connected to one end of the primary winding . However, it does not seem to be connected to the winding. It reads open circuit between the other taps and gives no voltage output when used as an input. It's the same for both transformers.

 

It's supposed to be connected so I suspect that when you cut the cross over wire you may have also disconnected that end of the coil.

Just a guess at this point but ultimately yes it should be connected to all of the other taps on the primary coil just as the schematics show.

 

It's supposed to be connected so I suspect that when you cut the cross over wire you may have also disconnected that end of the coil.

Just a guess at this point but ultimately yes it should be connected to all of the other taps on the primary coil just as the schematics show.

You're right. There are actually two wires together coming from the primary in the crossover connection. When I cut the connection, the wires were separated. When I joined them back together the COM wires are now connected to the primary as on the schematic.

Thanks for all your help with this.

 

I tried several wiring schemes and tested them to determine the voltage output with various loads. The best one gives ~135V with no load and drops less than 3V with a 100W incandescent light bulb. The transformers are rated at 150W.

The 135V seems too high to be used with most 120V devices. Is there a simple way to reduce the voltage by 10V or so and keep the AC sine wave output without wasting much power?

 

Taking a number of windings off the secondary would be the easiest to get your voltage down to a more acceptable 120 - 125 VAC at no load.

 

Taking a number of windings off the secondary would be the easiest to get your voltage down to a more acceptable 120 - 125 VAC at no load.

Is there an easy way to remove windings without disassembling the transformer?