Using transformer primary as an autoformer

stoolpigeon · 22nd April 2007, 12:55 AM

Hi,

I have 230 volt mains and need to test a 115 volt item. Can I use the primary of a power transformer which has 2 x 115 volt windings as an autotransformer?

I would connect the transformer primaries in series (of course) and then tap off the mid point to supply 115 volts.

The transformer is much bigger than that in the 115 volt item.

If this is ok do I need to do anything with the transformer secondaries?

Thanks!

BudP · 22nd April 2007, 01:56 AM

stoolpigeon

Go ahead and do this for a short while. The 115 volt winding, that is the tapped winding, will be forced to carry twice it's normal current loading. So, as an example, if your total VA is 100 and the power transformer is rated for 200 VA you should be fine. If this twice the size of the load is not the case, you can still run it, but for no more than about 15 minutes before that common current winding starts getting too hot. It will take more than an hour for you to feel the heat all over the transformer and by then you will have seriously degraded the life of the transformer.

Do not load the secondaries at all.

Bud

leadbelly · 22nd April 2007, 01:59 AM

?
I didn't think that a 240-120 autoformer would have equal windings.

BudP · 22nd April 2007, 02:14 AM

Leadbelly,

That is correct, there will be twice the voltage drop and it will be more like 230 / 110, so that might be a problem for extended use.

Depends upon whether the primary is a series / series or a series / parallel. The latter is far and away the most common and the former would suit the needs perfectly, as the 115 volt section is designed to carry twice the load current.

Bud

stoolpigeon · 22nd April 2007, 02:32 AM

Thanks Bud, it will just be a quick test to check the laser in a cd player.

Just out of interest, if the transformer was way bigger, there was no overheating and the voltage being tapped was close enough to the 115 volts required, could this arrangement be made permanent?

SP

BudP · 22nd April 2007, 03:40 AM

Yes, it could be a permanent hookup.

By the way, to tell whether it is a series parallel or series series, just measure the primary DCR of the windings involved, If they are within 10 to 20% of each other it is a series parallel, if they are off by 50 % or more then it will be series series, almost certainly.

Bud

I_Forgot · 22nd April 2007, 05:57 AM

Quote:

Originally posted by BudP
Yes, it could be a permanent hookup.
Bud

This is a really BAD idea. One of the things a "normal" transformer does for you is isolate you from the power grid. Autotransformers provide no such isolation.

Autotransformers are cheaper than isolating transformers so you know every piece of equipment would use them if it were allowed, but it isn't, for good reason. Don't do it.

I_F

jeff mai · 22nd April 2007, 06:05 AM

I think Bud's suggestion was to use it in addition to the isolating mains transformer, not instead of.

stoolpigeon · 22nd April 2007, 06:17 AM

Yes it was.

I wanted to use this "home made" autotransformer to run a cd player which was rated for 115 volts and obviously had inside a 115 volt transformer.

It worked just fine.

SP

22nd April 2007, 12:55 AM	#1
stoolpigeon diyAudio Member Join Date: Feb 2007 Location: Laputa	Using transformer primary as an autoformer Hi, I have 230 volt mains and need to test a 115 volt item. Can I use the primary of a power transformer which has 2 x 115 volt windings as an autotransformer? I would connect the transformer primaries in series (of course) and then tap off the mid point to supply 115 volts. The transformer is much bigger than that in the 115 volt item. If this is ok do I need to do anything with the transformer secondaries? Thanks!

22nd April 2007, 01:59 AM	#3
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	? I didn't think that a 240-120 autoformer would have equal windings. __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

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22nd April 2007, 01:56 AM	#2
BudP diyAudio Member Join Date: Feb 2007 Location: upper left crust, united snakes	stoolpigeon Go ahead and do this for a short while. The 115 volt winding, that is the tapped winding, will be forced to carry twice it's normal current loading. So, as an example, if your total VA is 100 and the power transformer is rated for 200 VA you should be fine. If this twice the size of the load is not the case, you can still run it, but for no more than about 15 minutes before that common current winding starts getting too hot. It will take more than an hour for you to feel the heat all over the transformer and by then you will have seriously degraded the life of the transformer. Do not load the secondaries at all. Bud

22nd April 2007, 02:14 AM	#4
BudP diyAudio Member Join Date: Feb 2007 Location: upper left crust, united snakes	Leadbelly, That is correct, there will be twice the voltage drop and it will be more like 230 / 110, so that might be a problem for extended use. Depends upon whether the primary is a series / series or a series / parallel. The latter is far and away the most common and the former would suit the needs perfectly, as the 115 volt section is designed to carry twice the load current. Bud

22nd April 2007, 02:32 AM	#5
stoolpigeon diyAudio Member Join Date: Feb 2007 Location: Laputa	Thanks Bud, it will just be a quick test to check the laser in a cd player. Just out of interest, if the transformer was way bigger, there was no overheating and the voltage being tapped was close enough to the 115 volts required, could this arrangement be made permanent? SP

22nd April 2007, 03:40 AM	#6
BudP diyAudio Member Join Date: Feb 2007 Location: upper left crust, united snakes	Yes, it could be a permanent hookup. By the way, to tell whether it is a series parallel or series series, just measure the primary DCR of the windings involved, If they are within 10 to 20% of each other it is a series parallel, if they are off by 50 % or more then it will be series series, almost certainly. Bud

22nd April 2007, 06:05 AM	#8
jeff mai diyAudio Member Join Date: Aug 2002 Location: Melbourne	I think Bud's suggestion was to use it in addition to the isolating mains transformer, not instead of.

22nd April 2007, 06:17 AM	#9
stoolpigeon diyAudio Member Join Date: Feb 2007 Location: Laputa	Yes it was. I wanted to use this "home made" autotransformer to run a cd player which was rated for 115 volts and obviously had inside a 115 volt transformer. It worked just fine. SP