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Old 22nd April 2007, 12:55 AM   #1
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Default 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!
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Old 22nd April 2007, 01:56 AM   #2
BudP is offline BudP  United States
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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
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Old 22nd April 2007, 01:59 AM   #3
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I didn't think that a 240-120 autoformer would have equal windings.
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Old 22nd April 2007, 02:14 AM   #4
BudP is offline BudP  United States
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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
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Old 22nd April 2007, 02:32 AM   #5
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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
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Old 22nd April 2007, 03:40 AM   #6
BudP is offline BudP  United States
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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
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Old 22nd April 2007, 05:57 AM   #7
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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
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Old 22nd April 2007, 06:05 AM   #8
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I think Bud's suggestion was to use it in addition to the isolating mains transformer, not instead of.
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Old 22nd April 2007, 06:17 AM   #9
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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
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