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Old 11th August 2012, 03:28 PM   #1
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Default secondary in parallel/series with primary

What are the risks involved if I hook up a secondary in series with primary to theoretically increase primary windings to reduce output voltage?

vs.

What are the risks involved in hooking up the secondary in parallel and out of phase with primarys to reduce output voltage?
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Old 11th August 2012, 03:32 PM   #2
AJT is offline AJT  Philippines
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such pratice is not encouraged here unless you are using isolation transformers, electricution hazards are real.......
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Old 11th August 2012, 07:16 PM   #3
DF96 is offline DF96  England
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Secondaries should only be connected in series with the primary, and only if those secondaries are not required for any other purpose and if they have sufficient insulation for mains potential. I would recommend attaching a label to the transformer to warn unsuspecting people that this has been done. In a few years time you may have forgotten what you did.

Never connect two windings in parallel (primary or secondary) except when they are identical.
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Old 11th August 2012, 08:02 PM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by DF96 View Post
Secondaries should only be connected in series with the primary, and only if those secondaries are not required for any other purpose and if they have sufficient insulation for mains potential.
It is not generally the case: the mains safety isolation barrier is between the primaries and all secondaries.
The secondaries themselves often rely on the wire enamel for sole isolation material.
There are exceptions: transformers for oscilloscopes, variable frequency drives, etc, but no basic general purpose transformer is built like that.
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Old 11th August 2012, 08:29 PM   #5
DF96 is offline DF96  England
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Yes, I suspected that might be the case so the answer to the OP's question is No - unless you are absolutely sure your transformer is suitable.
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Old 12th August 2012, 12:02 AM   #6
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So, the secondary is to only be hooked up in series if this is to be done.

The transformer is in a housing and sealed in an epoxy like substance (I assume to reduce vibration). When hooked up properly to 120V the (unused) secondary has 20V across it. I'm unable to see the actual wire that's wraped around the core, but the lead comming from it is rated for 300V, I don't trust that to be the rating of the secondary.

Is there another way to tell what the secondary's voltage rating is?

Last edited by scutterflux; 12th August 2012 at 12:04 AM.
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Old 12th August 2012, 01:05 AM   #7
AJT is offline AJT  Philippines
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use a dmm set to voltage to measure it....


a picture of your traffo can help....
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Old 12th August 2012, 03:52 AM   #8
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I do it all the time, works great when you have unused windings (especially that 5V winding intended for a tube rectifier). When you need to bring that voltage down just a little.

Best bet is to connect the secondary winding at the neutral end of the circuit, but ultimately I don't think there are extreme insulation concerns. If you were using the 5V winding (just as example) on a directly heated rectifier, no one panics about the insulation rating.

Keep in mind if your transformer is equipped with an electrostatic shield, using a secondary winding in the primary essentially negates the action of the shield.
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Old 12th August 2012, 12:21 PM   #9
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Quote:
Originally Posted by Elvee View Post
It is not generally the case: the mains safety isolation barrier is between the primaries and all secondaries.
The secondaries themselves often rely on the wire enamel for sole isolation material.
There are exceptions: transformers for oscilloscopes, variable frequency drives, etc, but no basic general purpose transformer is built like that.
Yes, and that's why it is potentially lethal and definitely illegal. Unless you are absolutely sure it is not.
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Old 12th August 2012, 04:43 PM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by Tony
use a dmm set to voltage to measure it...
Most DMM measure voltage. I have never seen one which measures the voltage rating of insulation. For that you need something like a Megger.

Quote:
Originally Posted by zigzgflux
If you were using the 5V winding (just as example) on a directly heated rectifier, no one panics about the insulation rating.
That winding will have been built for high voltage operaton, because the transformer designer will know that a 5V heater winding is intended for a rectifier. Not all secondaries are.
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