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Use Centre-tapped transformer without using the centre tap...possible?
Use Centre-tapped transformer without using the centre tap...possible?
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Old 4th December 2013, 12:13 PM   #1
Crom is offline Crom  United Kingdom
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Default Use Centre-tapped transformer without using the centre tap...possible?

Hi,

This is probably a stupid question but I can't find the answer...

Can I use a centre-tapped transformer without using the centre-tap? For example, if I have a transformer with a secondary wired as 40-0-40 then can I just wire the two '40' wires and leave the '0' wire unconnected?..or should I connect the centre-tap to earth.

I'm just trying to save some money by re-using an existing transformer that was previously used in a transformer with a full-wave centre tap psu .

I understand the different current/voltage multiples that I need to calculate (Toroid, Switchmode, Power Transformers Mount Manufacturers from MCI Transformer Corporation) but I just want to check before I do something stupid ;-)
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Old 4th December 2013, 12:24 PM   #2
jamesfeline is offline jamesfeline  United Kingdom
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Hi

You don't need to earth the center tap, but you voltage across the live terminals will be 80v (not a split +ve -ve rail)
The idea for a center tapped earth is for building site power tools which are normally 55v ov 55v ... so the idea is that if a person becomes exposed to a live conductor to earth the exposed voltage for electric shock is 55v which is classed as extra safe LV .. in this situation the center tap is earth .. hope this helps
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Old 4th December 2013, 12:29 PM   #3
richie00boy is offline richie00boy  United Kingdom
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If you just want an 80v winding then yes you can ignore the centre tap.
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Old 4th December 2013, 12:36 PM   #4
Crom is offline Crom  United Kingdom
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Hmmm, I want 2 x 40v windings...I guess this isn't going to work. Thanks for your help guys.
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Old 4th December 2013, 12:39 PM   #5
Michael Bean is offline Michael Bean  United States
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What are you trying to do? Maybe we can help.

Mike
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Old 4th December 2013, 12:43 PM   #6
Crom is offline Crom  United Kingdom
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:-)

Well, I have a couple of transformers with dual centre-tapped secondaries wired like this: 2 x 40-0-40vac (so this would give me 4 x 40-0-40vac supplies) . I need 4 x 40vac supplies to feed psu's with bridge rectifiers (ie no centre-tap required). I was wondering whether there's a way of re-using the existing transformers or whether I need to buy new ones.
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Old 4th December 2013, 12:52 PM   #7
doctordata is offline doctordata  Australia
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Quote:
Originally Posted by Crom View Post
:-)

Well, I have a couple of transformers with dual centre-tapped secondaries wired like this: 2 x 40-0-40vac (so this would give me 4 x 40-0-40vac supplies) . I need 4 x 40vac supplies to feed psu's with bridge rectifiers (ie no centre-tap required). I was wondering whether there's a way of re-using the existing transformers or whether I need to buy new ones.
See if yo can cut the centre tap winding where it's coming out of the transformer , then you have two separate windings . and use them separate or in parallel .Can you post a close up pic ?

Cheers ,

Rens
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Last edited by doctordata; 4th December 2013 at 12:57 PM.
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Old 4th December 2013, 12:55 PM   #8
Michael Bean is offline Michael Bean  United States
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Yeah, but what are you trying to do? Do you have a schematic? If I understand, you have two seperate transformers, and each one has two separate 80VCT (40-0-40) seconaries, for a total of four 80VCT (40-0-40) secondaries? If so, yes you can use just one half of each one to get 4 X 40VAC, but transformer rating must be halved. But if you're going to use a full wave bridge rectifiers on each secondary for a DC power supply (4 X 55VDC), then you would be better served using two discrete rectifier diodes and the center tap for full wave 55VDC at full transformer VA rating.

Mike

Last edited by Michael Bean; 4th December 2013 at 01:05 PM.
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Old 4th December 2013, 01:22 PM   #9
doctordata is offline doctordata  Australia
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Quote:
Originally Posted by Michael Bean View Post
Yeah, but what are you trying to do? Do you have a schematic? If I understand, you have two seperate transformers, and each one has two separate 80VCT (40-0-40) seconaries, for a total of four 80VCT (40-0-40) secondaries? If so, yes you can use just one half of each one to get 4 X 40VAC, but transformer rating must be halved. But if you're going to use a full wave bridge rectifiers on each secondary for a DC power supply (4 X 55VDC), then you would be better served using two discrete rectifier diodes and the center tap for full wave 55VDC at full transformer VA rating.

Mike
That's a good one , ( discrete rectifier ) Old school electronics !
Forgot about that along time ago !

Cheers ,

Rens
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Old 4th December 2013, 01:48 PM   #10
Michael Bean is offline Michael Bean  United States
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Yup - I'm Old (school)
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