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Transformer with center tapped primary
Transformer with center tapped primary
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Old 30th October 2017, 10:05 PM   #1
cspirou is offline cspirou  United States
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Default Transformer with center tapped primary

I bought a transformer from eBay fully expecting 2x115v primary windings that I could wire in parallel for 115v or series for 230v. Instead the transformer is wound as 0-115v-230v with 115v as a center tap. Iím sure itís fine wired for 230v however I have 115v mains and using the tap leaves the winding unused.

So would it be okay to use this type of transformer at 115v?
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Old 30th October 2017, 10:14 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by cspirou View Post
would it be okay to use this type of transformer at 115v?
Yes, just follow the secondary rating.
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Old 30th October 2017, 10:37 PM   #3
JMFahey is offline JMFahey  Argentina
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To be certain you should measure winding resistance.
Since transformer is expected to put out same power either on 115V or 220V , then 0>115 winding must use thicker wire (and have lower resistance) than to 115>230V one.
So measure both.
No need for an accurate value here, just to make a comparison and confirm/deny this possibility.
So maybe your transformer is, say, 3 ohms 0>115, 5 or 6 ohms 115>230 and 8 or 9 ohms 0>230 taps.

Please measure and post results.
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Old 30th October 2017, 11:01 PM   #4
cspirou is offline cspirou  United States
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Here is the resistance I measured

0v - 115v, 19 ohms
0v - 230v, 56 ohms
115v - 230v, 38 ohms

Edit:

Transformer is 68VA with 2x18v secondary windings

Last edited by cspirou; 30th October 2017 at 11:21 PM. Reason: Additional info
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Old 30th October 2017, 11:25 PM   #5
anjump123 is offline anjump123  United Kingdom
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Has the centre tap got two wires in it or does it lead to two wires at a relatively external point?
If so you could wire it as you would with two windings by cutting/ unsoldering and checking with a multimeter.
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Old 31st October 2017, 03:46 AM   #6
cspirou is offline cspirou  United States
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I could check, but I would have to unwrap a few layers. Since the resistance is in proportion to what JMFahey mentioned, I’ll use it as is. Just bothers me a little that there is an unused winding.
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Old 31st October 2017, 04:17 AM   #7
JMFahey is offline JMFahey  Argentina
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fine, you can use it.
In the old days, International transformers were multitapped and each section was wound with the proper diameter wire, period.
Look at this VOX guitar amplifier (used by Beatles, Stones and Queen among 1000 others)
To work *anywhere* on the World it had primary taps at: 115/160/205/225/245V.
Click the image to open in full size.

Then cutthroat competition chimed in and transformers became dual series/parallel primary, matching only 2 voltages and "sort of" close enough to others.

Of course, saving some 20% weight and cost

Unused windings are he norm there and well accounted for.
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Old 31st October 2017, 04:18 AM   #8
anatech is online now anatech  Canada
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Transformer with center tapped primary
Hi cspirou,
Just smile and use it as-is. If you must, you could run a neon lamp off the 115 to 230 V winding. That should make you feel better!

-Chris
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Old 31st October 2017, 03:54 PM   #9
cspirou is offline cspirou  United States
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Yeah, I just thought it would be some issue with parasitic inductance or capacitance. But if the design accounts for it then it really shouldn't bother me.
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Old 31st October 2017, 04:23 PM   #10
anatech is online now anatech  Canada
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Transformer with center tapped primary
Nope, it shouldn't.
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