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I have been looking for a center tap transformer for use in a dual +/-15 volt supply using 7815 and 7915 linear voltage regulators. I have obtained a transformer from an old stereo amplifier, which has a 52 volt coil with center tap, which will give me a nice solid 26 volts which I can rectify and feed my voltage regulators.

However, there is an additional coil on the transformer as seen in the diagram below from the service schematic of the amplifier. I measure about 50 volts across the orange and blue transformer output. I don't need the output from this second coil.

So the question is, what are the consequences of leaving this coil unconnected? Would this have any impact on power supply noise in my circuit?

Transformer diagram from amplifier service manual

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There is no problem with leaving an unused secondary winding unconnected - but be sure to tape up the wires so that they will not accidently short to each other or to anything else.

The 26/26 volt secondary will give you about 36 volts DC, which is over the the Absolute Maximum input voltage of the 7815 and 7915 regulators, so this transformer is unsuitable for your application.

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  • \$\begingroup\$ Could you help me understand where the extra DC voltage is coming from as part of rectification? \$\endgroup\$ – Ashton Snelgrove Sep 18 '15 at 15:23
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    \$\begingroup\$ AC voltage is always given as the RMS (Root Mean Square) value (a sort of average). The peak voltage of the sine wave is about 1.4 times the RMS value, and the capacitor following a rectifier will charge to the peak value (minus 0.7 volts dropped in the diodes). \$\endgroup\$ – Peter Bennett Sep 18 '15 at 15:46
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    \$\begingroup\$ The voltage would probably be even higher with no load as no transformer has perfect regulation. For this size of transformer (guesstimated from the 1.6A fuse), probably 10-15% higher. \$\endgroup\$ – Fizz Sep 22 '15 at 21:22
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Leaving the winding unconnected will not harm the transformer or do anything else bad. Zero current is acceptable.

However, you may have another problem. 26.3V rectified is 26.3V*1.41=37V, which isover the maximum allowed input voltage for the 7815. Also, unless you plan to only use a few miliamps on the output, the regulator will dissipate a lot of power, so it will need a big heatsink (and a fan possibly).

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  • \$\begingroup\$ Where does the 1.41 multiplier come from? \$\endgroup\$ – Ashton Snelgrove Sep 18 '15 at 15:24
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    \$\begingroup\$ To calculate the peak voltage of a sine signal when the RMS voltage is known (and the transformers, mains voltage are all in RMS), you need to multiply it by Sqrt(2), which, approximately, is 1.41. \$\endgroup\$ – Pentium100 Sep 18 '15 at 21:26

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