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I'm trying to get a stable 48 VDC power source with at least 500 mA. I have a power transformer with two secondaries, each with 24 V 0.73 A. The transformer is rated at 35 VA.

What I've tried is connecting the middle lugs of the secondaries together to give me 48 VAC, but it's giving me closer to 52 VAC. After the rectifier bridge it's giving me around 52 VDC, but when it gets to the circuit's power capacitors (680 μF across the power rails), it goes up to around 70 VDC. It's supposed to power an audio amplifier and some extra LEDs.

Is there a way to get a constant 48 VDC from this, or am I better off regulating each of the secondaries on their own to 24 VDC, and then wiring them in series? If so, what's the best solution to get the 24 VDC? I was thinking of using 7824s or something similar.

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    \$\begingroup\$ The rectifier will charge the smoothing capacitors to the peak voltage of the AC waveform, not the average voltage. If the input is 52 V AC (RMS), the peak voltage will be SQRT(2) * 52 V = 73.5 V. A full bridge rectifier will drop approx. 0.7 V for each conducting diode, bringing the no load capacitor voltage closer to 72.1 V DC. The transformer is likely designed so that the output voltage is 48 V AC at the full rated load current, that's why you see 52 V at no load. \$\endgroup\$
    – jms
    Commented Mar 7, 2016 at 14:27
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    \$\begingroup\$ Audio amplifier running on 24V split power supply with 0.5A current capacity?? What will be your speaker's impedance? Common 8 Ohm speaker will seriously overload your power supply... \$\endgroup\$
    – soosai steven
    Commented Mar 7, 2016 at 14:54
  • \$\begingroup\$ A transformers output voltage will drop as it is loaded, the ratio of RMS output voltage at no load to output voltage at rated current is called regulation. In general larger transformers have better regulation. The other comment about voltage at no load after rectification is also true. \$\endgroup\$
    – Icy
    Commented Mar 7, 2016 at 15:11

2 Answers 2

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Keep both secondaries separated. Use bridge rectifiers on each secondaries. Use 680uF smoothing cap for each bridge's outputs. Use 7824 regulator for each side, keep the ground isolated. Now you should have two isolated 24V power source.

Finally link these in series and take the mid point as your common ground. Don't forget heatsinks (maintain isolation) for both 7824. Output smoothing caps of 1000uF will help too. These linear regulators needs reverse protection diodes (check datasheet) to prevent current flow into the output terminals. Beware of that.

enter image description here

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  • \$\begingroup\$ You can't use 7824s for the negative rail, as they won't sink current. You'll need to use 7924s, which are designed for negative voltage. Also, there are far better modern linear regulators out there that should be explored. 78xx/79xx stuff is pretty crusty... \$\endgroup\$
    – Jay Carlson
    Commented Mar 7, 2016 at 17:12
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    \$\begingroup\$ Jay Carlson, its not on the negative rail... Read my instruction carefully again. \$\endgroup\$
    – soosai steven
    Commented Mar 7, 2016 at 21:33
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You should use a buck converter to convert 70V unstable to 48V stable. Considering the power you need, I would suggest to use a switching regulator instead of a linear one, like LM7824. You'll have better efficiency and get much less waste heat to deal with. A switching regulator with 48V output shouldn't be difficult to find.

Converting each secondary voltage to 24V is possible, but you'll have to make two converters instead of one. I would only do it if I had another 24V load to power.

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  • \$\begingroup\$ Dmitry.... Your suggestion is 100% correct. But the original query is from the person who has only 7824s available. \$\endgroup\$
    – soosai steven
    Commented Mar 15, 2016 at 9:23
  • \$\begingroup\$ @soosaisteven No, the OP says he was thinking of using 7824s or something similar. Sounds like he's open to suggestions. \$\endgroup\$
    – Dmitry Grigoryev
    Commented Mar 15, 2016 at 9:35

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