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I have a device that needs a (regulated) 12VAC power supply, which turns out to be close to impossible to find.

But, I also have an unregulated 12VAC 1A PSU, which reads 14.4V without load. So, since the device will only draw its specified 320mA, and the PSU is marked at 1000mA, I'm guessing the voltage of the PSU probably won't drop close to 12V, but rather around 13-point-something at that load. So my question is, is this PSU close enough to do the job, or could the possible excess voltage fry something?

Here's a screenshot of the part of the schematic where the PSU goes:

schematic

Edit: Here's the missing bit of the circuit that uses the (half-rectified?) 12VAC:

schematic2

PS: I edited this bit of the circuit out of a larger schematic (there's more stuff to the left), and it probably makes sense to put any input jacks on the outer side of the whole schematic, which is the right-hand side in this case. So yeah, it's not an Arabian schematic :D

PPS: Thanks a lot for the quick and very helpful answers! I'm new to this community and really didn't expect such a helpful and thoroughly explained analysis.

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    \$\begingroup\$ Only you have that PSU at hand and can do the actual measurement of its output and only you know what you are powering with it and if it can live with the higher voltage and ripple \$\endgroup\$
    – PlasmaHH
    Commented Apr 27, 2016 at 11:38
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    \$\begingroup\$ What's with the close votes? The circuit to be powered is given, as well as all relevant data from the powersupply. \$\endgroup\$
    – JRE
    Commented Apr 27, 2016 at 11:45
  • \$\begingroup\$ @JRE Where? I can see you're making assumptions in your answer (rectifiers? regulators?), but I don't get it where that data comes from. \$\endgroup\$
    – Dmitry Grigoryev
    Commented Apr 27, 2016 at 12:08
  • \$\begingroup\$ From the schematic in the question. It clearly shows a power jack feeding into two regulators. It is using half wave rectification to produce +5VDC and -5VDC from a 7805 and 7905 (respectively.) \$\endgroup\$
    – JRE
    Commented Apr 27, 2016 at 12:11
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    \$\begingroup\$ @DmitryGrigoryev: Arabian schematic. I'll have to remember that. \$\endgroup\$
    – JRE
    Commented Apr 27, 2016 at 12:37

1 Answer 1

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It is impossible to find because there ain't no such beast.

A 12VAC supply is a transformer, and is not regulated.

The circuit it is powering has two rectifiers and two regulators to make two regulated DC voltages (+5VDC and -5VDC.)

The 78XX devices can accept upto 25V and 79XX series devices can accept down to -25V, so you are safe using a 12VAC transformer that puts out 14.4VAC.

Also note that the input capacitors are rated for 25V, so the designer allowed for higher voltages going in.

In response to comment:

As Asmyldof points out, there is something else connected to the unregulated voltage at the input to the 7805 regulator. I missed that. You will need to see what else is connected there. I expect it will still be OK, but you ought to check anyway.

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  • \$\begingroup\$ But they will dissipate more. Device draws 320mA max, and you are let's say 2 volts above the nominal input max. This makes 0.64W of additional dissipation. Maybe this needs to be checked too. \$\endgroup\$
    – dim
    Commented Apr 27, 2016 at 12:40
  • \$\begingroup\$ The device does NOT draw 320mA. The original transformer was probably rated for 320mA, but regulators used are specified for 70mA typical. So, worst case an additional 140mA. Assuming original 12V to 5, that's 7v*70mA dissipation=490mW. The regulators are rated for an absolute maximum of 700mW. 490+140=631mW total worst case. So, still OK. \$\endgroup\$
    – JRE
    Commented Apr 27, 2016 at 12:48
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    \$\begingroup\$ While the answer is very likely correct, I'd like to point out the mystery trace going off to who knows where from the input of the 78L05. As a vastly boring sidenote; a 12VAC creates approximately 14.3VDC with a 1.9V ripple at 60mA with those caps, 14.4VAC creates approximately 17.6VDC with a 1.9V ripple. Assuming constant current drain. So the power maths is a bit off. \$\endgroup\$
    – Asmyldof
    Commented Apr 27, 2016 at 14:07
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    \$\begingroup\$ @Asmyldof: Whoops. Missed that wire running off to the side. So, maybe it does draw 320mA, but not through the regulators. \$\endgroup\$
    – JRE
    Commented Apr 27, 2016 at 14:09
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    \$\begingroup\$ As I say, it's extremely likely the answer itself is still spot on. Just wanted to point it out. Dangerous to take clipped circuits for granted in all situations. \$\endgroup\$
    – Asmyldof
    Commented Apr 27, 2016 at 14:12

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