I have to design a power supply that takes AC, converts to DC and outputs 9V and -9V with respect to ground. The current being handled is no greater than 100mA (Possibly much smaller than 100mA). The power supply is for an audio application (that's why the power supply has to output negative and positive voltage). I'd imagine there is a standard circuit topology or IC that can achieve this.

I found this question, which is basically trying to do the same as me, but they are doing DC to DC. Does AC to DC make it easier? or should I follow the approach the mentioned there? Also, the question was last active 4 years ago. Maybe there have been new ICs/improvements on how to do that.

I'd appreciate if someone can share an updated way to do this.

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    \$\begingroup\$ have you done any research on the internet before posting here? \$\endgroup\$
    – jsotola
    Nov 1, 2021 at 20:09
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    \$\begingroup\$ This probably is the most common and simple power supply type on this planet. A simple search for "power supply schematic" will kick up thousands of schematics. Surfing through them, you will see that many of them fall into one of two basic types. That is a start. Also, what is the power source for the supply? \$\endgroup\$
    – AnalogKid
    Nov 1, 2021 at 20:25
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    \$\begingroup\$ A transformer and 7809 and 7909 regulators, some caps and diodes. \$\endgroup\$ Nov 1, 2021 at 20:28
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    \$\begingroup\$ 1) The question you linked to is quite irrelevant. 2) I'd appreciate if someone can share an updated way to do this. What you're asking is for a very basic "symmetric power supply", the simplest and safest way to make one is by using the same design which has been in use for about 30 years. A simple analog supply. There is no reason to do this in a more "modern" way. Look at: electroschematics.com/12-volt-dual-power-supply and replace the 7812 and 7912 by 7809 and 7909, done. \$\endgroup\$ Nov 1, 2021 at 20:28
  • \$\begingroup\$ Just to clarify, I did research before posting here. I just didn't use the right terms because it sent to me DC-DC converters such as the question I linked. Thanks for the help. \$\endgroup\$
    – Schach21
    Nov 1, 2021 at 20:38

1 Answer 1



If the tansformer is spec'd for 12-0-12 then the voltage into the regulators will be +&- (12 * 1.414) - 1.5 V where the 1.5 V is the approximate drop across the bridge. So, the voltage into the regulators is +&- 15.5 V but this will rise a bit under light loading due to transformer regulation.

There will be some ripple voltage, on top of the voltage into the regulators, which will have an amplitude of:-

dV = (Idc * dt)/C = (100mA * 8ms)/330uF = 2.4V and the simulation verifies this calculation.

So, the minimum voltage into the regulators is 15.5 V - 2.4 V = about +&- 13 V which is fine because the regulators require a headroom of only 2 V (even less under a load of 100mA).

So, the regulators require a minimum input of +&- 11 V and this design provides a minimum, under full load, of +&-13 V.

With 100mA dc current, the transformer's rms current will be 100mA * 1.8 = 180mA and so the transformer's VA rating should be 180mA * 24V = 4.3 VA. I have suggested 6 VA to give a margin.

The power dissipated by the regulators, under full load, = (14.25 - 9) * 100mA = 0.525 W and so the regulators will warm but should be ok without heatsinks. I would suggest heatsinks if the power dissipation was over 1 W for a TO220 package.


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