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Can a non-switched power supply be used in place of a switched one?

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May 5, 2005
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Kind of an emergency situation here. I am with a band and the power supply to the electric keyboard died last night. They have a gig tonight and nobody sells replacement power supplies locally.

I am wondering if we can find another 12VDC 3.5A source (assuming unswitched) if that will work with the keyboard or if there is risk of damaging the keyboard. Here are the specs on the power supply.




Yes, you can use a non switching instead of a switching power supply.
Be sure it has regulated output.
Many non swiching have higher output voltage when no load or light load is connected. This higher voltage may harm your connected device.

One problem can be the earth connection. Especially for music equippment there should be no ( or high impedance) connection from earth to secondary sidde.

One alternative could be a fully charged car battery. It should bring enogh power for 10 hours or more.
But a fully charged battery may have up to 13.x V.
Although it is not likely that the 13.x Volts may harm your device, i can't give a guarantee on that.

Good luck

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The keyboard cares only that it receives DC 12V. It does not care whether the power supply is a switcher or linear.

Whatever type you get, it is essential that the plug be the right size and type. Have the old one with you, to check whether the new one matches. There are more than one size and type of power plug.

3.5A is substantial. You might find the power supply you need at a musical instrument store, office supplies, Radio Shack, automotive store, etc.

Even if you can't buy one, an automobile battery is 12V (give or take). You can attach clips, solder on a power plug, and the crisis is met. A big battery supplies extremely smooth DC.

It's a good idea if you can monitor the voltage. Start charging it if it drops below 11.5 V or so. The charger will probably cause buzzing from the musical instrument, so don't charge during the performance.

- - - Updated - - -

I see KlausST was a step ahead of me. I agree with his post.

Thank you for your replies. Right now I am looking into a tutorial on hacking a PC power supply for 12V. It is my understanding that the amperage output is plenty for the keyboard. Does that sound like it would work?

- - - Updated - - -

Well, I got the PC power supply hacked, splicing together 7 12V yellow wires and 7 black wires (GND) and the reading is 11.85VDC. The keyboard powers up and seems to work just fine with this. I assume this should be plenty for 3.5A, does that sound about right?


As mentioned I got the computer power supply hack to work with the keyboard, but then on a Korg forum someone made this comment regarding the power supply:

amperage rating (usually mili-amps) should be exactly the same to avoid damaging the computer components.

It's always been my understanding that power supplies technically do not put out amperage and the connected device uses only as much as it needs. That's how it works with standard electricity, so does it matter if the power supply I am using is capable of more than the required 3.5A when it comes to delicate electronics?

Their comment is wrong!

You are right, the current is drawn from the source and provided the source is capable, as much current as needed can be taken. An UNDER sized power supply will probably croak and die under the load but one with higher rating just has a bigger safety margin.

If you are in a hurry, I wouldn't advise tinkering with a PC power supply. First problem is they 'talk' to the motherboard to get a signal to turn them on so you would have to fake it with a link between pins on the big plug. Second problem is their 12V probably isn't regulated very well, in most PCs it only powers the motors in disk drives so economics says why regulate it accurately when the drives are speed controlled anyway. Third problem is some PC power supplies will not start or they 'trip' unless there is a load on their 5V outputs, obviously that would always be the case in a computer. Finally, the output probably wouldn't be 'clean', you may get buzzing or whistling from interference they produce when used in association with the amplifier in the keyboard.

It's worth giving it a try but for now I would go with the battery solution.


Well, I do have a car battery in the basement hooked to my backup sump pump but it's registering 13.2V and I am a little uneasy about sending too much voltage into the keyboard. Seems like there is a risk no matter which way I go.

The PC power supply hack included grounding the green wire so the power supply is always on.

Any 12VDC 3.5A regulated source should work or even a 12Vdc 20A source with a ground jumper to enable power.

5% tolerance is typical. No damage with 10%. Check for buzz conducted interference.

These are often used by scanners and found in PC shops.

Yes, that is correct, normally the wire is driven low (not actually grounded) by the start-up circuit on a motherboard.

Check the 12V though, the way most PC PSUs work is they assume all the output voltages will stay the same relative to one another so they only regulate the 5V or 3.3V outputs. The theory is that if that output is stabilized, the others should be right as well. It doesn't work very accurately, especially when the load on the monitored voltage isn't present. You may well find that the 12V rail is less accurate than your battery.

A 'dirty' but simple trick if the voltage is slightly too high is to add a diode or two in series with the battery. The voltage dropped across the diode will stay fairly constant despite the current through it so each one you add will drop about 0.7V. The diodes should be rated to handle the load current. If you can't find single diodes rated at 3.5A you can use a bridge rectifier and fix it to a heat sink. Ignore the AC connections, wire the battery positive to the bridge negative pin and take the output from the bridge positive pin. That will give you a parallel set of two series diodes.


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