enter image description hereMe and my dad have a small project at the moment for one of his amplifiers he uses with his band, on a hot day the amplifier overheats and conks out so we decided to fix an old pc fan we had lying around and blow air at the heat sink. but to make it one compact unit we want to get power from the same source as the amp. So I found an old 12v ac to dc power supply and thought simple job solder it onto the pins on the inside of the casing and job done.

Now how it looks is that there is standard 3 pin power cord going into it, the socket of the 3 pin power cord is connected to a small pcb with some pins coming out and a few pins have cables attached, I can't remove the pcb so I can't see how it was soldered to find out the positive and negative terminals. So I get out my multimeter to find out. I found the positive terminals, there are 3 and 2 have cables attached, great that leaves one spare. The same goes for the negatives.

I'd like to connect my fan to the switch as well the problem is that the voltage difference between the negative and the positive is 120v as far as my multimeter shows, even though it should be 240v. The only reason I can think of that maybe the system is designed for America. But whats even more confusing is if I try other pins it goes back to 240v which is what I need for my power supply.

Does anyone have an explanation for this, I'm only a little bit of tinkerer and I'm definitely not an expert, I don't tend to handle large voltages like this only when I fit some sockets on the wall but there must be some reason why the voltage drops from 240 to 120.

I hope I explained it well enough, I can get pictures if anybody needs me too.

enter image description here

The pins in the red circles are positive

The pins in the blue circle is negative

The pins in the green circle is earth connecting to the casing

The one in purple is also negative put it says its 120v

If I try to connect the purple to any red I get 120v If I try to connect the right most blue with any red I get 120v which is also the negative wire going to the switch. If I try to connect the middle empty pin with the upper empty red pin I get 240v but If I try to connect the right blue pin with the empty red pin I get 120v

I would like to connect my negative by the switch so the fan switches on or off based on the switch but it only gives me 120v out of some reason and I can't figure out why. The 3 pin power cord supplies 240v and this pcb connect directly to the 3 pin power cord just behind it so its not like there is a step-down transformer behind it.

  • \$\begingroup\$ I have no clue what you are on about from your wall of text and there is no question there to answer. Can you clear it up, show with a picture and boil it doen to a question? \$\endgroup\$
    – winny
    Mar 31 '17 at 21:08
  • 2
    \$\begingroup\$ You are possibly looking at a single (or split) phase 240 volt line. If you measure either leg to neutral you'll see 120 volts. If you measure between the two 'hot' legs you'll see 240 volts. \$\endgroup\$
    – BobT
    Mar 31 '17 at 21:12
  • \$\begingroup\$ Does the amplifier have a switch to select either 120 or 240 Volts supply? If there is a dual-primary power transformer to deal with 120/240V, that could explain the 120 V that you measured. Can you find a manual or schematic diagram for the amp? Manufacturers often have documentation on their websites, even for discontinued products. \$\endgroup\$ Mar 31 '17 at 21:20
  • \$\begingroup\$ Pictures would be useful, especially of the mains input PCB. If it's awkward to remove, see if you can use an inspection mirror to get a picture of both sides. At the very least, take a picture of where you plan to make your connection. Readers can then give practical advice on how to design a safe modification. \$\endgroup\$
    – user133493
    Mar 31 '17 at 21:21
  • \$\begingroup\$ There is no switch to select between 120 or 240v, the power comes directly from the socket into the amplifier, there is no transformer involved. \$\endgroup\$ Mar 31 '17 at 21:38


simulate this circuit – Schematic created using CircuitLab

I strongly suspect you have an amplifier with an input transformer with taps for both 240V and 120V, as in the diagram. That way, it can be configured in the factory for either 240V or 120V use, without having to re-design anything - just swap a wire from one terminal to another.

If that's the case, then you're seeing the primary of the transformer act as an auto-transformer. If you put 240V into the 240V tap, then you'll get 120V out of the 120V tap.

  • \$\begingroup\$ If there is it would have to be tiny because there is no space behind the pcb, the pcb is directly attached to the 3 pin plug. \$\endgroup\$ Mar 31 '17 at 22:41
  • \$\begingroup\$ @SimonMoore Only three leadwires from the transformer would go to the vicinity of the plug; the transformer wouldn't be located there. \$\endgroup\$
    – Whit3rd
    Apr 1 '17 at 5:34
  • \$\begingroup\$ @SimonMoore Have you looked around inside the amplifier for a transformer that could be wired to the PCB? I wouldn't normally expect it to be running straight off the mains - when rectified, that would give about 340V DC, which is seriously high. \$\endgroup\$
    – Simon B
    Apr 1 '17 at 21:52

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