Me 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.
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.