Transistors, what am I doing wrong?

Other Hardware General Electronics
1

Basically what I'm trying to do is turn on one computer power supply with another. When you short pin 14 (PS_ON) to GND on an ATX power supply, it turns on. My reasoning was that I could take a transistor, connect the PS_ON pin to the emitter, the collector to gnd and the base to one of the 5V pins on the other power supply. When I turn on the first power supply, the 5V pin would turn on and allow current through the transistor, shorting the pins on the second PSU, turning it on. I can't get it to work though, and in messing with the transistor further I've become even more confused. I've used these transistors before as "button pushers" to short the pads of a button on a remote control using the Arduino. It's been years, but if I remember right I set the base pin low, then when want to push the button I set it high. It's a PNP transistor, I have a few different ones but I'm using a 2N3906.

I set up a very simple test with the Arduino. I haven't had a working computer in a while so I don't know what code is on my Arduino, but when I turned it on the LED on pin 13 flashed once per second so I think it's just the blink sketch. In a breadboard I put 5v (from one of the power supplies, not the Arduino) to the emitter, the collector to a resistor then the LED, then gnd of the power supply. The Arduino was powered by the same power supply, 5v to Vin and gnd to gnd. I took a resistor and put it between pin 13 and the base of the transistor and the LED on the breadboard blinked with it, as it should have. Then I took the jumper out of pin 13 and put it directly to both 3.3v and 5v on the Arduino, and nothing happened. Why does the transistor "turn on" when connected to 5v through pin 13, but not when connected to 5v through the 5v pin?

2

I really hate ploughing through verbal descriptions. Please post a diagram.

3

I have done similar.
Pin 14 to collector, emitter to GND, base to 1k then to +5 or 0V as needed.
I used 2N3904.

4

I'm sorry, I do too. Problem is I don't have a working computer, I'm posting this from my phone. Basically I just need to close the circuit between PS_ON and GND, but automatically when the first power supply turns on. I figured using a signal from the first power supply to "turn on" a transistor would work.

LarryD:
I have done similar.
Pin 14 to collector, emitter to GND, base to 1k then to +5 or 0V as needed.
I used 2N3904.

With the 3904 did the power supply turn on when the base was given 5v, or 0v? That's an NPN, I'm using a PNP. My understanding was that bringing the base pin of a PNP high would close the circuit, and vice versa for an NPN.

5

You need to use a NPN transistor as you need to pull down the voltage on pin 14.

// Per.

6

Zapro:
You need to use a NPN transistor as you need to pull down the voltage on pin 14.

// Per.

Shoot, so the only way to do what I need is either with an NPN or a relay. I only have PNP transistors, and I threw away my relays in a move recently.

Alternatively, if I could program my Arduino, could I detect the first power supply coming on with an input pin and pull pin 14 on the second power supply to gnd with an output pin? Basically just connect a random 5v pin from the first power supply to an input pin and detect the signal going high...?

7

You could connect the pin14 directly to a pin on the Arduino and set that pin low. It would have the same purpose.

This is actually what is happening on the PC motherboard. The signal goes directly to the chipset without any transistor in the signal way.

// Per.

8

And detecting the 5v from the first power supply would be safe, for the Arduino? As well as sinking pin 14 through the Arduino. This answer on stackexchange has some information from the ATX spec, as well as some tables detailing the current and voltage of the PS_ON pin:

9

Should be fine.

10

I'll give it a shot! It's not ideal, using the Arduino for this purpose, but it'll have to do in the meantime. Thank you for the help!

I'm not sure if you'd be able to answer this, but I plan on running different components in the same computer off of these two PSUs, one for harddrives and fans, the other for motherboard and graphics card. My 500W PSU went out last year and I haven't been financially stable enough to buy a new one, but I have a 350W and a 300W from different computers that work fine. If I do this, should I ground them together or do something to, make it safer for the computer? I know this configuration isn't recommended, but I also know it should work, I just want to make it as safe as possible.

11

Running two power supplies: As long as they are bolted to the same computer case, that should be enough grounding.

// Per.

12

Zapro:
Running two power supplies: As long as they are bolted to the same computer case, that should be enough grounding.

// Per.

Cool beans. I'll probably have to run a wire from the secondary one to the case, but sounds good to me. Thank you again!

13

HaLo2FrEeEk:
Shoot, so the only way to do what I need is either with an NPN or a relay. I only have PNP transistors, and I threw away my relays in a move recently.

Well now.

14

HaLo2FrEeEk:
I threw away my relays in a move recently.

Rookie error..... (Unless there's a Mrs HaLo2FrEeEk who had something to do with that, then you're excused.)