PNP Transistor Theory

[jb33]

I have A1015 PNP transistor on a circuit board used to power a 12V coil on a relay that allows AC to flow to a motor contactor. The emitter and base both have 12V, but the collector is floating around 2V which is not nearly enough to power the relay. I have tested the transistor with a meter and it checks good. Any ideas why this is happening if the transistor is good? Does the emitter voltage need to be higher? Maybe the problem lies with the TD62003AF 7Ch Darlington Sink Driver that sits in front of the transistor? Thanks in advance for any help!

 

[Papabravo]

If the base and the emitter are both at the same potential, then the transistor is OFF. Without a schematic diagram of the circuit, I have no idea if there is any way to turn the transistor on. Were you hoping I could jinn up a circuit in my head that would help you?

 

[jb33]

Part of what I'm doing is creating a schematic for the board, but I need to fix this issue first. Tomorrow, I can post the schematic I've created for the transistor/relay portion, which is the output to the motor contactor. I believe the transistors are being driven by the TD62003AF 7Ch Darlington Sink Driver which has a flip flop in front of that. I'll post some pictures tomorrow. Thank you for the reply.

 

[atferrari]

First the schematic.

 

[Pommie]

but I need to fix this issue first.

The issue may come to light once a schematic is available.
How are you going to fix it without a schematic?

Mike.

 

[jb33]

That's what I'm doing, reverse engineering these things so I can build a schematic. I know the problem is the transistors not turning on, so I have to figure out the why.

 

[Nigel Goodwin]

That's what I'm doing, reverse engineering these things so I can build a schematic. I know the problem is the transistors not turning on, so I have to figure out the why.

Hence our request for a schematic of that part of the circuit, as we've no idea what is supposed to be switching the PNP transistor ON.

Presumably it will be similar to the sixth picture on my webpage here:

Nigel's PIC Tutorial Hardware Extras

Which switches a higher voltage supply from a logic voltage. From that example you would read the voltage at the bottom end of R3, and see if that is high or low - if it's low then the PNP should be ON, if it's high then T1 isn't switching, and you would check the voltage input to that. The same likely applies to your situation.

 

[jb33]

I have created the schematic for the portion I am having trouble with. The Flip-Flop is getting the required 5V. Thanks again everyone.

 

[Nigel Goodwin]

Measure the voltages on pins 1, 2, 15 & 16 of U2 and post the results.

 

[jb33]

Problem fixed. There was an entire wiring harness missing that provides an extra 12V supply to the base of the transistor that nobody told me about. It's sourced through another connector that was never there. It was the entire reason the transistors weren't turning on. I was beginning to lose my mind. Anyway, thank you everyone for the efforts!

 

[Papabravo]

That's why good engineers never take a dump WITHOUT a schematic!