very simple question about grounds in circuits...

[lh1]

Hi, I have a development board powered by 12VDC that has a CMOS 5V output pin that I can control.

I want to use the 5V output to switch on a 12V relay. So I am using a transistor to avoid drawing too much current from the CMOS to switch on the relay. My problem is that the CMOS 0V is 0.7V higher than the negative terminal of the 12VDC supply.

How do I reconcile this? Do I connect the emittor of the transistor to the 0V of the CMOS or to the negative terminal of the 12VDC supply?

Many thanks, sorry if this is a trivial question.

 

[ericgibbs]

Hi, I have a development board powered by 12VDC that has a CMOS 5V output pin that I can control.

I want to use the 5V output to switch on a 12V relay. So I am using a transistor to avoid drawing too much current from the CMOS to switch on the relay. My problem is that the CMOS 0V is 0.7V higher than the negative terminal of the 12VDC supply.

How do I reconcile this? Do I connect the emittor of the transistor to the 0V of the CMOS or to the negative terminal of the 12VDC supply?

Many thanks, sorry if this is a trivial question.

hi,
Look at the other thread on this topic, answer suggested.

https://www.electro-tech-online.com/threads/ground-problem.103336/

EDIT:
Is the 12Vdc supply powering any other devices.?
Also how have you measured this =-.7V below ground voltage on the 12V supply.?

 

[lh1]

Thanks for that... The 12VDC is supply the development board that has the CMOS 5V pin output. It is also supplying the relay that I want to trigger via the 5V pin output. That's all.

Sorry but I'm quite a beginner and not sure what the answer is from that thread.

hi,
Look at the other thread on this topic, answer suggested.

https://www.electro-tech-online.com/threads/ground-problem.103336/

EDIT:
Is the 12Vdc supply powering any other devices.?
Also how have you measured this =-.7V below ground voltage on the 12V supply.?

 

[lh1]

I am using a multi-tap transformer as a power supply. I have rectified and regulated oneof the taps to generate a 12vdc steady power supply. I did this because my dev kit takes 9-12vdc that it then rectifies/ regulates down to 5v to power a programmable IC. This IC has a bunch of input/output pins. In one block, there are 8 IO pins plus 2 other pins: a Vcc and a 0v. I use the multimeter to measure the voltage between Vcc and 0v and I get 5v as expected. I then used the multimeter to measure the voltage between the negative terminal of my little 12v power supply and the 0v pin expecting it go
be pretty much 0v however it was +0.7v... I didn't build the dev board so I don't know what is going on with it...

I wanted to use these IO pins as logical 1s and 0s to switch on/off a 12v relay. Because the relay pulls quite a bit of current to energies it's coil, I didn't want to burn out the programmable IC on the dev board. So I want to use a transistor to handle the current.

My question is: which ground do I use for the emittor of the transistor?

Hope that is clearer! Thanks

 

[atferrari]

Sorry for the question but is it any chance that the 0V you mention (CMOS) is an output pin in low state?

Maybe a schematic would help...