Output Driver Circuit

[tonlokstr]

Hi,

I am working on a project that has a board that uses a TIP125 PNP darlington transistor as a switch to control the outputs and drive the relays. I went through and recreated the circuit and have don't understand the function of the zener diode and how the optical isolator supplies current for biasing the base of the transistor since it is connected to the output voltage. At first I assumed it would be from the path of the component but it will turn on even with no output connected.

The circuit is set to work on 12-24 VDC, our interface is 28 VDC. The issue we have run into on a system is that two of the outputs contactors that require a higher pull in current with a reduced hold current. On the bench at above 31.5 VDC input which is inside the range of the power supply the zener conducts and turns on the output without a command. The higher current draw heats up the TIP125 and it stays on with the largest contactor and the smaller one it cycles off and on based on the temperature.

Was thinking of adding a pull up resistor from 1 to 3 and changing the zener but don't understand the circuit enough to be sure. Power supply is fixed and not able to change so a component change on the board would be best.

Any suggestions or insight into what the zeners purpose is and how the actual optical isolator drives the emitter base would be super helpful in my journey.

As a note this design has been in the field for 10+ years without an issue. The system the issue occurred on had two different power sources applied, unfortunately it occurred in the field and I don't have notes suggesting either power supply was over the 28VDC unless it was AC ripple my meter didn't read.

The attached image is a single output circuit. There are 16 total broken down into groups of 8 and then commons into groups of 4.

Thanks in advance.

[magic]

OK, first regarding the drawing:
Please draw positive voltages at the top next time, the circuit around Q1 is upside down.
The 1N4005 diode seems to be drawn backwards, though it doesn't really matter for this discussion. Nevermind, I'm not sure what this diode is even doing there.

Regarding base current, it flows through the opto, into the relay and then to ground, obviously. As the output voltage reaches about 2V below input voltage, there no longer is enough input-output difference to push current through the base and opto and therefore output voltage stops rising. I guess you can verify it with a DMM.
The usual way of driving darlington switches is to have access to ground on the board and send the base current straight to ground. Then the output can rise to less than 1V below input.

The zener probably is meant to protect the optocoupler from overvoltage. Check its maximum secondary side collector-emitter voltage.

[tonlokstr]

Magic,

Thank you for your insight it has been very helpful. Apologies for the way the schematic was drawn, used OrCAD default orientations as I forgot that I could flip and change them. The 1N4005 looks correct based on the physical hardware. Not getting a lot of support from the OEM on the matter, requested the output only portion of the circuit and I have to assume by no response that they are not willing to share anything so I had to use a meter, my eyes and a flashlight to verify the connections to the best of my ability. The board is multi-layered in the logic side but not the output side from what I can see.

Looked up the data sheet for the optical isolator and it looks like it has a emitter to collector breakdown voltage of 7 min and 10 typical. Looks like i could change the 1N4750A 27VDC Zener part out to a 1N4751A (30VDC) or even a 1N4752A (33VDC) based on my 29.5 VDC input voltage.

Will mock up and do some bench testing.

Again, thanks for all of your insight.

[Ian.M]

The H11AA4 optocoupler has an output side breakdown voltage BV_CEO of 30V.  The 27V Zener is there to protect the optocoupler so if you increase its voltage the opto will probably die.   Unless you can find pin-compatible high voltage optocouplers, the whole circuit is going to need to be redesigned to limit the voltage across the optos *WITHOUT* turning on the TIP125.

[magic]

4N35 seems pin compatible, same 5.3kV isolation voltage, CTR 100% minimum and Vceo 70V.

[tonlokstr]

Ian and Magic,

Thank you so much for taking your time to help someone out, it is much appreciated and I hope at some point to be able to pay it forward to someone starting up as much as you have helped me. Best wishes in all that you do.