I have a couple 'counter boards' that consists of multiple logic ICs. It's function is to simply count products coming off an industrial machine & reset once the predetermined count is reached.

The input voltage is 15V and output voltage is around 13.5V without a load.

Both boards have the same issue - once a 12V relay is applied, the output drops significantly to 1.9V & will relay will not engage. I have tried putting a 10uf cap in parallel with the output, as well as using a solid state relay to minimize the current draw but no difference.

I am stumped. I am willing to do a modification to the board but not sure where to start.

Any suggestions? (I have attached the schematic, sorry it's scanned)

* The output the relay is connected to is Pin 9 which isn't shown well in the schematic but it is on the far right side of the schematic connected in series with R10 - 39ohms.

 

What is the current rating of the power supply feeding the board ? Does this power supply supply the power to drive the relay coil or does the output of the counter board drive a transistor or mosfet that switches the current throughthe relay ? What is the current rating or the relay coil or it's resistance resistance ? I am guesseing that relay is driven from the right hand end of R9 or R 10. Is this guess correct ? Looking at the data sheet for the SN75492 it can only sink current. It cannot source current.

Les.

 

Welcome to AAC!

Relevant portion of the schematic:

How is the relay connected? What is the coil resistance? Where are you measuring the voltage? At the output of the darlington or on the other side of R10?

 

What is the current rating of the power supply feeding the board ? Does this power supply supply the power to drive the relay coil or does the output of the counter board drive a transistor or mosfet that switches the current throughthe relay ? What is the current rating or the relay coil or it's resistance resistance ? I am guesseing that relay is driven from the right hand end of R9 or R 10. Is this guess correct ? Looking at the data sheet for the SN75492 it can only sink current. It cannot source current.

Les.

Hi Les,

Thanks so much for the reply.

The power supply has 3A rating.The board itself draws 230mA. The coil resistance on the relay is 230ohms (52mA). I have also tried a solid state relay that only drew 14mA and the exact same issue occurred - the voltage dropped from 13.5V to 1.9V.

The output of the counter board does not drive a MOSFET or transistor to switch the current. Would a transistor at the output fix this?

And yes, you are correct, the relay itself is driven right from R10.

 

Welcome to AAC!

Relevant portion of the schematic:
View attachment 138425
How is the relay connected? What is the coil resistance? Where are you measuring the voltage? At the output of the darlington or on the other side of R10?

Hi Dennis,

Thanks so much for the cropped version of my schematic. I would've done the same but my printer/scanner crapped out last week.

The relay is connected after R10. I am measuring the voltage on the other side of R10.

The coil resistance is 230 ohms (12V relay).

Thank you for your reply

Anna

 

Try connecting the relay between the output and +15V.

 

I agree with MrChips. To get 1.9V at the end of R10 implies the relay coil was between there and ground when you took the measurement.
Do you have a spike-suppression diode across the relay coil?

 

I agree with MrChips. To get 1.9V at the end of R10 implies the relay coil was between there and ground when you took the measurement.
Do you have a spike-suppression diode across the relay coil?

Thanks so much for your input.

I tried a 12V zener at the output but did not try it across the coil.

I just tried what Mr.Chips suggested, and the coil did not engage. Would that be because the voltage difference between the output and the 15V supply is only 2.5V in the difference? Sorry if that's a stupid question...I may be over complicating this.

 

Looking at the waveform on the schematic the signal will only go low when the required count is reached. So with the relay connected between +15 volts and the output it sould only pull in when the required count is reached.

Les.

 

Looking at the waveform on the schematic the signal will only go low when the required count is reached. So with the relay connected between +15 volts and the output it sould only pull in when the required count is reached.

Les.

YES. Of course!!! I completely disregarded the low input pulse for the count in.

The relay engaged once predetermined count was reached and output only dropped to 12.5V!!

Thank you SO much everyone!!! AAC rocks.

 

It looks like an SN75492 drives the relay coil. (though the coil not being in the schematic makes me uncertain of this). I was only able to find a very poor image of the datasheet but it looks like that chip can drive 20 mA. If true, then 50 mA is too much and it could fail. Also, if connecting the coil between output and ground, why didn't it turn on immediately? (it could be that the chip doesn't source much at all but sinks more. though the drop on the PSU output doesn't make sense, then).

I would probably use a driver transistor circuit for safety's sake. The board looks old and probably no longer in production. Would suck to loose it.

 

Hi philba,
This data sheet gives the maximum output current (Sink) at 250 mA. It gives the collector emitter voltage for the on state witha test current of 50 mA.

Les