I'm installing a gauge to watch battery levels and auto-start a generator when batteries fall to 50%

The gauge has connections for a generator start stop - via a normally closed - N/C, normally open - N/O and common wire

N/O contacts will close to start the generator, and when batteries are charged the N/C circuit will become open. I plan to use a small relay on the N/O side to close two contacts and turn off the generator off.

The issue is that keeping the circuit closed for the off state also triggers the diesel fuel pump and I do not want that running continuously during the "off" command state.

The generator only requires that the contacts to be closed very briefly to shut the generator down and trigger the off state.

I'm looking for absolute simplicity to create a circuit which triggers the "off" function and just for a second or so.

Do folks have experience with this? I was thinking some combination of capacitor/resistor to charge a cap and then go open circuit again, but I do not want it cycling.

Thoughts appreciated.

 

A capacitor in series with the relay coil should do what you want. The value will depend on the relay voltage and current rating. The timing of the length of the pulse will not be accurate but from what you say this does not matter. You would also need to add a resistor to dischage the capacitor when the contacts opened. this would connect between the end of the capacitor that was connected to the contact and the end of the relay coil that was not connected to the capacitor. If you choose a resistor value that discharges the capacitor quickly you will also need to put a diode in parallel with the relay coil so the discharge current does not pull in the relay. You will need to provide information on the relay voltage and specifications if you want us to suggest a suitable value capacitor. How quickly do you need the capacitor to be dischaged after the contacts feeding it open ?

Les.

 

A capacitor in series with the relay coil should do what you want. The value will depend on the relay voltage and current rating. The timing of the length of the pulse will not be accurate but from what you say this does not matter. You would also need to add a resistor to dischage the capacitor when the contacts opened. this would connect between the end of the capacitor that was connected to the contact and the end of the relay coil that was not connected to the capacitor. If you choose a resistor value that discharges the capacitor quickly you will also need to put a diode in parallel with the relay coil so the discharge current does not pull in the relay. You will need to provide information on the relay voltage and specifications if you want us to suggest a suitable value capacitor. How quickly do you need the capacitor to be dischaged after the contacts feeding it open ?

Les.

Thanks for this Les.

Simplicity is my design goal. Everything is 12V and I intend using a regular automotive type relay similar to the one in the photo, so the current to fire the relay should be quite small.

As for the discharge time a bleeder resistor which discharges the cap in several seconds would be entirely sufficient.

If you had time for a quick sketch that would be appreciated, but from what you are saying, it seems a cap wired in series with the 12v to the relay coil would conduct until charged and then appear as an open circuit so the relay would stop functioning and the contacts would open again correct?

In the event a bleeder resistor passes the charge to ground, surely the capacitor would begin charging again once discharged, if the N/C circuit is still open?

I need to experiment a little and check how long the signal for the changed state is.

Conceivably if it cycles again and again it would t be too much of an issue unless I am trying to start the generator manually and the circuit is cycling reign to turn the Hensey off?

The other scenario is that I also have another existing circuit which auto starts the genset in the even of mains power failure and would not want to interfere with that by a circuit which is continually cycling to force off.

 

For my suggestion I was thinking of a small relay such as this one. It has a 720 ohm coil so a capacitor of 1000 to 2000 uF would be a good starting value to try. I considder your relay quite large. Looking on ebay for that sort of relay I could not find any adverts that gave the coil resistance (Or current.) So I have no idea of what size capacitor would be required. (I think it would be at least 10000 uF.)


The resistor would be drawing current for all the time. I misread you original post. When you said that you would use the NO contact to trigger the stop relay I was thinking you meant the closeing of these contacts would trigger the stop pulse. For the above circuit to work you would have to use the NC contacts.

Les.

 

First a question, which is "what does the relay to stop the engine actually do? What I think you need is a relay to switch the fuel pump ON when the engine needs to run, and then for the relay to be kept on by the output from the generator. Then, when the engine is to be stopped, the relay powering the fuel pump would be released.