Hi

I want to build the circuit shown below to shut off the natural gas to my generator in the event that the engine stops running. I will use a 12 volt battery to open the normally closed solenoid valve through toggle switch SW-1. Once the generator is running its 12 volt output will keep the solenoid open and I will disconnect the 12 volt battery. What size diode would you recommend so I don't back feed the generators 12 volt output.

Thanks

 

Both power sources are connected in parellel, so voltage will be the same, but the currents are added. What 12V battery will you be using? What is the power output of the generators 12V supply?
You don't want to overload the solinoid with the currents you can get from a battery + supply.

 

The 12v battery is from a garden tractor, I don't have its capacity, but it starts a 12 horsepower motor. The generators 12 volt supply is capable of ten amps.

 

I would be more consurned with putting a resistor in series with the solinoid than including a diode.

The solinoid needs 420mA of current at 12 volts, and so using Ohm's Law and rounding to the nearest common value I get 30 ohms. Now the resistor also needs to be "big" enough for the power it will govern. So we take current of the resistor times the voltage and get 5 watts. My preferance sometimes is to multiply the wattage by 2 to as
sure lonnnnnnng life of the resistor.

 

(Referencing the circuit diagram in post #1)
I recommend a 25amp stud mount diode in series with the generator output because that will allow some heat sinking. A DC isolation diode carrying te full ten amps DC will get warm or even hot. And connect the solenoid gas valve across the generator output. You will probably need a normally open push button switch to bypass the diode for starting the engine.If the generator output is DC from diodes then that should not be a problem.

(Edited later to include what I forgot the first time.

 

How is the solenoid powered now?

You need a diode in series with the battery also, so you don't back-feed from the generator to the battery.

The diode needs to be large enough to carry the 0.42A solenoid current so a 1A diode, such as a 1N400x should suffice.

 

The solinoid needs 420mA of current at 12 volts, and so using Ohm's Law and rounding to the nearest common value I get 30 ohms.

The solenoid apparently has an internal resistance of 30Ω.
Why do you think it needs an added resistor?

 

This is my first attempt adding the valve.

I will put a diode before the battery as you suggest. BTW once the generator is running the battery is removed from the circuit.

Thanks

 

The solenoid apparently has an internal resistance of 30Ω.
Why do you think it needs an added resistor?

I never thought it needed a resistor, nor did I think it needed much more than a one amp diode.

 

I never thought it needed a resistor, nor did I think it needed much more than a one amp diode.

Thanks again for the sound advice.

 

This is my first attempt adding the valve.

Okay.
That clears up my confusion.
Sounds like you are good to go.

 

How many amps at 12 volts is the generator capable of producing?
Tractor battery is probably 12V 40Ah.
You want to hook up the battery and turn the switch on then start the generator. Doesn't the generator have its own gas solenoid?
Once the generator is running I'm assuming you intend to disconnect the battery.
Are you planning on charging the battery from the generator? If so - 12 volts will not charge the battery.
What chemistry is the battery? Lead acid? (probably). Sealed Lead Acid (SLA)? (possible)
The diode you want is to block the battery from back feeding the 12V circuit in the generator. That's a good idea. As long as the diode is capable of handling the amount of amperage needed. Since you can't charge the battery from a 12V source and all you really need is the 12V for the gas solenoid - a 1A diode such as a 1N4001 would be plenty strong. The solenoid is only drawing 420mA, less than half the capacity of the 4001.

Here's my interpretation of your drawing - without all the negative wiring shown. Instead of the negatives I've used the common ground symbol. Makes drawing easier to draw and read.

 

Forgot to ask: The solenoid; is it rated for AC or DC use? That's important.

 

"This is my first attempt adding the valve. "

Are you adding your own solenoid? If it is not rated for natural gas and/or not installed by a licensed gas fitter, you are running a huge risk of something going wrong. In fact if anything connected to that gas line/generator goes "bad" and damages anything, including your house, your insurance will not cover you any more. Insurance companies look for any excuse to get out of any settlement. Your own gas fitting would be considered "illegal" by your insurance company (and any regulator agency as well).
That all said, if you get your final setup inspected by a licensed gas fitter and they ok the whole gas setup, then you should be ok, maybe...

 

How many amps at 12 volts is the generator capable of producing?
Tractor battery is probably 12V 40Ah.
You want to hook up the battery and turn the switch on then start the generator. Doesn't the generator have its own gas solenoid?
Once the generator is running I'm assuming you intend to disconnect the battery.
Are you planning on charging the battery from the generator? If so - 12 volts will not charge the battery.
What chemistry is the battery? Lead acid? (probably). Sealed Lead Acid (SLA)? (possible)
The diode you want is to block the battery from back feeding the 12V circuit in the generator. That's a good idea. As long as the diode is capable of handling the amount of amperage needed. Since you can't charge the battery from a 12V source and all you really need is the 12V for the gas solenoid - a 1A diode such as a 1N4001 would be plenty strong. The solenoid is only drawing 420mA, less than half the capacity of the 4001.

Here's my interpretation of your drawing - without all the negative wiring shown. Instead of the negatives I've used the common ground symbol. Makes drawing easier to draw and read.

View attachment 362671

Yes the solenoid is DC. The battery is just to open the valve until the generator can power it.

 

WHY IN THE WORLD?? would the TS need a gas fitter to install a solenoid gas valve on a portable generator, considering that portable generators are ONLY USED OUTSIDE??? AND it is quite likely that the TS is entirely competent to correctly do the pipe connections.

 

The diode you want is to block the battery from back feeding the 12V circuit in the generator.

I also think a diode from the battery is a good idea, as I previously suggested, to avoid backfeeding the battery and generating sparks when he removes the battery connection.

 

I also think a diode from the battery is a good idea, as I previously suggested, to avoid backfeeding the battery and generating sparks when he removes the battery connection.

The battery is just to open the valve until the generator can power it.

I would agree man. Should I modify the drawing? Something like an OR configuration?

 

WHY IN THE WORLD?? would the TS need a gas fitter to install a solenoid gas valve on a portable generator, considering that portable generators are ONLY USED OUTSIDE??? AND it is quite likely that the TS is entirely competent to correctly do the pipe connections.

Because the TS said it was "natural gas". One has to assume he is going to tap off an existing gas line (from the house?) There are lots of regulations about that, including inspections for any changes. I was just pointing out that his home insurance "may" deny any coverage if something goes wrong because any "changes" were not inspected or done by a licensed gas fitter. It is still his choice as to what he does, but I thought I'd point out the risk, that's all....
If it was an external propane tank to an external generator, he would not be tapping into an existing natural gas line - different scenario....

 

This makes sense now. Thanks Crutschow. I'm sure the TS will appreciate it.