I have this circuit that works when I hook up a 12vdc led, but when I substitute a 9vdc solenoid it won't engage. This is a Normally Closed valve. I'm using a IRF540 mosfet. I want the solenoid to stay closed when the switch is closed and open when the switch is open.

 

You can't expect to operate a 12V solenoid with 9V.

Next time, just use a pencil/pen and draw your schematic on paper instead of trying to make it with ASCII characters.

And drive the solenoid properly. You should use the transistor to drive low side or high side. You wouldn't need the 100 ohm resistor (which could be causing problems depending on the coil resistance).

 

Something like this -



Switch from R1/R2 junction to +12/+9. Switch closed solenoid powered.

Make sure the solenoid coil R is limited and does not exceed ratings of
MOSFET. Use low RDSon MOSFET. Calc MOSFET power as IxIxRsolenoid,
to figure out of you need a heatsink. I = the solenoid current.

R1 ~ 100 ohms, R2 10K ohms should do.

Switch from"mcu" label point to +9.

Regards, Dana

 

Thanks for your responses. Just a point of clarification. I am trying to use a 9vdc battery to switch a 9vdc solenoid, not a 12vdc solenoid. So with the changes I should be able to turn off and on the 9vdc solenoid with the 9vdc battery ? I need the solenoid to stay closed when the switch is closed and the solenoid needs to open when the switch is open. This is a normally closed solenoid. Thanks again.

 

Do you know how much current the solenoid draws? Usually solenoids are quite greedy, so if yours draws appreciable current then a little 9V battery won't last long.

 

Do you know how much current the solenoid draws? Usually solenoids are quite greedy, so if yours draws appreciable current then a little 9V battery won't last long.

I checked their website and it says it is a Normally Closed, 9v dc, Power 6 watts. Using the formula Amps = Watts/Volts I get .66 amps. I'm using a 9v 300 mAh NI-MH battery. So if I'm correct this battery provides .3 amps. Maybe I need to hook two of them in series. Thanks for asking the question.

 

Followup. I also tried an Energizer 9v Lithium 1000ma battery. Both batteries will energize the solenoid connected directly to the solenoid. I'll try the circuit above also.

 

I'm using a 9v 300 mAh NI-MH battery. So if I'm correct this battery provides .3 amps.

In theory it will provide 0.3A for one hour, or 0.66A for 27 minutes. In practice, at such a high current, the battery life will be a good bit less than the theoretical value.

 

I'm using a 9v 300 mAh NI-MH battery. So if I'm correct this battery provides .3 amps.

300 mAh tells you the energy capacity of the battery, but that's typically for a 10 or 20 hour current draw, or longer.
At higher currents it will deliver less energy.
For that you need to look at the battery data sheet.