Hey guys, I have a 12v negative triggered solenoid that I need to power.

When the solenoid receives the ground signal I need it to go to full voltage for 2 seconds and then drop down to 400hz 60% duty cycle until the ground signal is removed to avoid burning up the solenoid.

What would be the best way to accomplish this?

 

Does it have to be a PWM signal or could you just use a series resistor to reduce the current?

 

Do you mean that you want to apply 12Vdc to the solenoid for the first two seconds, and then reduce the current through the solenoid to 60% of the initial current?

All you have is one input? What is it? A switch contact to ground?

 

Do you mean that you want to apply 12Vdc to the solenoid for the first two seconds, and then reduce the current through the solenoid to 60% of the initial current?

All you have is one input? What is it? A switch contact to ground?

Correct on both accounts.

 

What is the DC resistance of the solenoid?

I'm guessing less than 10Ω?

 

8Ω

 

Where does the 2 seconds come from, is this just an arbitrary figure? e.g could it be 1 second or 0.5 second?

 

Here is how I would do this:

The simulated "switch to ground" closes at 0.1s, and opens at 4.1s.



This shows a close up of the solenoid current during PWM. Note the freq, duty cycle, and average current... I used 6Ω as the solenoid resistance, so the current initially is 2A. 1.27/2 is 63% during PWM.

 

If all you want is to ensure that the solenoid receives full power to fire initially and then reduced power to hold it in place without burning out; you could try fitting an 8 Ohm 10W resistor paralleled with a large >=10,000uF electrolytic capacitor, in series with the solenoid.

 

If all you want is to ensure that the solenoid receives full power to fire initially and then reduced power to hold it in place without burning out; you could try fitting an 8 Ohm 10W resistor paralleled with a large >=10,000uF electrolytic capacitor, in series with the solenoid.

It will take a huge capacitor, and then the current profile is still an RC time constant, where it spends very little time near the peak current. Also, R1 has to be a 10W resistor (also huge).

 

This is presumably a AC solenoid? why can't you use a DC solenoid and avoid the burn out problems, an AC solenoid should normally never see a reduced voltage.
Max.

 

Yes, I believe I already said the resistor should be rated at 10W, which I would not consider "Huge", Nor would the capacitor be "Huge" due to its low voltage rating.

It's just a suggestion, something the OP could try if he just wants a simple solution rather than an electronics project.