I've been searching and reading about various circuits to drive solenoids, but still have a couple questions.
The solenoids I want to drive are on a vehicle, so 12vDC (~14v with engine running). They are rated at 1.6 amps, but obviously the driving circuit should be oversized. Also, I want to drive them with a PWM signal. These solenoids control a hydraulic valve that will steer the vehicle, so only one will be on at a time.
Initially I wanted to use some solid state relays, but it appears they don't do switching fast enough for PWM to really work right.
I have since started looking into the transistor circuits, such as:
Using a common ground between the arduino circuit and the solenoids is fine for this application.
I see that the TIP120 (rated at 5A) should be sufficient for the load I have, but will need to have a decent heat sink attached to it.
What I'm curious about is the protection diode - how many amps does that need to be rated at?
Because that is exactly what they are designed for. It is a proportional hydraulic valve, so the stronger the magnetic field, the more open the valve is, and the more oil it flows.
I would use one that was at least twice the rating of the coil.
As it is only pulse current rating that needs to be this big a diode with a smaller continuous current rating could be used, see the data sheet of the diode you want to use.
I'm using a TIP120 w/ a 1N4001 diode to drive the pwm idle valve on my car. I'm also using an IRLZ44 to drive the purge control solenoid, with a 1N4002 diode. This is commonly used for boost control solenoids:
Both components are mounted on a heatsink, but since neither run at 100% dc they don't get terribly hot. In fact, the TIP120 isn't actually mounted on a heatsink right now....
Thanks for the insight, guys. I connected the solenoid up to a test circuit today, controlled by a TIP120, with a 1N4004 parallel to the solenoid. After a minute of use, the TIP120 was warm to the touch (no heat sink), and the diode was still room temp. At 100% PWM duty cycle, I was seeing 1 volt of drop across the TIP120. At ~1.5 amps, it was creating 1.5 watts of heat.
I plan to replace the TIP120 with a FDP6030BL mosfet, which has a RDSon of 0.018 ohms. I think that should allow me to power the solenoid without a heat sink. At a typical 1.5 amps, the mosfet should produce 0.04 watts of heat. Even at a worse case 5 amp load, it would only have to dissipate 0.45 watts of heat.
I'm having trouble finding a diode with a 5+ amp rating. Any recommendations?