Driving a large 12V load

DavidGreg

Hello All-

I'm interested in controlling a fairly hefty solenoid with a propeller. Its a 12V coil, nominal resistance of 1.5 ohms, approximately 8-10 amps at full voltage.

I was hoping to use the full voltage to get the solenoid moving, and then reduce the supplied power via PWM in order to cut down on the power required.

I confess a fair amount if ignorance on issues like fly-back voltage. I was considering the VB921ZVFI/VB921ZVSP based on its use in a DIY automotive kit I've built in the past.

Is there a Solid state solution to what I want to do? or is this the realm of relays only. I would welcome any comments suggestions.

Peter Jakacki

The solenoid is easily handled by most mosfets and driven directly from the Prop. There is a good mosfet to use that can be driven directly from the 3.3V output of the Prop but I will have to dig up the number, maybe someone can mention it. The part that you specified is not suitable for your solenoid use.

The switching frequency for the solenoid can be quite low and in the region of 100 to 1000 Hz so you don't need special circuitry to drive it.

*Peter*

Phil Pilgrim (PhiPi)

The IRF3708 is characterized for a V_GS of 2.8 volts (R_DSon = 0.029 ohms) and should handle the required current at the Prop's available gate drive voltage. However, to PWM it at a reasonably high frequency, you may still need a MOSFET driver to handle the current requirements of a rapid gate charge/discharge cycle. As Peter suggests, though, 100 - 1000Hz should be no problem, assuming the solenoid's inductance is high enough to keep it from chattering or "singing" at those frequencies.

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 9/23/2009 3:31:00 AM GMT

pmrobert

I'm familiar with your previous automotive DIY project. The VB921 is not available anymore, they're using the Bosch BIP373 for the coil drivers now. Impressive little device and very difficult to kill. PM me if you need a source for single quantity, I believe there's a minimum order of 5K or some such through the official suppliers.

DavidGreg

Phil Pilgrim (PhiPi) said...
The IRF3708 is characterized for a V_GS of 2.8 volts (R_DSon = 0.029 ohms) and should handle the required current at the Prop's available gate drive voltage. However, to PWM it at a reasonably high frequency, you may still need a MOSFET driver to handle the current requirements of a rapid gate charge/discharge cycle. As Peter suggests, though, 100 - 1000Hz should be no problem, assuming the solenoid's inductance is high enough to keep it from chattering or "singing" at those frequencies.

-Phil

Thanks Phil! These guys are working great. I think I've settled on 1000Hz/65% duty cycle. They do sing if the duty cycle is set too low - I wasn't sure what you meant by that before I heard it. Also, the 3708s seem to get much hotter at PWM frequencies. 1000 is not too bad though.

Thanks Again!

-David

Toby Seckshund

Most solenoids will be better off with a dual current drive. One that comes on at full whack for a short time to get the actuator moving and once the thing is virtually fully closed the second, lower holding current, will be sustained (the attraction force is inversly proportional to the distance). This will get the thing closed and then hold with minimum current and heat. AC solenoids rely on reactance to cut down on the heating, and so do not like DC.

1KHz tone will get annoying, very quickly. We use it to get rid of people that dont get the hint and leave us alone whilst working.

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Rayman

You can see what I did here:

http://forums.parallax.com/showthread.php?p=687346





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My Prop Info&Apps: ·http://www.rayslogic.com/propeller/propeller.htm