Why is PWM needed for proportional control solenoids?
Why is PWM needed for proportional control solenoids?
(OP)
Probably a very simple question but I can't find an answer. For current controlled proportional solenoids such as those used on hydraulic directional control valves, why is a PWM signal needed? Why can't I use a variable resistor in series?





RE: Why is PWM needed for proportional control solenoids?
If it was a big solenoid, then the variable resistor might have to be huge to dissipate the power safely.
RE: Why is PWM needed for proportional control solenoids?
12VDC
Coil resistance 3.1 Ohms
Current from 500mA to 2500mA.
Does that seem like it would cause heat issues?
RE: Why is PWM needed for proportional control solenoids?
Beware that the "12VDC" is actually likely to be a vehicle voltage that might be more like 14 volts. That is enough of a difference that it's worth using 14 instead of 12.
So 7 volts (one half of 14) over the resistor set to 3.1 Ohms is about 16 watts. 20 watts is the nearest common power rating, but a bit too close for my liking. So you might need to use a 50 watt variable resistor, if you can't find a 30 watt.
I'm not sure how easy it would be to find a variable power resistor with these sorts of values for resistance and power.
Also, you might find the taper of the resistance curve to be unfeasible. In other words, all the action might end up happening within a very small range crammed into one end of the total range of motion.
Also, one must consider that the power dissipation in the worst case might also be confined to a small section of the total variable resistor. In other words, 16 watts of heat crammed into one small end of the resistor.
One wouldn't want to simply install a variable resistor without carefully analyzing it at all the critical settings along the adjustment range.
You can probably start to see why PWM might actually be easier.
RE: Why is PWM needed for proportional control solenoids?
Bill
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"Why not the best?"
Jimmy Carter
RE: Why is PWM needed for proportional control solenoids?
RE: Why is PWM needed for proportional control solenoids?
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Why is PWM needed for proportional control solenoids?
Yes, there is a distinct vibration when I put my screwdriver in the middle of the coil.
RE: Why is PWM needed for proportional control solenoids?
Now with pulse width modulation there is something else in the background: Current control. Fewer turns with heavier gage wire so it does not get hot, and more current giving the same Force to the Solenoid. The current is controlled and the Power Dissipation is much lower. As the pulse is applied current in the coil rises and with pulse width modulation when the pulse is turned off the current falls but only by a small amount before the next pulse comes along. Thus applying an average current of whatever the solenoid requires to maintain its position with some amount of ripple. A transistor is used as a switch to turn the current on and off and the Power Dissipation in it is very small. This transistor acts like the variable resistor in series with the coil.