Is there a better alternative for PWMing a solenoid valve at about 20 Hz than a SG3525? To get this low frequency with a SG3525, the timimg cap needs to be an order of magnatude greater than the max. recommended value. Is this a stress issue on the discharge circuit?
I think the main concern with large timing capacitors is the energy dumped into the discharge transistors. You should be able to add extra resistance between the discharge pin 7 and the capacitor measuring pin 5 to limit the energy dumped into this transistor each cycle.
But there may be a solenoid driver chip that includes the power switch transistor.
I'd consider using a CMOS PWM chip, which would allow using higher resistor values. BTW, what's wrong with a higher-frequency PWM for the solenoid?
It likely has a very lossy core. 100Hz may work but not 100KHz.
But the a.c. flux will be low, so losses shouldn't be much.
For strong pull-in force then hold-in at a reduced power, open- looping these things (initially on, followed by a fixed duty cycle PWM) is often good enough.
Cheers, James Arthur
The solenoid does not respond to PWM signals at higher frequencies. With increasing frequency, the response time becomes an increasing portion of the on - off cycle until it just turns on or off. The intellegence is in the duty cycle, current limiting or conservation is not a factor. In other words, the the solenoid has to bang on and off in relative proportion to the duty cycle.
Is it true, the long term average power through the discharge circuit may not be much different at 20 Hz than from a proper size cap discharging at a rate magnatudes higher? If not, tell my old brain why? Adding a discharge resister causes an unwanted anomaly on the output pulse train, unless it is a few ohms.
Regards,
BobS
Have you estimated how long your valve will last, mechanically, if it continuously opens and closes 20 times a second?
robert
It won't be too bad if the valve is designed for that service. The valves in an engine going at 3000 RPM will operate at a 3000 / (2 *
60) = 25Hz rate. The valves in an engine used continuously last for years. The valves that control the water flow into an automatic washing machine wouldn't last very long doing this.
The solenoid's plunger won't move, but its coil will still integrate the effects of any applied voltage, creating a holding force.
When the plunger is out of the coil, the poorer magnetic circuit means it takes a lot of juice to exert a given mechanical pull-in force on that plunger. Once the plunger has moved within the coil, more force is produced with less current. To save power (or heat in the coil), then you can start chopping.
I've seen this done with relays, which also combine coils and a magnetic circuit with a gap that closes.
I don't understand. It almost sounds like you're trying to get proportional motion from the solenoid, which it won't easily do-- that's a linear motor.
Discharge resistor? Please elaborate.
Cheers, James
timimg
The end result is proportional pressure from the valve, which is made to operate in the 15 - 20 Hz range. The valve is in a closed loop so it does not need to be too accurate.
The discharge resistor can be used in the SG3525 frequency oscillator timing circuit. To obtain 20 Hz the timing capacitor can be larger than the max recomended value. A discharge resistor is used to limit the current though the discharge circuit.
20
timimg
circuit?
Ah, the timing resistor. Gotcha.
Rather than fret about SG3525's guts, why not roll your own controller from an LM339 or such? Easy at these frequencies, & you'd have a section or two left over.
E.g.: (oscillator to produce ramp)-->(threshold comparator) = pulse width modulator, with another section for the error amp.
Then again, you could always use a PIC...
Cheers, James Arthur
You may be able to do all that is needed with just two LM339 or other comparitors. At these frequencies (20Hz) an LM324 would serve as a comparitor.
The error amplifier stage is as per normal. The oscillator stage needs to be a bit funny.
R1 10*R1 1*R1 Error sig ----/\\/\\----+-------/\\/\\-----+---/\\/\\/----Vcc ! ! --!+\\ ! ! >--------+-+--- Out GND--!!------+--!-/ ! C1 ! ! Vcc--/\\/\\----+----/\\/\\------ R3(huge) R2
C1 and R2 have to be larger than normal. At the two extremes the frequency decreases.
The R3 ensures that the Error Sig can completely stop the output. It just has to overcome the biggest offset voltage you can see. An R4(huge) to Vcc could also be added to make the duty cycle go to 100%.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.