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Solenoid PWM Duty Cycle and Other Questions

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nodee

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Hi All,

Im hoping to use a simple 555 circuit with mosfet (low side) output to drive a solenoid, Im confused over the specs of the solenoid i will need. Am I right to think that I can replace a 24V 12W solenoid with a 6V 3W on a 25% duty cycle at 24V?

The main trouble with the 24V 12W is the heat it generates when continuously on, would the pwm solution run hotter/cooler? Also what is a ideal frequency for the solenoid pwm signal, I'm trying for 20kHz and up to avoid being able to hear it is this the right idea?

Thanks,
Nodee
 

FvM

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You'll need a freewheel diode to actually save power, for 20 kHz a schottky diode would be best. The minimum pwm frequency is given by the requirement to keep the current almost constant, a few kHz will be usually sufficient and doesn't create much noise, switching losses will be slightly lower than at 20 kHz.

If you intend to drive the solenoid at reduced voltage, you should bypass the PWM circuit for a few 10 ms until the solenoid is fully engaged.
 

nodee

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Im hoping to drive the 6V 3W solenoid at 24V on a 25% duty cycle, to achieve the same effect of the 24V solenoid on constantly, this is a correct assumption?

Ill drop the frequency and give it a go I guess, the freewheel diode would be across the solenoid facing the supply rail yes? Is it necessary to place a diode across the MOSFET also?

Thanks,
nodee
 

FvM

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Ideally, you can multiply the supply voltage with the pwm factor, so in fact, 25% of 24V gives 6V average voltage. The freewheel diode is across the coil, no additional diodes are needed. The circuit is acting as a buck converter with the coil inductance as storage inductor.
 

andre_teprom

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...The freewheel diode is across the coil, no additional diodes are needed. The circuit is acting as a buck converter with the coil inductance as storage inductor.

Maybe the wide distance between solenoid and switching circuitry at PCB may disseminate a large amount of ESR noise to enviroment around.
Special care must be take at solenoid cabling ( shielding / twisting ) if that solution will be accomplished.

+++
 

elactis

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Hi Nodee,
You need to make a circuit little differently. A 6V 3W coil run at 24V will heat like hell (You will have 48W). At 25% duty cycle it will give 12W as the original coil. Normally, once the solenoid has switched, you can decrease the power consumption by 75 - 90 % and eliminate the heat. The easiest is to use the 24V coil and after the switching (say 100-200 ms) you apply a 50% PWM. Do not forget the freewheeling diode. If you wish, I can send you such a board.
 
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