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solenoid driver

Hi I wanted to desig a simple solenoid driver. The solenoid's operating voltage is 6V. The resistance is 20Ohm and inductance is around 60 mH. I want to operate it at 20 Hz 50% duty cycle. The input voltage will vary from 6V to 15V. So i am plannig to use 6V zener to clip the voltage first of all. And to drive the solenoid i am plannig to have astable multivibrator and a transistor driver for driving the solenoid. In that case how should i select the zener and the series resistor. I have done some rough calculation but the wattage seems to be higher. Could you help me in selecting the zener and resistor values?

Regards, M.Nagarajan.

Reply to
naga.deer
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The voltage drop across the resistor under worst case (15V) conditions is 15-6-= 9. The resistor value must = 9/.02 = 450 Ohms. The power dissipation in the resistor will be I^2R = (.02)^2 X 450 = 0.18 Watts.

Reply to
Jon

Thank you Jon. What is it about the 0.02 in the denominator of resistance calculation. How did you arrive that value? Could you exsplain me?

I Guess you calculated the current like this 12/450 = 0.02A. But this

20 mA current will not be enough to actuate the solenoid right.

I did the calculation like this.

Mininmum Reqd to actuate the solenoid = 6/20 = 300mA

At input voltage of 7 Volts the resistor should drop only 1 Volt to drive the solenoid still at 6V. So the resistance value would be 1/0.3 = 3.33Ohms.

At input voltage of 15Volts, the power dissipation will be (15-6)*(15-6)/3.33 = 24.32watts.

Is my calculation is correct?

Regards, M.Nagarajan.

Reply to
naga.deer

No good. At 15 volts, you need to drop 9 volts across 3.33 ohms so that your solenoid gets 6 volts. That means that the zener must draw

2.7 amps. At 6 volts for the zener, that's over 16 watts it would need to dissipate. Such a zener (6 volts at over 16 watts) is unobtanium. (The resistor will need to dissipate the 24.32 watts you computed.)

I answered this elsewhere with a circuit that will work. A simple series resistor will not, and a series resistor with a zener calls for an unobtainable zener.

Ed

Reply to
ehsjr

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