simple 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
nag
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Hi, Nag. Your basic question concerns what is called a shunt regulator. You're saying you want to make this (view in fixed font or M$ Notepad):

| ___ | o-|___|-o--------o | | | Vin /-/ Vout | 6 - 15VDC ^ 6V, 0 - 0.3A | | | o-------o--------o | (created by AACircuit v1.28.5 beta 02/06/05

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and want to know a value for the R and a component choice for the diode.

Your problem can't be solved. Any resistance R will drop voltage when your solenoid is on, making an output voltage of 6V impossible for an input voltage of 6V when your solenoid is on. And if you choose R = 0, when your input voltage exceeds 6V, there will be no limiting of the current going through the zener, which will burn it out.

20Hz service for your solenoid is going to require at least nominal voltage for your solenoid. That would make a low dropout series pass regulator a less than ideal choice for your project, too.

Not only that, but the inductive energy which will need to be dissipated when your solenoid turns off makes using a step-up switching power supply a little problematic, too.

Your power supply seems to be the hang-up here. Is it possible you might be able to find a power supply that will provide a somewhat higher minimum input voltage?

Chris

Reply to
Chris

To use a zener in this application, you need to do this :

NPN

  • -----+----- -------+ | c\ /e | [R] --- | | | | +-------+ [your circuit] | | | [Zd] [C] | | | | Gnd ---+-------+---------+

Use an NPN power transistor on a heatsink.

Make R 100 ohms. Use a 6.8v 1 watt zener. (The transistor will drop the voltage a bit) If you have a lot of ripple on the input you can use a fairly large cap from base to ground as shown in the diagram.

This won't be perfect, because your input voltage is too low at 6 volts to allow stiff regulation. But for a 300 mA solenoid, it will be ok. The fact that the input voltage drops to 6 volts precludes using a shunt regulator.

Ed

Reply to
ehsjr

Thank You very much John and Ed.

John, If my operating region is from 8 to 15 volts means my problem could be solved some what right. In that case what would be the value of the resisor and Zener.

Ed, Is it possible to drive this series transitor itself on and off at

20Hz? if so how should i give base signal to this transistor along with this voltage regulation circuit?

But i have seen some circuit which drives the 2 coil solenoid from 6 to

15 volts range with the limited current. I heard that the second coil is used especially for the current limiting purpose only. Do you have any idea on that? if so could you expalin those? i will try to get the coil deatils also

Regards, M.Nagarajan.

Reply to
nag

The transistor is not being used as a switch. It regulates by acting as a variable resistance, so you don't want to drive it on and off.

Use a separate circuit to switch the solenoid on/off - something like this:

+6 ---+---[Solenoid]---+-------+ | | | +------|

I don't know what circuit you have in mind, so I can't comment on it.

Ed

Reply to
ehsjr

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