# need to activate a solenoid quickly 2-10 times / sec

Discussion in 'Electronic Basics' started by [email protected], Oct 17, 2005.

1. ### Guest

I've got an application where I'm trying to get a 24vdc solenoid to
activate at high speed. It will activate 2-10 times per second.

The solenoid works, but it doesn't activate quickly enough. I believe
it needs a spike current or voltage to activate it quickly. Could
someone point me in a direction.

2. ### petrus bitbyterGuest

Apparently the solenoid has not been made to switch that fast.

You can find out what the nominal current should be. (So what current does
is take in the on state?) Then increase the voltage to the double while
limiting the current to its nominal value. This way the current through the
solenoid will raise faster and so it will switch faster too. Can't say
whether it will switch fast enough. Don't know either whether the mechanics
of that solenoid will bear this treatment on the long run.

petrus bitbyter

3. ### ChrisGuest

Hi, Jim. If you want a solenoid (or any other inductor, for that
matter) to operate quicker, try a higher voltage with a series
resistor.

As an example, let's say you've got a 24VDC solenoid whose DC
resistance is 100 ohms. To make it pull in faster, you could drive the
solenoid with a 48 volt power supply with a 100 ohm series resistor.

At the moment of turn-on, the inductor will inhibit current. This will
cause the whole 48V to appear across the inductor. This will mean the
current will ramp up quicker. But as current increases, the voltage
divider of the resistor and the DC resistance of the solenoid will
prevent problems.

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That's the answer to your question, but I don't think it's the whole
times a second (please check manufacturer's specs -- that's kind of
fast), you might have a diode across the inductor to catch the
inductive kick of the solenoid on turn-off. This diode can be very
helpful, but There Ain't No Such Thing As A Free Lunch (TAANSTAAFL).
The cost of this blessing is that current will recirculate around the
solenoid for a longer time, leading to much slower turn-off.

If you have to deal with this problem (it's fairly common in getting
data acquisition relays to turn off quickly), you might want to put a
power zener in series with your diode, like this (view in fixed font or

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This will allow the Lenz's Law voltage to increase from Vcc + 0.7V to
Vcc + 0.7V plus the zener voltage. This will help you get faster
turn-off. The energy stored in the inductor has to be dissipated.
Make sure the zener is rated for the instantaneous surge current of the
solenoid (equal to its on-state current), and that the total power
dissipation of the zener is not exceeded. For a real-world solenoid, a
1N4749 zener won't do. But since you didn't provide information on
your solenoid, zener selection will have to be left as an exercise for
the reader. If you need help, post again.

Also, make sure the transistor is rated for a V(ceo) greater than Vcc +
0.7V + Vz.

Actually, I would try the power zener before I tried bumping up Vcc and
adding a series resistor. Most solenoids as well as relays turn off
slower than they turn on anyway, and if you've got a diode across the
solenoid, the recirculating current through the diode keeping the
solenoid on is a more likely problem.

Good luck
Chris

4. ### Bob MastaGuest

You might want to check out Electronic Design, 10-13-05,
page 62: "Smart Solenoid Driver Reduces Power Loss"
by S.V. Nakhe. It adresses the problem of how to supply
a high activation current but a low holding current.

Best regards,

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

5. ### ALBERT C. GOOD JR.Guest

May I suggest that if the solenoid where to be fired with a capacitor of a
appropriate capacity and charged to a voltage higher than the rated voltage
of the solenoid.

This would raise the voltage high enough to over come the initial inductive
lag and inertia of the solenoid.

The capacitor would discharge during the cycle, at a rate that would
maintain constant current through the solenoid during its activation.

The circuit should be such that the circuit would be fired by a SCR and of
heavier than usual wiring and mounted physically close together to assure
minimal resistance in the wiring between the solenoid, capacitor and SCR.

Depending on the application, it might be possible to arrange the timing of
events in the project so that the solenoid's slow closure, would not be
critical to the operation of the success of the project.

In electronics as with many other endeavors, you don;t get something for
nothing. In this case the capacitor will have its own delays in getting
ready for the next cycle so there will be limits as to how rapidly the
combination of capacitor, solenoid, and SCR can be cycled.

Albert

6. ### ALBERT C. GOOD JR.Guest

That is understood. You had originally stated that you needed it to work
faster and the technique that I outlined before should do that, with in the
scope of one operation.

In that you indicated that you need it to operate up to 10 times a second,
it might be necessisary as one of the others had indicated, to implement
other techniques to assure that the solenoid releases rapidly enough.

If the solenoid is used or has been sitting around awhile, you should
consider making certain that it is free at the friction points of
contaminated and thickened lubricants.

If in your judgement, lubricant is required then use a good grade of clock
oil.