Freewheeling Diode Across an Inductive Coil

That's what the dioade is supposed to do! Like the free wheel on a bicycle...
It makes use of the otherwise wasted energy allowing the current to keep flowing through the coil. If your application was a vehicle drive you would be getting more range out of you battery by utilising that current.
In your application a capacitor or capacitor /resitor network will be better (across the switch contacts or the coil)

There may well be better answers as I am just a cat
Del

When the supply is turned off, the magnetic field starts to collapse. The effect is to try to sustain the current that had been flowing through it. In the absence of the freewheeling diode, the electons "pile up" at the contacts of the switch that opened, and jump across the gap as a spark. Adding the diode means that they can chase round the loop through the diode until the magnetic field is gone instead, and sparks don't happen; adding a resistance in series will have little effect as the energy is already dissipating in the resistance of the coil winding.

The air released doesn't actually increase in quantity, though it might be happening a split-second later than it would without the diode. Why is this an issue here, please?

Thanks.

Energy stored in the inductive coil of solenoid valve continue to dissipate in its own coil resistance via freewheeling diode resulting magnetic field to reduce gradually, this gradual reduction in magnetic field does not allow to release the plunger immediately making operation of solenoid valve little sluggish.

is there any way reduce this sluggishness of valve.

thanks

Divyesh patel

Again, why is this an issue?

There are solutions, though some context would help. Remember, CR4 readers cannot see the installation as well as the Original Poster.

Red Fred is correct. The diode does nothing on the contact (control) side of the circuit. It only protects the relay driver from the back EMF caused by the collapsing field in the relay coil. If the contacts are welding together, all you need is a suitable capacitor across the contacts. (Just like the old condensers on vintage auto distributors) As redfred points out, you will need an O'scope to evaluate where the delay is occurring. It could be the relay control circuitry, hysteresis in relay output, or even lag in the valve itself.

Thanks but this is not a relay with contacts being driven but a solenoid that opens some air valve that affects a brake. I'm glad you recognized my point that an oscilloscope will identify if the electronics or magnetic field collapse is causing the delay.

Your problem, most likely is higher mechanical friction of the solenoid plunger, in the bush. The diode across the coil, has nothing to do with it. If you have a skilled fitter, ask him to dismantle the valve, throughly clean it, assemble it with a light smear of lubrication at bush.

...and once that has been done, consider installing pneumatic quick-release valve(s) [QRV] on the air motor(s) to save the valve chest(s) needing to vent back through the air supply line(s). If noise is a problem, add a silencer to each QRV made from a sintered material; brass and plastic are widely available and other materials may be as well, depending on the environment and the ambient conditions prevailing.

_{This is why some context as to why the valve colsure time is an issue is so valuable in determining what to recommend.}

Note these other factors that can cause a failure to close when de-energized:

or page 7 of this document:

http://www.westmillindustries.com/files/Asco%20Solenoid%20Valve%20Trouble%20Shoot%20Manual.pdf

The method some of the DC-coil contactor manufacturers use when requiring fast turn off is to use a resistor, not a diode to attenuate back EMF from the coil. It does result in more current but if that doesn't bother you then it a reasonable solution. Resistor snubbed contactors generally draw about 30% to 50% more current so I would suggest a resistor value of coil resistance times about 1.3. Make sure it has an adequate power rating.

Please clarify your question: Do you de-energize the valve to release air, or do you energize valve to release air?

Is this perhaps a blow down solenoid for an air compressor,operating on a timer?If so, make certain that the valve is installed in the proper direction of flow.When properly installed, the pressure will assist in closing the valve.If improperly installed, the pressure will resist the internal spring of the valve, making it close slowly. Reverse the valve and see if it makes a difference.Most, but not all valves have a direction arrow.

I presume this valve has 3 ports..1 supply, and one normally open and one normally closed.It will work if piped wrong in some applications, but it will not operate quickly.

If this is a 2 port valve,same thing applies.It is made to work with flow in one direction.The incoming air normally goes through an orifice,the exhaust is unrestricted.

See how this could give the trouble you describe?

The flyback voltage into the free wheeling diode is so fast that there will be no noticeable delay in the operation of the valve, so that is not a problem.I have seen relays operating thousands of times per second with these diodes on them.Leave it.

It serves a good purpose.

Please give more info and feedback.

Thanks

Hope this helps.

The entire circuit is like this ;

this is basically electro-pneumatic brake control, where application solenoid valve release the air to the brake drum till desired level when voltage is applied to it and holding solenoid valve maintain that pressure in the drum.

turn on command to the application solenoid is through software and turn off is though feedback of pressure sensor i.e solenoid dis-energized whenever demanded pressure reaches in the drum.

When there was not freewheeling path connected, it was observed that, pressure in the drum reaches exactly demanded and functioning perfect of course spikes across the static contact observed.

now when freewheeling path introduced in the circuit, voltage spikes removed but pressure in the drum reaches little above than demanded and due to that some corrective action needs to be taken to remove that extra pressure by operating holding valve unnecessarily which is not desirable/acceptable.

coil inductance is approximately 4 H and having 500 ohm resistance.

regards

Have you tried adjusting the set point on the pressure switch?

Ok that's most of the useful information _{^{(not the entire circuit)}} which allows for some preliminary analysis. A 4 H coil with 500 ohms of series resistance (that's a big coil) has a time constant of 4H/250Ω=16 msec. Even when one considers the five time constants (80 msec) to drop the coil field to less than 1% of the original field one could easily think that this is sufficiently fast to close your valve. Yet it clearly is not fast enough. Why?

Remember that the solenoid is actually moving a small pin a very short distance of possibly no more than a few millimeters of travel. So a 16 msec time constant may mean that this pin is gradually being released during this travel time. The pin appears to not be reaching its designed closing velocity when seating and thus not quickly sealing.

Also my earlier 10 ohm resistor suggestion would clearly not make a significant change in time constants. It would allow an oscilloscope to document the field collapse time. So my new resistance suggestion is to try a 2.5 Kohm resistor in series with the diode. This will mean a field collapse time constant in just under 1.5 msec. Now if you had identified the current drawn, ON voltage applied or solenoid activated valve part number then the voltage spike at the solenoid could be calculated. Identifying the solenoid driving circuitry could then verify if this spike was trivial or potentially damaging.

This still does not preclude that something mechanical (debris in valve, wrong flow direction, low air pressure, etc.) may be the actual root of the problem. Regardless of the free wheeling diode condition it is obvious to me that a marginal change in field collapse time is making your system unusable. If nothing is found to be failing then the system as designed will be marginal at best.

It sounds like you are using a very brief pulse and due to the diode maintaining current flow briefly as it dissipates the stored energy, you are not able to achieve the short open time you desire.

I suggest you add a resistor in series with the diode and increase the voltage rating on the transistor (or implement a high voltage transistor switch if you are using some other kind of driver. If you need to you could even eliminate the diode all together, but I wouldn't if you can find a happy medium.

The usual reason for using the diode is to prevent the high voltage spike that occurs from damaging the driving circuit. If that is a problem you may just need to increase the voltage tolerance of your driving circuit. It's not the cheapest answer usually but it should solve your problem as long as you use a big enough transistor so that if you were to turn it right back on in the middle of the voltage spike you don't experience second breakdown failure on the transistor which is a limitation on the amount of simultaneous voltage and current a transistor can tolerate.

We had similar problem in D.C brakes fitted with Electric Hoist. Due to residual magnetism in the circuit, the brakes remained un engaged and it resulted in load slipping even after hoist was stopped. We solved this problem by breaking the circuit by incorporating additional air break electrical contactor in the circuit. This stopped flow of the residual magnetism and engaged the brakes instantly.

What is the supply pressure?

What brand and model number of solenoid valve?

What is the air volume of the brake?

What is the length of the lines from the brake to the solenoid exhaust port?

What is the diameter of the lines?

What is the part number of the diode?

A schematic would be helpful also.

What is the critical timing for the brake?(How long is desired for operation and stabilization?)

What is the duty cycle of the brake and solenoid?

This info is critical to provide a more critical analysis of the problem.

The diode is inserted in the circuit across the coil so that it is reverse bias during normal operation. When the power is applied or removed from the circuit the sudden change of current flow causes a reverse current to flow that can cause several problems. This is the same action that is seen from an auto-transformer and an example is the coil used in the ignition of older gas powered cars. One of the problems is relay chatter caused by the ringing or freewheeling current when the power is applied or removed. Another problem is that depending on the size of the coil the reverse current can cause a voltage several times the applied voltage which can damage switches or semiconductors used to control the device. The diode selected must be of sufficient specifications to handle the resultant reverse current and able to withstand the voltages applied and induced. I would not recommend a zener diode for this application since a zener diode is designed to breakdown at a predetermined voltage and operate in the avalanche zone with current through the diode controlled by a series resistance.

Here is a link that will allow you to calculate the proper value for a snubber circuit according to the information you have.Good Luck.

www.hiquel.com/fileadmin/userfiles/AppNotes/Englisch/HIQUEL_Snubber_AppNote_EN_0100.pdf

J agree with Dell, put a capacitor or capacitor resistor combination across the relay contacts, forget about the free wheeling diode it does not help in your situation.

Please report after this trial.

If the turn off time was acceptable without the diode but you want to protect the contact, I suggest you use a transitor to drive the valve, and a resistor across the valve coil. Use a resistor of 10 times the DC resistance of the coil and a transistor with a Vce 10 times the voltage supplying the valve. The resistor will dissipate 1\10 of the wattage used by the coil when ON. The turn off time increase will be 1/10 of the increase with the diode. No oscilloscope needed.

Suggestions:

24Vdc, 240 ohm valve

2400 ohms 1 watt resistor

ZTX457 $0.68 NPN 300V 500mA hfe 25@100mA

10000 ohm resistor on the base

Francis.

Look carefully at the coil specifications.The coil is not an A/C coil is it?

If so, they have a built in shading coil that is designed to hold the coil in position during the switching of the voltage as it crosses zero.

When the voltage is removed, the emf generates an opposite current in the shading coil to provide a steady pull on the armature.

If someone has accidentally replaced the coil with an A/C one, you would have this problem.

I have used Freewheeling diodes for over 40 years, and never encountered this problem.Not saying it cannot happen,I've just never seen it.Apparent Sluggish solenoid valve action is usually attributed to dirty internals of the valve,weak return spring on the valve, a mushroomed end on the solenoid valve armature, a worn armature or armature slides or guides, sticky linkage on final element actuator(brake) or poor design, with long length lines from valve to actuator, or a feed back pressure sensor at the wrong point in the process.

Free wheeling diodes are widely used in very fast and sophisticated processes,including digitally switched outputs to very fast relays,a lot faster than a solenoid valve.

One other thought:How far from the coil of the solenoid is the diode?It should be as close as possible to the solenoid coil.If not, you are creating a capacitor with the long lead wiring that could create a delay in the voltage decay in the solenoid.

If all of the checks listed above do not solve the problem, go to the link I provided to calculate the design of a snubber circuit.(This part I leave to you and your ingenuity.)