OT- Resistor and diode on this coil...why ?

Replacing blown contactor coil (72 VDC) on my electric forklift and wondering if I should order a new "little black box" while I'm at it. This thing connects between the two terminals of the coil and contains one resistor and one diode.

They both 'work' and seem to give proper readings, but there is evidence of extreme heat in the past in there. Seems unlikely this would be a spark suppressor, if not, what the heck is it for anyway ?

Any energized coil has stored energy in the magnetic field. Something has to absorb that energy when the excitation current is removed. For example, an ignition coil gives up that energy in the form of an arc discharge - hopefully controlled and to the spark plug. My guess is the module you are looking at is to absorb the coil energy when it is turned off.

Bruce, I'll bet that is it...with this device, when the voltage is removed the magnetism in the coil lingers for shorter period and the contactor drops out quicker.

Not so much to drop the contactor out quicker, but to absorb the high voltage spike generated by the collapsing magnetic field, preventing damage to other components connected to the same power supply.

A reverse diode is a good idea on ANY relay/contactor coil operated from a DC supply. Particularly if the coil is switched by a solid state device (like a PLC output).

Used to be called a spark quencher.

I had a Toyota minivan once that died due to a this effect. The fuel computer quit one cold morning before dawn in the middle of nowhere. The mechanic kept replacing the computer and it would run for 10 or 15 minutes before quitting again. I finally delved into the details and after much consternation discovered the problem to be a failing fuel pump causing the fuel pressure to be a little low and the computer was compensating by driving the injectors harder. Somewhere in Japan there is a Toyota engineer that never learned about putting snubber diodes across inductive loads and the output transistors of the fuel computer were blowing under the increased stress. Isn't electronics wonderful?

Just an interesting point on stored energy in coil produced magnetic fields. Several years ago, when I worked for GE, the company was thinking seriously of entering a new power storage business. The idea was to store energy during low demand and supplement generated power with stored power during peak demand. It was based on storing energy in the magnetic field of superconducting magnets - similar to those used in MRI machines. The point is a coil can store quite a lot of energy - especially if designed to do so.

Hi all;
Commonly known as a “freewheeling diode”; yup it’s there to conduct back emf when the magnetic field of a coil collapses, and the resistor to set a time constant such that the diode doesn’t see too much current.

And now a quick story of what happens when the diode opens up. I’ve an electro-magnetic chuck on an ancient “Cincinnati” planer. Said chuck is 2 foot x 6 foot and takes 180 vdc. Primary ac voltage supplied by a variable autoformer (It’s an old system… like I said).

In operation you ramp the voltage up and down with the autoformer, never energizing or de-energizing the chuck at full voltage. In theory there is only a couple of volts on the chuck at “turn-on” or “turn-off”.

Well, one day I guess the freewheeling diode failed. I ramped the voltage down to minimum and turned off the power-supply. The next thing I knew I was on the floor several feet from where I had been standing, vaguely aware of their having been a very loud noise and some visual “event”. Shop smelled metallic and seemed way too quiet.

The cover of the switch box and switch were simply gone, the inside of the box was plated with some sort of metallic deposit, amidst burn marks and some sort of crusty deposit. In the adjacent enclosure the brush-gear and large segment of the variac winding were likewise gone.

Cyclotronguy

061016-0950 EST USA

Basic electrical theory.

The current in an inductor can not change direction or magnitude instantaneously.

If a current is flowing in an inductor and you attempt to interrupt the inductor current the inductor will do "whatever is necessary" to maintain exactly the same current flowing and in the same direction.

"Whatever is necessary" means the inductor will produce a voltage across the inductor sufficient to maintain that instantaneous value of current. Note this requires a voltage reversal across the inductor terminals. This is why a reversed biased diode works as a snubber.

So if you open a switch to an inductor carrying a current the switch looks like a near infinite resistor and the inductor will try to generate an infinite voltage, but long before that the air gap in the switch will breakdown and conduct the inductor's current. Thus, some sort of snubber is needed if you do not want large voltages and arcing.

The time to discharge the stored energy in the inductor is a function of the initial current, inductance and resistance to dissipate the energy. A lower resistance is a longer time.

If you use a diode only as the snubber, then the resistance is the internal resistance of the inductor, but the terminal voltage will be no higher than the diode drop.

If you add external series resistance, then the terminal voltage will be the current times the external resistance plus diode drop, and decay time will be less than for the diode only.

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The voltage generated by the collapsing magnetic field of a coil is V(t) =L di/dt where L is the inductance of the coil , di/dt is the change of current I with respect to time, and V(t) is the voltage developed across the coil and is a function of time. If the coil is energized and a constant current is flowing, there is no voltage developed by the magnetic field because there is no change in current. But when current is cut off, then there is a change in the current and a voltage is developed across the output of the coil. The more abrupt the change in current, the larger the voltage developed. The voltage developed as a result is the opposite polarity. That is why a diode is used. The diode conducts in one direction and is installed to conduct in direction of the V(t). The resistor is used to dissipate the energy (convert the energy into heat) conducted by the diode. The voltage V(t) can be quite large. I have measured several hundred volt transients V(t)
using an oscilloscope of a small 24VDC relay coil. Snubber diodes are a necessity to prevent the generated transients V(t) from damaging control circuitry.

Switching circuit engineers typically talk about "volt-seconds" to describe the energy input to or removed from a coil......

The relay will hold in as long as there is sufficient magnetic pull to hold the relay "clapper" in.

If you allow a higher voltage in your "damper" circuit, you remove more energy per second.... there are 10 x more "volt-seconds" removed per unit time at 20V than at 2V.

Your damper box gizmo is no doubt set up to be the optimum compromise between a high voltage spike, and holding the relay "in" longer.

IT MAY ALSO affect the speed at which the relay clapper actually moves when it drops out, and THAT may affect the life of the relay. The faster it drops out the shorter the arc that will occur in the main power contacts, since they will be faster to break the arc.

If it is too slow, it may burn the contacts faster than it should. The 72V can create a very good welding arc when the contact opens.

You want a nice snappy opening of the contacts, with no hesitation and no more arcing than is unavoidable. They probably figured that damper circuit out to do that.

So I WOULD suggest you go ahead and replace the "damper" assembly and not think twice about it.

The short answers


The network is required to protect other circuitry which drives the coil

or......

The engineer that built that thing would not have spent the money if it wasn't needed.

All valid points! I would like to mention that all DC motors should have a diode connected across them to dissippate the commutation back EMF. Unless they are reversible then a pair of zener diodes connected anode to anode and rated at the supply volts + 20%, the cathodes connected to the motor terminals.
Frank