I just purchased some pinball machine solenoids and was experimenting with them; DC resistance is about 30 ohm, they actuate at about 30 volts, and they hold at about 6. I tried controlling them with 10A relays and found that the a relay sometimes latched arced even though I have flyback diodes, so I looked at the solenoid voltage with a scope. One side of the solenoid is connected to the positive supply via relay and a PTC fuse; the other side is grounded. The scope is directly across the solenoid.

It appears that the when the solenoid is active the voltage flies up to over +200 volts. Not the reverse voltage that would appear when releasing a solenoid with no flyback diode--forward voltage. I would guess that the coil is effectively magnetizing the slug, and that when the slug then moves into the coil it generates back EMF; because the coil is crossing more lines of force at it gets closer to the slug, the back EMF is not limited to the driving voltage as it would be with a conventional motor. Would such back EMF imply that the solenoid current would be dropping to zero during the stroke? Is such behavior typical for solenoids?

If such behavior is typical for solenoids, it would seem that all of the "useful" energy, except what might be needed to hold the solenoid (if desired), would be imparted before the current dropped to zero, and one could reduce energy consumption enormously by watching current usage. I would guess that if mechanical factors prevent the slug from moving quickly, the current might not drop all the way to zero, but watching for the derivative of current to go positive-negative-positive should still provide an identifiable "optimal turn-off" point. Are there any solenoid-driver circuits that exploit this? Certainly end-of-travel contacts could help provide such behavior, but those add mechanical complexity. Are all-electronic solutions practical?

  • \$\begingroup\$ All coils and all devices that use coils have back EMF. It's just the side effect of having a coil. The slug doesn't even need to be magnetized for this to work. \$\endgroup\$
    – AndrejaKo
    Commented Apr 3, 2012 at 14:52
  • 2
    \$\begingroup\$ The effect you describe is unusual but potentially makes sense. I have not heard of this happening and a quick gargoyling through internet pages does not provide any indication that it is an accepted effect.It would need to rely on the armature being attracted more solidly than required so that it enters a mode where it would "coast" home if the drive was removed part way through the stroke. Check you circuit carefully, check numerical results, use a scopt to be sure you are not fooling yourself. Patent it :-) :-) \$\endgroup\$
    – Russell McMahon
    Commented Apr 3, 2012 at 15:14
  • 1
    \$\begingroup\$ @AndrejaKo: An inductor which is energized will initially drop voltage equal to the applied voltage until current starts flowing an infinitesimal time later; I guess one could call that back EMF. A non-moving inductor, however, will have current that monotonically increases as long as the applied voltage is positive. Trying to reduce the current in an such an inductor will cause the voltage drop to go negative. My question is whether it's typical for the forward voltage on a solenoid to exceed the supply voltage, and whether this can be exploited when controlling them? \$\endgroup\$
    – supercat
    Commented Apr 3, 2012 at 15:16
  • \$\begingroup\$ This is a very similar question to this one, and its answer might also help you. \$\endgroup\$
    – Cerin
    Commented Jul 30, 2017 at 14:42
  • 1
    \$\begingroup\$ @Cerin: I didn't notice anything in that question, answer, or linked blog entry, that mentioned whether the movement of the slug affected the behavior of the solenoid, which was the detail I was particularly interested in. Was there something I missed? \$\endgroup\$
    – supercat
    Commented Jul 30, 2017 at 16:26

2 Answers 2


There could be some small back EMF effect when the slug moves. However, I seriously doubt that is what is causing the high voltage on closing. There may be other effects:

  1. Relay contacts bounce. That means the solenoid will be disconnected multiple times even during a overall "on" operation. These short disconnects that happen after some current has built up could cause high voltage for a short time.

  2. Ringing. There is inevitable capacitance in the system accross the coil. When the coil is switched on, it is like energizing a tank circuit. In ideal conditions, this could ring up to twice the input voltage especially with contact bounce. In practise, the DC resistance of a solenoid is usually substantial enough to damp the system well enough, and the R and L of the solenoid dominate.

  3. It's not really there. The scope may be showing you things that aren't really happening at transients and especially with poor probe grounding.

I don't know what exactly is happening, except that I'm quite skeptical the EMF is really going to 200 V. I also don't like the PTC fuse being in series for testing these things. Try shorting it out and see what that does. Also try putting a reverse diode immediately accross the solenoid, not at the other end of a wire or on the other side of the PTC fuse. This should be a fast diode.

  • \$\begingroup\$ I would expect that relay contact bounce would cause forward EMF, rather than back EMF. I hadn't thought about capacitance causing ringing; that seems like a definite possibility. I have a 300V diode directly between the solenoid terminals, but it's probably not a terribly fast one. I could short out the PTC fuse, but I wouldn't think that would be having much effect. The scope ground is attached to the negative solenoid lead, and the tip to the positive. Even if there were some "transformer" effect, the number of turns in the coil should dwarf the one "turn" of ground-clip loop. \$\endgroup\$
    – supercat
    Commented Apr 3, 2012 at 15:41
  • \$\begingroup\$ I was looking at things a bit more, and it seems to be ringing related to contact bounce. It seems a little odd that the ringing would swing up much further above the supply than below, and perhaps the movement of the slug played a role, but I don't think any such effect would seem reliable. Three of the four digits have contacts which open when the solenoid is active; I'd like an approach which could work uniformly for all four digits, but I think my best bet is to get three digits working with the contacts, and then find a contact for the fourth. \$\endgroup\$
    – supercat
    Commented Apr 4, 2012 at 13:17

This is called a transient. Transients are caused by sudden changes in fields.

Energy is being stored in the magnetic field. The voltage is caused by the change in the magnetic field over time. If you connect or disconnect the circuit, the field changes causing a voltage. This transient can be stored in a capacitor for later use. Moving iron past a conductor will not generate electricity. Changing a field with respect to time will.

A capacitor is the same. It stores energy in the dielectric field. Amperage is the rate of change in the electric field. The faster the discharge, the higher the current.

  • \$\begingroup\$ Moving a non-magnetized piece of iron through a field won't generate electricity, but moving a magnetized piece of iron through a field will. When a solenoid is energized, I would expect that the iron would become magnetized as a consequence, and that such magnetization could cause it to produce a current as it moves, but I have no idea what the actual behavior is. Conceptually, I would expect the moving slug to generate a voltage opposing current flow, but I have no idea of the actual behavior. \$\endgroup\$
    – supercat
    Commented Nov 5, 2013 at 18:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.