Solenoids not de-energizing


Thread Starter

Tom Ludwig

The voltage across the solenoid was still 4.4 VDC which was enough to hold the solenoid in its energized state.

I remember from 6 years ago when I worked at Solar Turbines we used to put a passive device like a resistor or a diode across the solenoids . Hopefully I will find a copy of one of my projects at home that has the information.

My question is this. Why is it that solenoids require special attention? If it is a resistor or a diode, why are they required for ordinary 24 VDC outputs.


Greg Schneider

Are the outputs solid state? Do you have high-impedance solenoids? Solid state outputs, when OFF, usually leak enough current to energize neon lights, solid state annunciators (like Sonalerts), and other devices that operate on low current. I think you will have to put resistors in parallel with your solenoids.


Steve Myres, PE

The problem is not specifically with solenoids. You can have this problem any time you use a solid state output with a device that has a significant discrepancy between starting and holding power (relay, solenoid, contactor, etc.) or one that will produce some output at low percent of voltage with very little current. I have found this situation with piezo horns.

The basic problem is that the solid state output never turns completely off. Even in the nominally off state, a small amount of current still leaks through the device. If the load to which it is connected is sensitive to this amount of current, like a relay or solenoid with an unusually low holding power requirement, the load will stay on. The solution is to provide more load. This can be done by adding a resistor in parallel with the output load (solenoid).

Size the resistor to consume about the same amount of power as the solenoid with the output on, and make sure the resistor has a sufficient power rating and the output has a sufficient current rating (although the resistor and solenoid currents will be significantly out of phase and so do not need to be added arithmetically).

An alternate solution is to use relay outputs, if the switching frequency and relay contact life will allow such. You will probably need to add a snubber or varistor in parallel with the solenoid anyway. You can also add a small relay between the output and the solenoid. This will also protect the output.

Aniruddha Joshi

Firstly solenoids are inductive devices. Any inductive device has a tendency to maintain its state. Some potential gets built into the solenoids' coil, and the direction of this potential is reverse to what is applied. The best way to avoid any disruption is to instal a free wheeling diode across the solenoid coil, in such a way that the diode is in reverse bias. The moment the supply is removed, the only voltage is the voltage generated in the coil and this gets dissipated in the free wheeling diode, removing the residual voltage. In AC supply the voltage source is itself alternating, so a diode would
conduct in one cycle causing a short across the solenoid coil, thus possibly causing chatter. Hence diodes cannot be applied in this case.

Surendra Mehta

Douglas Firlotte

One note of caution when using resistors for leakage current, be sure to properly size the resistor, and be sure your maintenance personnel understand the issues.

Last year we had two termination cabinets and one electronics cabinet, with 500+ I/O points, totally destroyed by fire. In addition, cabinets containing another 2000 I/O points were damaged by smoke. The cause was identified as an inappropriate loading resistor, in an inappropriate location. The resistor was most likely added several months earlier by a tradesmen, attempting to resolve field device problem.


Sounds like you have a latching type solenoid.
This means that you energized it to close or move shaft out and you have to reverse the voltage polarity to release it. Refer to the manufacturer's data sheet for this reverse voltage level. The "dropping" resistor is put in series between supply and the coil for release as a simple way to reduce voltage which is usually lower and negative. For faster closing, a diode is put in parallel with the resistor with anode [band] toward the negative supply terminal. By the way, a diode across [anode to +] is used to clamp voltage spikes for unidirectional coils. For this application, if you want to still clamp spikes, use a bidirectional TVS diode [essentially two enlarged zeners back to back]. If you have the data sheet and need more help sizing the resistor feel free to contact me or your supplier.

Good luck,