I'm trying to actuate a solenoid lock with the following specifications from the vendor's page:

  • Working voltage: 12 Vdc
  • Starting current: 0.9 A
  • Steady current: 0.12 A

For this application, I'm using a circuit based on the answer to the question in this link: MOSFET to run 12 V, 0.5 A door lock My circuit diagram is as follows, with the solenoid wires connected to the J2 borne connector: enter image description here

However, the IRLML2502 MOSFET is not able to actuate the solenoid lock when a logic pulse is supplied to its gate. The original idea is to operate it using 3.3V, but I also tested it using 5V to no success.

From what I understood, the datasheet gives a maximum of R_{ds(on)} = \Omega for V_{GS} = 2.5 V, resulting in a voltage drop of 0.08 at 1 A.

Using a DMM, I measured the solenoid's current draw from activating the MOSFET gate using 5V logic on a protoboard. As soon as the 5V pin switches, the DMM shows the current of 9.9 mA, then goes out of the 200 mA range and settles at 182.1 mA for the rest of the pulse. The solenoid does not fully actuate in this setup, only when supplied 12.1 V (measured) directly from a 2.5 A DC power supply.

I'm suspecting here of the MOSFET not being able to handle the higher starting current. Am I missing any information here? What parameters should I consider in this case? Is it possible to use IRLML2502 MOSFET for this application? If not possible, any substitute parts reccomendations are greatly appreciated.

  • 1
    \$\begingroup\$ I don't see why it would not work, but I don't see where the 5V or 3.3V signal comes from. Did you measure Gate voltage and Drain voltage? What happens if you short-circuit Drain to Source? Can you post a photo of the wiring setup, lock datasheet, and where does the 12V come from? \$\endgroup\$
    – Justme
    Oct 29 '20 at 17:43
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    \$\begingroup\$ A simple troubleshooting thing is to directly short the drain and source pins at the MOSFET with a wire. Does the lock activate? If not, then the MOSFET has no chance. But if it it does, then the MOSFET must not be switching on properly. \$\endgroup\$
    – mkeith
    Oct 29 '20 at 17:50
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    \$\begingroup\$ Also, when the gate is high, measure Vds. It should be very low. If not, the MOSFET is not switching. If it is low, then the problem is likely elsewhere. \$\endgroup\$
    – mkeith
    Oct 29 '20 at 17:52
  • 1
    \$\begingroup\$ How long is your pulse to the gate? Is it long enough for the lock to overcome its inertia and physically move? \$\endgroup\$
    – Aaron
    Oct 29 '20 at 18:41
  • \$\begingroup\$ What is the solenoid resistance? \$\endgroup\$ Oct 29 '20 at 19:00

One thing that jumps out at me, there's a chance your door lock is either rusty, or isn't broken in. Put some 3 in 1 or low viscosity synthetic lubricant on it and then actuate it by hand a couple times.

Once the lock is moving freely by hand. Then feel your solenoid actuate. Is the power of the solenoid enough to match the force required to engage the door lock?

If yes. Make sure your the lock bolt is able to freely traverse and doesn't hit the jamb plate on your door.

If no... It may work if you pulse the solenoid repeatedly.

Lastly, solenoids in most applications actuate with the same force regardless of polarity. However, some have "favorite" directions. If it has a ferrous casing, mount, return spring, etc. it may slightly change the excursion profile.

  • 1
    \$\begingroup\$ I'll do what you recommended and try it again, thanks. \$\endgroup\$ Oct 29 '20 at 20:26
  • \$\begingroup\$ After lubricating the solenoid and using the repeated pulsing technique, it started working with a new IRLML2502 MOSFET on a different microcontroller. Thanks for the tips! It might have been a combination of the solenoid not being broken in and faulty parts... \$\endgroup\$ Oct 30 '20 at 12:12

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