Controlling 12V Magnetic Latch Solenoid with RaspberryPi 3b

Question

As the title says I am trying to control a solenoid with a raspberry pi. I am currently using an npn transistor (will post link with parts at the bottom) however I am having a hard time getting it to work. The collector is at 12V and the gate is controlled by the raspberry pi gpio pin (runs at 3.3V and 16mA (this is the max amount I believe)).

Attached is a picture of a circuit I used to verify my solenoid circuit. It is essentially the same except that it has LEDs and that it runs on 3V instead of the 12V for the solenoid. Unfortunately I do not have any pictures with the solenoid. It would look roughly the same except that the two positive legs on the solenoid replace the LEDs and the single ground wire is hooked up to the negative row on the breadboard.

This verification circuit works perfectly, but the solenoid circuit does not. I have read online that MOSFETs can also be used as a switch, would this be better? In the case that it is, I would like to try to get the NPN transistor to work as I would rather not use more money.

Any help is greatly appreciated.

Thanks.

Solenoid (15mm version): https://www.pneumadyne.com/way-normally-closed-latching-solenoid-valves-p-1448.html#1-YToxOntzOjQ6ImdyaWQiO2k6MDt9

Transistor: https://www.mouser.com/ProductDetail/Diodes-Incorporated/ZTX688B?qs=sGAEpiMZZMshyDBzk1%2fWizF0eVyYE44ZxbGt8e1SnMg%3d

Images: https://i.sstatic.net/C1p7z.jpg

I have included a diagram of what my solenoid circuit looks like. For reference on the transistor labeling, as I am not sure what is common. B - base (gate) C - collector E - emmitter

Answer 1

Your circuit does not work for the solenoid or supply voltage that you have.

You have a 3 wire (2 coils) self latching solenoid that specifies the common as the negative pin. The data for the solenoid shows that you have a 4W drive requirement. This means that at 12VDC you need about 340mA to activate each of the solenoid coils (latch and release).

The transistor you selected is completely unsuitable for your task. It has enough Collector current capability but has a V(CBO and V(CEO) of only 12V. You would never use a device at its absolute maximum rating.

You are going to have to buy suitable devices to drive your solenoid.

A potential circuit would be as follows and you would need two GPIO pins to drive the latching solenoid:

^{simulate this circuit – Schematic created using CircuitLab}

It would be up to you (in software) to ensure the A or B coil is activated for the appropriate length of time to latch and unlatch.

Note: You must have the pulldown resistors on M3 and M4 to ensure that neither coil is activated as you turn on and boot the R'Pi. The GPIO pins are all set as inputs on power-up, so the 10K Ohm resistors keep M3 and M4 off.

Update: The documentation for the solenoid switch you use specifies the polarity of activation (I assume this is required since the latching is by a magnet, so pole polarity sensitive):

Answer 2

When using transistors in switching applications, it is always a good idea that they are referenced to a voltage that will not move on you. If you notice on @peter bennett ‘s suggested circuit, the NPN is driven by the GPIO of the RasberryPi referenced to GND (emitter is at GND, and the base is driven by the GPIO which supplies the base current via the base resistor as (Vgpio - 0.7)/Rbase), and the base of the PNP is driven referenced to the “fixed” battery voltage at its emitter. Notice as @Jack Creasy pointed out, that you need a current limiting resistor in series with the collector of the NPN used to drive the PNP, or you will probably fry them both. Otherwise, that should be a perfectly good circuit for driving a regular solenoid.

I need to review how the latching solenoids work—-if i remember, once they no lnger need to be driven anymore . However, to disengage them, i think you need to drive the current through the solenoid in the opposite direction. If so, @Peter Bennet’s circuit will not achieve that. You could accomplish this by using an H-bridge.

Answer 3

As mentioned by Peter Bennett, you should change the transistor so that it doesn't work at its limit, and wire it as shown here below. I would also add a diode to protect the transistor from the spikes caused by switching the inductive load. BTW, the transistor value is just the default part that comes up when I drew the schematic. Please look up the datasheet for the absolute maximum values.

I suggest that if you're are in doubt of how a circuit works (or doesn't) try simulating it in LTSpice; It is free, and fairly easy to use, and it will save you a lot of headaches.

^{simulate this circuit – Schematic created using CircuitLab}

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As it has been pointed out that the solenoid is polarised, and that OP is a newbie, I shall present the easiest and probably cheapest solution possible. it also includes a protection from activating both coils at the same time.

^{simulate this circuit}

The relay board can be found for just over a dollar on Aliexpress

Answer 4

First, the Absolute Maximum collector-to-emitter voltage for that transistor is 12 volts. It may be barely adequate for your circuit - I'd prefer something rated at 20 volts or more - you should never run anything at its Abolute Maximum rating.

Second, you are apparently using the transistor as an emitter follower - you have the load between emitter and ground. Unfortunately, the emitter voltage will always be about 0.7 volts below the base voltage - since the Pi operates on 3 volts, your circuit can't apply more than 2.3 volts to the 12 volt solenoid.

You should ground the emitters, and place the solenoid between the collector and the positive supply. You also need resistors (1K or so) between the Pi outputs and the transistor bases.

edit: It appears that the common wire on that solenoid must be negative, and you need to apply a positive pulse to the brown or blue wire to switch the solenoid. Something like this would work:

^{simulate this circuit – Schematic created using CircuitLab}

The transistor and diode types are defaults in the CircuitMaker program - not necessarily appropriate for your use.