Controlling a solenoid with 3.3V logic

Hi I am new to the forum!

I have been working on a project that uses the Spark Core development board. I am essentially controlling a solenoid that changes the direction of air flow.

I need to control this solenoid with the 3.3v logic from the board. I have heard that the best thing I can use is a MOSFET because of its lack of power drain.

Can anyone help me with some simple tips for controlling the different solenoids.

Thanks

You want to use a logic-level N-Channel MOSFET. The part BS-170 would most likely be a good choice.

The Drain pin connects to your system +V supply (Edit: unless you're using a different supply for the solenoid. See my next comment below)

The Source would connect to the +V pin on your solenoid.

You also want a Schottky diode across the solenoid terminals, with the anode toward ground and the cathode toward the MOSFET Source pin (it looks backwards). This is called a Flyback diode.

The Gate also connects to a GPIO pin on the Spark Core. The Gate connects to a resistor, which connects to ground.

Logic low on the GPIO turns the solenoid off, and logic high turns it on. This is an "Active High" configuration. If you prefer "Active Low", then you would use a similar circuit with a P-MOSFET.

A few other details to consider:

The BS-170 can drive about 500 mA continuous, and about 1200 mA in a brief pulse, so if your solenoid draws more than about 250 mA to stay on, you'll need to look for a part with a higher current rating.

After answering, it occurred to me that you might not be powering the solenoid with the system 3 Volt supply. If this is the case, connect the Drain of the MOSFET to the +V of your solenoid power supply, not the Spark Core +V power supply. Everything else stays the same. Make sure the ground of the Solenoid power supply is connected to the ground of your Spark Core power supply.

You need to also consider keeping relay switching transients away from your micro board, this can apply to the ground as much as the positive supply, so pay careful attention to grounding practices,
cheers,
richard.

Thanks a lot for the help, deciding which components to use is my main problem. You were right about a higher supply voltage for the solenoid I will be using +9v.

Can you explain me why I would use a flyback diode? i understand they only allow current to flow one way but I dont understand their use all that much.

Thank you

I am assuming this is to help with noise and power drop out? What would be my best option when grounding?

Thank you

Luke - You need a flyback diode anytime you switch an inductive load, like a solenoid or motor.

When the MOSFET switches off, the current through through the coil will drop. When that happens, the magnetic field collapses which generates current flow in the opposite direction. Essentially, you get a big spike of reverse voltage, called flyback or back EMF, that will often blow out the switching MOSFET. The flyback diode shorts this out, protecting the MOSFET.

Don't mix your high(relay) and low current grounds, and put a capacitor across the relay supply.
Cheers,
Richard

Having finally received the transistor from RS (nothing but problems with them) I have tried this circuit and for some reason with a 6 volt supply the FET only outputs .5v when the gate is triggered. What could this be??

Thanks

Luke

Double check the pin-out in the datasheet. I've seen some with the pin-outs reversed from what is normal.

I think what you need is sold by SparkFun. It has all the components Screaming Circuits is referring to. Check out https://www.sparkfun.com/products/12959.

Just question...

In this case, wouldn't it be better to use the FET as a low-side switch, being an N-channel? The reasoning is that the N-channel FET needs a great enough gate-source voltage to turn it on. In the above case the gate voltage needs to be higher because of the voltage drop across the relay, i.e. Vrelay + Vgs.

If the FET is used as a low-side switch, i.e. the relay is connected between the drain and the supply and the source connected to ground, the gate voltage can be lower.