Best MOSFETs for Driving 12V Solenoid With 3.3V Logic From Spark Core Board

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#1 21673751 26 Mar 2015 14:57

Luke Weston Luke Weston

Anonymous

Post #1
21673751 26 Mar 2015 14:57

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
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#2 21673752 27 Mar 2015 11:01

Screaming Circuits Screaming Circuits

Anonymous

Post #2
21673752 27 Mar 2015 11:01

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.
#3 21673753 27 Mar 2015 11:17

Screaming Circuits Screaming Circuits

Anonymous

Post #3
21673753 27 Mar 2015 11:17

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.
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#4 21673754 27 Mar 2015 13:26

richard gabric richard gabric

Anonymous

Post #4
21673754 27 Mar 2015 13:26

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.
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#5 21673755 28 Mar 2015 04:47

Luke Weston Luke Weston

Anonymous

Post #5
21673755 28 Mar 2015 04:47

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
#6 21673756 28 Mar 2015 04:49

Luke Weston Luke Weston

Anonymous

Post #6
21673756 28 Mar 2015 04:49

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

Thank you
#7 21673757 28 Mar 2015 07:50

Screaming Circuits Screaming Circuits

Anonymous

Post #7
21673757 28 Mar 2015 07:50

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.
#8 21673758 01 Apr 2015 20:36

richard gabric richard gabric

Anonymous

Post #8
21673758 01 Apr 2015 20:36

Don't mix your high(relay) and low current grounds, and put a capacitor across the relay supply.
Cheers,
Richard
#9 21673759 23 Apr 2015 13:38

Luke Weston Luke Weston

Anonymous

Post #9
21673759 23 Apr 2015 13:38

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
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#10 21673760 23 Apr 2015 13:56

Screaming Circuits Screaming Circuits

Anonymous

Post #10
21673760 23 Apr 2015 13:56

Double check the pin-out in the datasheet. I've seen some with the pin-outs reversed from what is normal.
#11 21673761 16 Jul 2015 06:31

Joel Funk Joel Funk

Anonymous

Post #11
21673761 16 Jul 2015 06:31

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.
#12 21673762 17 Jul 2015 00:42

Pieter Kruger Pieter Kruger

Anonymous

Post #12
21673762 17 Jul 2015 00:42

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.
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Topic summary

The discussion focuses on selecting suitable MOSFETs for controlling 12V solenoids using 3.3V logic from a Spark Core development board. A logic-level N-Channel MOSFET such as the BS-170 is recommended for low-voltage gate drive compatibility. The MOSFET should be connected as a high-side or low-side switch depending on the circuit design, with the low-side configuration often preferred for easier gate drive at 3.3V. A flyback (Schottky) diode across the solenoid terminals is essential to protect the MOSFET from voltage spikes caused by the inductive load when switching off. Proper grounding practices are critical to avoid noise and power dropouts, including separating high-current and low-current grounds and possibly adding a capacitor across the solenoid supply. The BS-170 can handle continuous currents up to about 500 mA; for solenoids drawing higher current, a MOSFET with a higher current rating is necessary. Users should verify MOSFET pinouts carefully to avoid wiring errors. Additionally, pre-assembled driver modules, such as those available from SparkFun, can simplify implementation.
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Best MOSFETs for Driving 12V Solenoid With 3.3V Logic From Spark Core Board

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Topic summary