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.
#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
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#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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FAQ

TL;DR: Use a logic‑level N‑MOSFET (e.g., BS170: ~500 mA continuous, ~1.2 A pulse). “Make sure the ground of the solenoid power supply is connected to the ground of your Spark Core power supply.” This setup works with 3.3 V control and a higher solenoid voltage. [Elektroda, Screaming Circuits, post #21673753]

Why it matters: This FAQ helps makers drive 6–12 V solenoids from 3.3 V GPIO safely and reliably, avoiding blown parts and noise issues.

Quick Facts

Pick a logic‑level N‑channel MOSFET; BS170 is a common starter choice for 3.3 V GPIO control. [Elektroda, Screaming Circuits, post #21673752]
BS170 capability: Approx. 500 mA continuous, ~1.2 A pulse; choose a higher‑rated MOSFET if your coil needs more. [Elektroda, Screaming Circuits, post #21673753]
Always include a flyback (Schottky) diode across the solenoid; anode to ground, cathode to MOSFET side. [Elektroda, Screaming Circuits, post #21673752]
Keep relay/solenoid and logic grounds separate in layout; add a capacitor across the relay supply. [Elektroda, richard gabric, post #21673758]
If using a separate coil supply, tie grounds together for a common reference. [Elektroda, Screaming Circuits, post #21673753]

What MOSFET works best to drive a 12 V solenoid from 3.3 V logic?

Use a logic‑level N‑channel MOSFET. The thread recommends BS170 as an accessible option for 3.3 V GPIO control of small coils. If your solenoid current is higher, select a MOSFET with a larger continuous current rating. [Elektroda, Screaming Circuits, post #21673752]

How should I wire the MOSFET—high‑side or low‑side?

Use the N‑MOSFET as a low‑side switch. Put the solenoid between +V and the Drain, and connect the Source to ground. This keeps the required gate‑to‑source voltage low and ensures full enhancement from a 3.3 V GPIO. [Elektroda, Pieter Kruger, post #21673762]

Why do I need a flyback diode, and what problem does it fix?

When you switch off an inductive load, the collapsing magnetic field generates a high reverse‑voltage spike (back‑EMF). That spike can destroy the MOSFET. A flyback diode provides a safe path for this current and protects the switch. “You need a flyback diode anytime you switch an inductive load.” [Elektroda, Screaming Circuits, post #21673757]

How do I orient the flyback diode across the solenoid?

Place the diode directly across the solenoid terminals. Connect the anode to ground and the cathode to the MOSFET/solenoid positive side. It looks reversed in normal operation, but it conducts only during turn‑off to clamp the spike. [Elektroda, Screaming Circuits, post #21673752]

What current can a BS170 handle, and what if my coil draws more?

BS170 supports about 500 mA continuous and around 1.2 A in brief pulses. If your solenoid’s hold current exceeds ~250 mA margin, choose a MOSFET with a higher continuous rating and lower RDS(on). That reduces heating and improves reliability. [Elektroda, Screaming Circuits, post #21673753]

Can I power the solenoid from a separate 6–9 V supply and still use 3.3 V control?

Yes. Drive the MOSFET gate from the 3.3 V GPIO, power the solenoid from its own supply, and tie the grounds together. “Make sure the ground of the solenoid power supply is connected to the ground of your Spark Core power supply.” [Elektroda, Screaming Circuits, post #21673753]

How do I reduce noise and protect my microcontroller board?

Do not mix high‑current and logic grounds in the same traces. Route coil current separately, then join grounds at a single point. Add a decoupling capacitor across the relay/solenoid supply to tame transients and dips. [Elektroda, richard gabric, post #21673758]

Why is my MOSFET only showing ~0.5 V on the output when I turn it on?

Check the MOSFET pinout against the datasheet. Some parts flip the usual Gate‑Drain‑Source order, causing miswiring and weak conduction. Correct pin mapping typically restores full switching behavior. [Elektroda, Screaming Circuits, post #21673760]

Is the control signal considered active‑high or active‑low in this setup?

With the suggested N‑MOSFET low‑side switch and pulldown on the gate, a logic high at the GPIO turns the solenoid on. A logic low turns it off. That’s an “Active High” configuration. [Elektroda, Screaming Circuits, post #21673752]

What gate resistor or pulldown do I need?

Include a gate‑to‑ground resistor so the MOSFET stays off during reset or boot. Connect the GPIO to the gate, and the pulldown to ground. This prevents accidental activation from a floating gate. [Elektroda, Screaming Circuits, post #21673752]

Can I buy a ready‑made driver board instead of wiring parts?

Yes. The SparkFun MOSFET power driver board integrates the MOSFET and protection parts, matching the thread’s guidance. It simplifies wiring and reduces common mistakes for beginners. [Elektroda, Joel Funk, post #21673761]

Will a 9 V coil supply work with the Spark Core’s 3.3 V GPIO?

Yes, if you use the MOSFET as a low‑side switch and share grounds. One participant confirmed planning a +9 V solenoid while keeping 3.3 V logic control. Add the flyback diode for protection. [Elektroda, Luke Weston, post #21673755]

Any quick wiring steps for a low‑side N‑MOSFET driver?

Connect solenoid between +V and MOSFET Drain.
Connect MOSFET Source to ground; tie logic and coil grounds together.
Drive Gate from 3.3 V GPIO; add gate pulldown and a flyback diode across the solenoid. [Elektroda, Pieter Kruger, post #21673762]

What’s an edge case that can still bite me even with a diode?

If the MOSFET is mis‑pinned on your board, it can half‑turn‑on and show a small voltage, misleading debugging. Always confirm the package pinout before powering. “Double check the pin‑out in the datasheet.” [Elektroda, Screaming Circuits, post #21673760]

Do grounding practices affect reliability as much as parts choice?

Yes. Poor ground routing can inject switching spikes into the micro, causing resets or sensor glitches. Keep high‑current return paths away from logic returns and use local supply decoupling near the coil. [Elektroda, richard gabric, post #21673754]

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