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I am using a circuit nearly identical to the one used in this question by Dyte. My version is pictured below:

circuit

I am powering the solenoid (water valve) with 12v, using a RFP30N06LE MOSFET (datasheet), and using a NodeMCU (3v3 logic) to drive it.

What I can't figure out is why the MOSFET drives the solenoid fine when I connect the gate to 5v, but not at all when I connect it to 3v3. I've measured the voltage between the drain and source when it's connected, and I am seeing 12v, so it seems that it should turn on the valve.

Is there another MOSFET I should be using, or something else that is wrong with my circuit?

pastudan
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  • You also need a common ground connection to your MCU – sstobbe Sep 11 '17 at 02:32
  • Gate threshold voltage. Also, draw schematic instead of vero-board picture. – winny Sep 11 '17 at 06:05
  • Note that the gate-threshold voltage (\$V_{GS-th}\$) is given for <1mA drain current (Check from datasheet, Electrical Specs on p.2). It seems that your MOSFET requires higher gate-source voltage to turn on due-to relatively higher load current. Although the level of current is not given in your question but probably it's in 1-3A range. – Rohat Kılıç Sep 11 '17 at 07:08

2 Answers2

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The threshold voltage of the BS170 is too high to operate off 3.3V.

if you look at the BS170 datasheet you will see from Fig1 on page 3 that at 3.3v it will only pass about 200 mA which is almost certain not enough to drive the solenoid.

The MOSFET will possibly pass enough current to heat up significantly so be careful operating in this state.

You depict a TO220 package - the only data sheet I could find was for smaller packages - is it really a BS170?

You need to find a device that will turn on adequately with only 3.3v drive or amplify the voltage up to a reasonable drive level.

Older MOSFETs we designed for 10V gate drive, a number of logic-level devices were the designed to be driven by 5V logic. There are even fewer that will work from 3.3v.

You need to know how much current the solenoid requires in order to select the MOSFET.

http://www.mouser.com/ds/2/308/BS170-1118810.pdf

Kevin White
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  • Oops, I think the wording I chose made this question confusing. The schematic is from another question and I have the same setup, but am using several different components. Specifically, the MOSFET I'm using is the RFP30N06LE – pastudan Sep 11 '17 at 04:36
  • I just updated the schematic in my question with the one I'm actually using. – pastudan Sep 11 '17 at 05:09
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Do you have an actual schematic of your circuit? The graphic you showed has the gate on the middle leg of the MOSFET's package, that isn't the case for the MOSFET whose datasheet you linked. Values of your pull-down resistors would be helpful too.

Without further information, I would say the threshold voltage is too large to be turned on by the 3.3V logic. The max threshold voltage of your transistor under tested conditions is 2V, this isn't a huge amount of leeway.

Measuring 12V between drain and source doesn't mean that the solenoid should be activated. If you measure across a 12V battery whose terminals are unconnected you will measure 12V. The transistor is not turned on so there is effectively an open circuit between the drain and source, you will measure 12V but not current will flow.

jramsay42
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  • I updated my question to include my actual schematic. I'm measuring 3.12v between the gate pin and ground, so I'm _fairly_ certain there's enough voltage to turn the MOSFET fully on, which is why I'm stumped. – pastudan Sep 11 '17 at 05:13
  • Hmm, have you tried switching a lighter load with the MOSFET, like say an LED? Just to say for certain whether it is turning on or not? – jramsay42 Sep 11 '17 at 07:02
  • good thought! Just tried that with my bench PSU. Seems the LED is not fully bright at 3.3v. It seems to stop getting brighter around 4.2v. What's strange to me is that the threshold max from the datasheet is 2v, so I still don't understand why 3.3v wouldn't fully turn it on. – pastudan Sep 11 '17 at 19:58
  • The rated threshold voltage is usually for a particular drain current/drain-source voltage so it's likely different for the current you are operating at. Furthermore, there are other factors like the MCUs pins not reaching all the way up to the supply voltage. – jramsay42 Sep 12 '17 at 00:37