FET switching question 4

[knowlittle]

I have a USB-based data acquisition card. I am using its digital output to turn on a solenoid. But there is a mismatch in power. Digital out can sink/source 15 mA. Solenoid needs 24 VDC 8W. I would like to build a MOSFET switch.
1. I will use a logic level N-type enhancement FET (RFP30N06LE). 60V 30A ESD rated, Rds 47 mOhm, Vgs (th) 1V min 2V max.
2. I will put the solenoid between +24V DC and Drain.
3. Connects all grounds together (FET Source, 24 VDC power supply ground, USB data acquisition board ground)
4. I forgot to add a flyback diode across the solenoid.

Link for the MOSFET datasheet is here

https://www.mouser.com/datasheet/2/308/rfp30n06le-1195941.pdf

Do you think this will work?

 

[Skogsgurra]

Do you have "ON/OFF" capability in the USB Contact? USB doesn't work like that, normally. And I doubt if your coard can be configured so. But if it can - yrs it could work. But I'd rater use a simple BJT.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[zappedagain]

I agree with Skogs; a BJT transistor will be much easier to implement. FETs are easily damaged by static. If you do use the FET, put a resistor (100 ohms or so) in series with the gate, otherwise you'll get lots of oscillations when the FET turns on and off.

You might also consider a relay if this doesn't have to switch very often.

Z

 

[itsmoked]

That will work with a FLYBACK diode.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MacGyverS2000]

I don't think he's trying to control it with the USB lines directly, guys.

With a back-EMF diode thrown in, I see nothing wrong with the circuit. I, too, would throw in a resistor on the gate... something above 333 ohms to keep the initial current rush below 15mA.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[VEBill]

I have a USB-based data acquisition card. I am using its digital output...

 

[VEBill]

The basic circuit concept as shown will probably work. I've used the same sort of design several times, such as a very crude audio amplifier (a bit square wave) where a low power tone from a sound chip was blasted into a large horn speaker via a big FET to form a siren.

One still needs to double-check the numbers, such as R-on times current = power. Plenty of details to check, even with a simple circuit.

 

[knowlittle]

Thank you for the advices. I have to buy the mosfet, but have a drawer full of transistors and found TIP 112 darlington transistors. Ic max 2A, more than my 0.5A need. So here is my first transistor calculation.

Emitter cutoff 2 mA max. Datasheet does not list min saturation current. Assuming 3 mA will push into saturation,
3 mA = (5V - 1.4V)/Rb. Rb=1.2Kohm.

In place of MOSFET, I will use this transistor with base resistor 1.2K. There will be a flyback diode across the solenoid coil, 50V 2A or higher. Will this work?

 

[IRstuff]

"Datasheet does not list min saturation current"

The curves in the datasheet provide ample information. Emitter cut-off current is <<not>> related to your calculation. The datasheet curves basically say that Ib over about 500 uA is sufficient for your case of 0.333 A Ic

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[knowlittle]

Thank you for the explanation. Now I know what those graphs are for. I didn't know Ib for saturation is so low. I will increase Rb to 2Kohms and go from there. Thank you, all, for guiding me step by step.

 

[IRstuff]

The datasheet also has the forward beta in excess of 1000, so nominally only 333 uA needed for Ib, but you need to allow for tolerances.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[MacGyverS2000]

Also, do not confuse the operation of a BJT and a MOSFET as being readily interchangable. They serve very different purposes. One can force a BJT to act as a MOSFET (and vice versa with the FET), but they're operation in those regions is not ideal by any stretch.

For a solenoid on/off switch, the FET is the proper tool.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[daveross100]

Darlington is not a good idea because of the 2 x VBE ,
will add to the Mosfet circuit 2 Resistors 1K ohm from Line to Gate and 500K ohms from Gate to Source=Ground ,
any feedback : daveross100@gmail.com

 

[electricuwe]

If you want the solenoid to drop the load fast then add a zener diode in series with the flyback Diode. Otherwise the current will circulate in the freewheeling path for up too a few hunderd ms and prevent release.

 

[ScottyUK]

Solid state relay isolates the power circuit from the control circuit. Simple, cheap, works first time.

Building the circuit is more educational though.