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Topic: Transistor Issue Plagues Me (Read 3 times) previous topic - next topic

boingaon

Hello,

I have spent about 2 weeks trying to figure out what should be a very simple problem. I'm trying to control a solenoid with my Arduino Mega 2560 using a transistor. I'm desperate for assistance.

When I connect the solenoid and Arduino to the transistor, the solenoid does not turn on at all. When connected directly to my DC power source, the solenoid opens & closes just fine. Here's what I discovered:

My DC power supply sits at 16.6V / 1.8Amps. When I read the voltage across the transistor, the voltage is a correct 16.6 volts. However, the amperage is a measly 20mA. As a test, I tried connecting a 7.2V battery to the system and got the same results- 7.2V reading with the multimeter but only 20mA for the amperage. When I use the Arduino's 3.3V power supply as the source voltage, I can control an LED flawessly (but this is because the LED can noticeably illuminate at 20mA). Just so you know, I have tried switching around wires connected to Drain/Source pins just to see if anything would happen but.......nothing.

Quick question about the circuit diagram I included below: Why is there a "ground" and a "solenoid supply ground"? Aren't they the same thing? There's only two power sources- the arduino digital pin on/off and the solenoid's power supply, both of which connect to common ground.

I want to provide some data:

MOSFET Transistor Data Sheet
http://www.onsemi.com/pub_link/Collateral/NDF05N50Z-D.PDF

Circuit Diagram
http://arduino.cc/playground/uploads/Learning/solenoid_driver.pdf

Blurry Picture of my Circuit (Ask if Unclear)
http://i1176.photobucket.com/albums/x334/raccoonking1/Random%20Uploads/DSC_0309.jpg


Power supply: 16.6V, 18A
Arduino control pin: 13

-PRESTON

shelleycat

The circuit diagram is for a NPN Junction Transistor, that is not the same as a MOSFET.  They do not work in the same way.

A NPN transistor will turn on with a current flowing from the base to emitter.  This will allow a bigger current to flow from the collector to emitter.  The base-emitter voltage will not get much bigger than 0.7V, so an Arduino output pin through a resistor will drive it.

A MOSFET will turn on when the voltage Gate to Source gets above the threshold voltage.  From your datasheet, it gives a figure for the threshold voltage of between 3 and 4.5 volts - depending on each device.  You may just not have enough voltage from a 3.3 V Arduino output pin to turn your device on.  The datasheet shows the device needs 10V to turn full on.

You need an intermediate transistor (NPN or a MOSFET that can be driven by a logic voltage) between the arduino and the power transistor; or change to a power NPN transistor as per the circuit diagram.

Runaway Pancake

#2
May 06, 2012, 02:30 am Last Edit: May 06, 2012, 02:36 am by Runaway Pancake Reason: 1
It's a "logic level" MOSFET, Vgs(th) is about 4V, which may be marginal.
But, Onward.

Quote
Why is there a "ground" and a "solenoid supply ground"? Aren't they the same thing?

If the arduino and the solenoid have separate power supplies, their ground/return should be connected together.

So, what happens if you touch/connect the 1K resistor to arduino's +5 (instead of a digital output)?

Quote
Just so you know, I have tried switching around wires connected to Drain/Source pins just to see if anything would happen but.......nothing.

In this case you probably just forward biased that "internal diode".  But, don't do stuff like that.
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Don't react - Read.
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boingaon

Runaway Pancake,

The Arduino digital pin should be connected to the Base of the transistor. Does the Arduino's ground need to be connected to anything then?

Runaway Pancake


The Arduino digital pin should be connected to the Base of the transistor. Does the Arduino's ground need to be connected to anything then?

The arduino ground and the solenoid power supply ground should be connected together.

When I suggested:
Quote
So, what happens if you touch/connect the 1K resistor to arduino's +5 (instead of a digital output)?

I was trying to get you to check the MOSFET circuit by applying the arduino 5V to the Gate.
Can you try that?
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

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