Help with transistor problem. TIP120 switching/control circuit for a motor/

Hi all first post on here. Got an Arduino Uno a short while ago and I'm really enjoying making wee simple circuits with it. I am from a programming background but always loved electronics in my technology class in school. I have forgotten pretty much all I know so I am really glad for forums like this.

I hope this is in the right place. The problem I think I am having is with the transistor but the circuit itself is to control the speed of a small DC motor. I am trying to follow the fritz from here http://www.instructables.com/id/Use-Arduino-with-TIP120-transistor-to-control-moto/

What seems to be happening is the motor is on all the time whether I am connected to the Arduino or not. I have changed the program just to setup pin11 and set that pin to LOW but the motor keeps spinning. I am sure I am making a silly mistake somewhere that is bypassing the transistor.

Here is my breadboard setup.

And here is my overall setup.

The yellow wire comes from pin11 on Arduino through 1k resister to the transistor base pin (left or pin 1). Transistor is TIP120 (T0-220).

The positive of the power is connected to the motor connected to the fan. The other pin of the fan is connected to the collector pin (middle or pin 2) via the green wire. Common with this rail is the cathode of the Diode (1N4004).

The rail connected to the emitter has the blue wire in between the emitter pin and the anode of the diode. This blue wire goes to another rail. The ground of the battery pack goes to this rail via one of the black wires and then the other black wire then takes this to the ground pin on the Arduino.

I really am lost. This always seems to be on. I can add the sketch if needed but it really is just setting the pin 11 to 0. Any help would be greatly appreciated. Thanks,

Robert

You have the diod at wrong place

image.jpg

I don't see anything obviously wrong with your circuit, although the diode appears to be mounted differently in the two images. It looks correct to me in the upper one.

Isolate sections of the circuit! If you disconnect the line from Arduino pin 11 to the transistor base, does the motor keep running?

The diode doe not go across the Emitter/Collector pins... it goes "across the motor pins" from the Battery Source, the cathode end, and the Collector pin (middle pin) which is the anode end.

Actually, whether the diode goes across the motor, or across the collector/emitter does not matter (as long as it is oriented the correct way). The diode will protect the transistor from inductive spikes in either configuration. In fact, many transistors (both junction and FET) have such diodes built in to the case.

Edit: The TIP120 is such a transistor and you don’t even need the diode. Here is the equivalent circuit:

tip120.gif

jremington: Actually, whether the diode goes across the motor, or across the collector/emitter does not matter (as long as it is oriented the correct way). The diode will protect the transistor from inductive spikes in either configuration. In fact, many transistors (both junction and FET) have such diodes built in to the case.

Edit: The TIP120 is such a transistor and you don't even need the diode. Here is the equivalent circuit:

NO You are wrong The diode should go across the motor. The built in diode don't protect for overvoltage. Look at the way it's oriented.

Back to school. Pelle

jremington: Actually, whether the diode goes across the motor, or across the collector/emitter does not matter (as long as it is oriented the correct way).

NO NO NO! The diode will NOT protect the transistor from inductive spikes if it is connected between collector and emitter. It MUST be connected across the load.

Perhaps you are confusing the situation of a motor switched by a single transistor (as here) with a motor switched by an H-bridge. In an H-bridge, the diodes ARE connected between collector and emitter - or more commonly, mosfets are used and the mosfet body diodes function as flyback diodes. This works because when the upper transistor or mosfet turns off, the lower diode acts as the flyback diode - and vice versa.

I stand corrected. I was thinking of the H-bridge situation.

Here is my second attempt. I still have the same issue. I must be doing something fundamentally wrong. I think fritzing diagram might be easier to see how it is setup rather than photo.

jremington: Isolate sections of the circuit! If you disconnect the line from Arduino pin 11 to the transistor base, does the motor keep running?

yes the motor still runs regardless of the pin11 being connected.

jremington yes the motor still runs regardless of the pin11 being connected.

And if you connect the base of the transistor to ground does it still run? If so the transistor is shot.

Grumpy_Mike:

jremington yes the motor still runs regardless of the pin11 being connected.

And if you connect the base of the transistor to ground does it still run? If so the transistor is shot.

I removed the resistor and wire going to Arduino. I replaced this with pin to ground. Still the same. This is brand new component. I bought pack of 5 and they are all the same. What is the minimum test circuit I could do to prove these components are not working. This still could just be simple mistake by me that is not clear on the fritz

What is the minimum test circuit I could do to prove these components are not working.

You have it here.
Maybe replace the motor with an LED and resistor but that’s it.

It turns out the first two of the 5 I bought were knackered! 3rd one I tried worked perfect!

Folks thanks for all your help. It is much appreciated. I am going to test the other two and get back to the supplier!

Here is a nice simple tester, including (about halfway down the page) instructions for testing a transistor.

JimboZA: Here is a nice simple tester, including (about halfway down the page) instructions for testing a transistor.

Thanks for posting that. It looks like a very useful page!

The supplier is sending out two more with stamped envelope to return old ones so that is fair enough.

It’s fair yes… but good business would be to give you a voucher for a nice discount off your next purchase.

but good business would be to give you a voucher for a nice discount off your next purchase.

Except in the UK where this sort of thing is illegal, it is something called the "Sale of goods act".

The seller must repair or replace the goods within a reasonable amount of time, incurring all costs necessary to perform this task.

from http://en.wikipedia.org/wiki/Sale_of_Goods_Act_1979

My belief is that they probably worked before you got the wiring right.