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Topic: another transistor quesiton (Read 3226 times) previous topic - next topic

robkiller

Ok so i tried to figure this out using all the other posted but have failed.

I have a tip120 (data sheet can be found at mouser elec PN 511-tip120)

I have a motor i am trying to run from the arduino so 5v

the motor is 12v and has max amps of .4amps

How can i use this NPN transistor as a switch. What do my resisters need to be?


The calculations i already tried to do did not work because the 12v always flows weather the 5v is added or not.

Thanks for answer this question again since i know you have other asking this from what i see but i still could not figure it out.


stuart0

#1
Feb 11, 2017, 08:51 am Last Edit: Feb 11, 2017, 10:48 am by stuart0
How can i use this NPN transistor as a switch. What do my resisters need to be?
- Connect the emitter to ground (zero volts).
- Connect the collector to one motor terminal (with the other motor terminal going to the +12 supply).
- Connect a suitable diode from the collector terminal (diode anode) to the other +12V motor terminal (diode cathode).
- Connect the base through a series resistor to one of the Arduino digital outputs.

That transistor is a darlington pair, so you only need a couple of mA base current. Something around the 1k mark should do.

Quote
The calculations i already tried to do did not work because the 12v always flows weather the 5v is added or not.
Show your attempt.

allanhurst

#2
Feb 11, 2017, 09:02 am Last Edit: Feb 11, 2017, 09:06 am by allanhurst
Use the enclosed.

I give 2 solutions - one with a TIP120, and one with a modern logic level mosfet.


The TIP120 will get quite hot as it is dissipating about 1/2 watt.

The IRL1104 won't.  A better solution

regards

Allan

stuart0

The IRL1104...  A better solution
Agreed 100%. I was just about to suggest the same thing but you beat me to it. :)

I still find small signal BJT's useful in various situations. But for something like the OPs application above, modern logic level mosfets are so just far ahead.

allanhurst

If you want variable speed for the motor you could use a PWM output from the arduino, but in that case the diode should be a fast one eg a UF4002.  The 1N400x series are slow recovery and could overheat.

regads

Allan

Grumpy_Mike

Quote
because the 12v always flows
Voltage NEVER flows, only current flows.


allanhurst

There speaks a lecturer.

He's right, of course.

Allan

Grumpy_Mike

Quote
There speaks a lecturer
:)

I have found that unless the right words are used, the thinking often remains muddled. It might sound pedantic if you don't fully appreciate the difference but you would never use this phrase if you had a good handle on things. 

allanhurst

#8
Feb 11, 2017, 09:03 pm Last Edit: Feb 11, 2017, 09:24 pm by allanhurst
Fair enough..

I often used the analogy of water pressure, pipe size (both length and diameter)  and flow rate to get to Ohm's law.

Allan.

robkiller

I am up for using what ever the best overall solution is be its the IRL1104 or something else.  I will look into that one. I will try to get a picture going of what i have read to make sure i am processing it correctly.

Thanks everyone

Wawa

The graph of a TIP120 shows a 0.75volt loss at 400mA motor current (with 1k base resistor).
I don't think that will be a problem with 12volt available (11.25volt left for the motor).
It will dissipate 0.3watt at that current (warm, but not hot).
A mosfet will only be a fraction better (in this case).
Leo..

stuart0

I am up for using what ever the best overall solution is be its the IRL1104 or something else.  I will look into that one. I will try to get a picture going of what i have read to make sure i am processing it correctly.
Yeah show what you've attempted so far, because the TIP120 should definitely work.

The thing about logic level mosfets is just that they are faster, more efficient and easier to drive (than BJTs). If for example you were trying to switch higher currents, or if you wanted to apply rapid PWM to the motor, then the mosfet could have significant advantages.

Having said that however, for simply static switching the motor on and off the TIP120 will still work just fine. So we need to see your original circuit to see what went wrong, otherwise you will just make the same mistake with the mosfet.

robkiller

Thats a great idea.                                                                                +12v to motor pos
                                                                                                         

                                                                                                          collector to motor neg
+5v from arduino--------------220 ohm resister-------220 ohm resister------!<
                                                                                                         emitter to ground

-5v ground


Hope this help and you can follow i could not attached anything.

Wawa

...because the 12v always flows weather the 5v is added or not.
Maybe you have connected the transistor wrong.
Or you have already killed the transistor, because I don't see a kickback diode in your diagram.

Post a real picture of the setup.
Leo..

TomGeorge

Hi,
Welcome to the forum.

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Thanks..  Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

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