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Topic: Transistor issue (Read 3 times) previous topic - next topic


Hello everyone, first post :)

I'm planning on making a H bridge for reverse motor control, so to get started, I bought a few transistors to play with. For reference, the transistor has BC546 written on its flat part, so shoving that into Google brings up this datasheet:


My first plan (just to get the hang of working with transistors) was to have the positive lead from my 9v battery connected to the collector, the base connected to pin 6 from the arduino, and the emitter connected to one end of the motor. The remaining ends of the motor and the battery both go to ground, which goes to the GND slot in my arduino. I anticipated that switching the battery on without powering the arduino whatsoever would have no effect on the motor, since the lack of current flowing through the base would not allow any current at all to flow through from the collector to the emitter, and then to the motor. Conversely, sending a signal down from the arduino to the base (with the battery on) should have turned the motor on due to the current flowing into the base.

However, I noticed that even when the base was NOT connected to pin 6 and both grounds from the motor and the battery were also not connected to the arduino, (in other words, the arduino is completed separated from the circuit) the motor begins to turn at some frequency when the battery is in the 'on' position. That is, instead of continuous power, the motor turns on, then off, then on, and so forth. I approximate the time between consecutive ons to be between about 0.4 of a second.

This makes absolutely no sense to me, because at school I always thought that no current would flow through from the collector to the emitter if no current flowed through the base. So if anyone could shed some light on this issue, it would be greatly appreciated.

Thanks :) 


Can you show us a schematic?  Your description leaves a lot of questions that can best be answered with a schematic of how you have things hooked up.
Facts just don't care if you ignore them.


Dec 08, 2012, 01:42 pm Last Edit: Dec 09, 2012, 12:21 am by Docedison Reason: 1
I apologize for an incomplete post, I have edited it to make a little more sense
It would help your attempt to have tried some of the examples provided in your IDE.. First. Doing so gives you a familiarity with properly using a port pin, the processor pins are Not indestructible. It's a good idea to follow some of those sometimes silly little exercises - Understand that they are there for a good reason and follow as many as you can find parts to do so...
Do them and understand why they work - This is really important as it is both microcontroller training, it is electronics training, it is Programming training And it is what you need most to work well with the Arduino product.
What I was trying to say is it is a good idea to get some familiarity with basic electronics first. Your connection to the Arduino was not the proper method to drive a transistor which is what Patduino was trying to point out. Your issue with the transistor conducting was most likely a floating base lead that is picking up a stray signal and conducting. This was in part why I advised you to study some basics first. Make a drawing (it's good practice and a good thing to learn to draw schematics) so you can post it here for commentary. I apologize for my rather terse note in the beginning... BUT you still need a little electronics training first. You have some data from an experiment and no way to understand it without some more training. There is an application called Fritzing that can be downloaded free from this forum that will help you with the schematic drawing and for making drawings of breadboards as well.
{Heavily Edited RKJ}

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This makes absolutely no sense to me

Due to interference.


Hi SprinterDrift,

Welcome to the forums, and congrats on your first post...

You'll need to include a 1k or 2.2k resistor inline between the GPIO pin and transistor base to limit current flow.  (These are common values you should be able to use for all of your future projects as you get the hang of them...)

The intermittent operation you describe is most likely caused by the fact that the voltages are floating - not tied together, and varying enough to trigger the transistor switch.

There are 10 types of people in the world, those that understand binary, and those that don't.

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