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[on:]

Hello everybody,

I've been struggling with this for the last couple of days and I can't seem to figure it out.
I have this circuit on the breadboard: http://s2.postimage.org/xhy2uqg89/1_17_2013_1_55_53_AM.jpg

The 2N5551 transistors provide enough current to BD243C. The amplification is so good that if I don't ground the base of the first 2N5551 with the 10K, the motor starts spinning if I barely touch the plastic of the jumper that leads to base of transistor 1 (Q1 on schematic). About 21V pass through to the motor, out of the 24 that the power source has, which is OK.

Now the funny part. I've connected the ground of the arduino board to the ground on my circuit, and connected the base of Q1 to pin 13 through a 1K resistor. The motor barely spins when pin13 is high. I have 4.3V coming out at the emitter of Q3. I tried removing the 10K resistor - nothing. I tried putting pin13 directly to the base of Q1. No change. This is something simple gone wrong, I thought I knew this much...

Does anybody have any suggestions?
Thank you!

[Reply #1 on:]

Motor should in collector Q3, don't forget diode. Base resistor R1 should be present.
http://ruggedcircuits.com/html/circuit__7.html

[Reply #2 on:]

The motor should be place in the collector side of the Darlington pair .
I don't think you will need both of the 2N5551s, try one with the BD243C

[Reply #3 on:]

Haha, this was lightning fast!

I placed the motor in the right place(like Larry D and Magician recommended - between collector of Q3 and +24V), worked for 1/100s then stopped. I had forgotten pin13 to ----ground sorry BASE of Q1 not ground ---- without resistor, from testing. Killed pin13 on my microcontroller. Oh well...that's how you learn.

I'm curious though...what's the explanation? Why would everything work with the motor between collector and ground when the arduino was not involved? Feeding the base of Q1 with power from my breadboard worked just fine. Hmm...

Thanks a lot guys!

[Reply #4 on:]

NPNs like to sink current, not source it.

[Reply #5 on:]

NPNs like to sink current

But can they be persuaded otherwise?- buy them flowers and stuff?

[Reply #6 on:]

Sure - just takes more effort

[Reply #7 on:]

I placed the motor in the right place, worked for 1/100s then stopped. I had forgotten pin13 to ground without resistor, from testing. Killed pin13 on my microcontroller.

This sentence is really a boggle.

There are multiple problems with your ckt. First, the original circuit is an emitter follower config,
 and you'll not get much more voltage on the motor than Vbase1 - 0.7 - 0.7 - 0.7 = approx 3V.

Secondly, as elsewhere noted, a 3 transistor Darlington probably has too much gain for life in
this universe. Leakage currents alone in Q1 will be multiplied greatly and trigger the motor. You
absolutely need R2 to ground to hold the ckt off with no input.

Thirdly, if you fried pin 13, you connected the motor up wrong, and it's not at all clear from your
"boggle" statement above what you actually did.

It would also help to know how much current the motor draws when running flat out.

I'm curious though...what's the explanation? Why would everything work with the motor between collector and ground when the arduino was not involved? Feeding the base of Q1 with power from my breadboard worked just fine. Hmm...

Did you mean "emitter and ground" here?

[Reply #8 on:]

Here's how I would explain it.  If you want to have your high voltage applied across the motor when you want it to be on,  then you will have voltage around 12 on the top side and near 0 on the bottom side.   You want to avoid having 12 V near the arduino.  So connect your transistor between the bottom end of the motor and the ground, instead of on the high DC voltage side.  That is what you will see in most diagrams.

Think of the transistor as a switch.  If the voltage between the base and emitter is low,  it's turned off.   If you increase the voltage between the base and emitter to 0.7V  and then a bit more,  it is turned on.   You use two transistors to amplify that effect.  I don't know why you'd need three.  There are plenty of examples to look at.  Then you figure out what resistors and diodes you need to regulate the current and avoid overloads and transient effects.

[Reply #9 on:]

That is what you will see in most diagrams.

Like this one

[Reply #10 on:]

Hi everybody,

thanks for the help

@oric_dan - I've read my posts and I see I wrote a bunch of shit there  It was pretty late at night. I've edited the first post (first boggle sentence). I agree didn't make any sense whatsoever.

As far as the second thing you quoted YES, exactly. I meant Emitter and ground. First I had it between emitter of Q3 and ground, then I moved it between collector of Q3 and +24V.

Secondly, as elsewhere noted, a 3 transistor Darlington probably has too much gain for life in this universe.

I agree. I tried adding the second 2N5551 because it appeared there wasn't enough gain with only one, to saturate BD243C. When I added the second 2N5551 and I got the same result I knew I messed up somewhere, and that's when I posted here.

It would also help to know how much current the motor draws when running flat out.

It draws 70mA with no load.



@JimboZA - link doesn't work, sorry.

@michinyon - Makes absolutely perfect sense, thank you!

[Reply #11 on:]

@JimboZA - link doesn't work, sorry.

Crap, sorry about that... I usually check them in Preview grrrr 

Try now