Need trouble shooting help (pics)

[alsetalokin4017]

"I see the current in question as ".056Ma" when it should have been .056A. The difference between Ma and Amps. 56Ma would have been correct. I knew what you meant."

Ah! I see what you mean. I used the wrong label.

Ah..... no.

Ma is not a usual electronics abbreviation. The capital "M" usually would indicate MEGA, that is, MILLIONs of amperes.

The correct abbreviation for "milliAmps" is mA. Not Ma.

So in your original post where you stated the current, you either misplaced a decimal and/or used the wrong abbreviation. I even quoted and bolded your text, up above, so you could see where you made your error. 

Believe it or not, these little nitpicking things are important, and it does a beginner a real disservice for someone else to "know what you meant" instead of pointing out the error so it doesn't get made again.


As far as reading and interpreting data sheets for transistors... usually on the first page on top you may find the most critical information you really need: the pinout,  the maximum collector-emitter voltage, and the maximum rated collector current. And the usual application information may help you select the proper transistor for your application. A "saturated switch" BJT would be better than an audio amplifier for turning on your motor. And of course a mosfet would suit the actual application better but would teach little about the use of a BJT.

[billbyrd1945]

"I goofed by not giving enough information. The video was https://www.youtube.com/watch?v=8DMZSxS-xVc. And I used the same transistor (PN2222A). The motor I have just happens to draw .056mA"

As far as I can tell, the only time I mentioned a current capacity was small m, large A. But it should've been amps rather than mA as Arthur pointed out. And don't think I don't appreciate your eye for detail. I retired from a medical career where such errors could kill. As to the datasheets, I can read what they say, just not yet sure what it means. But I'll get there. Do you think that the "audio" feature would have any bearing on the pulsations?

[ArthurDent]

In the video I don't see the 4.7K current limiting resistor in series with the +5 VDC and the transistor base. Not having the resistor in the circuit could be very bad for the transistor.

[alsetalokin4017]

One other thing that should be pointed out is this: That little motor when given 12 volts, unloaded and running freely, draws about 54 or 56 mA, well within the max collector current of the small transistors, as you've shown. But at initial startup it will draw more, maybe a lot more, maybe enough to blow or stress the little transistors. Ditto if the motor is heavily loaded or stalled. The initial startup surge will be over quickly once the motor is running at speed so the transistors may survive even if the peak current is over their "absolute max" rating.

The surging you see in your video could be caused by a number of things but my first guess would be the power supply sagging. I'd also change the transistor just on spec.


You will find either mosfets, or BJTs inside the typical compact fluorescent bulb. I've seen both, and same brand bulbs may use either circuit. You can experiment with either type in this present motor circuit.  You will usually find at least 4 diodes of the 1n400x variety or equivalent, the two transistors, a big electrolytic cap and some smaller film caps. I usually don't bother with the resistors or inductors in the CFL circuit but they too can be scavenged if you really need them for something. Be careful not to break the glass, there is a tiny amount of mercury inside.

[alsetalokin4017]

"I goofed by not giving enough information. The video was https://www.youtube.com/watch?v=8DMZSxS-xVc. And I used the same transistor (PN2222A). The motor I have just happens to draw .056mA"

As far as I can tell, the only time I mentioned a current capacity was small m, large A. But it should've been amps rather than mA as Arthur pointed out.

Or you could have moved the decimal three places to the right and said "56 mA" which would have been completely correct.

And don't think I don't appreciate your eye for detail. I retired from a medical career where such errors could kill. As to the datasheets, I can read what they say, just not yet sure what it means. But I'll get there. Do you think that the "audio" feature would have any bearing on the pulsations?

No, it just means the transistor is optimised for rapid oscillation (audio) and varying signal amplitude. A 'saturated switch' type is designed to be fully on or fully off, and wastes slightly less power.

I'm looking into the pulsations now.

[billbyrd1945]

"In the video I don't see the 4.7K current limiting resistor..."
Oh crap! I can't be trusted to do anything! I'll put it in now and try again.

[billbyrd1945]

Interesting. I placed the 4.7K resistor, connected the 12VDC without the 5VDC base voltage. The motor started immediately. That disappointed me because I assumed the previous magic had evaporated. Then, just for kicks, I decided to unplug the power supply and discharge the transistor with my fingers, a maneuver I only learned about a couple of days ago. I reconnected the 12VDC. The motor did not run. I plugged in the 5VDC and everything was as before. Yipee! Then I observed something new: I unplugged the 5VDC and saw the motor continue to run. No, wait a minute, it stopped. I repeated the process several times. One would swear it was not going to stop, but then it would. I decided the wall wort must have some kind of residual energy after being unplugged. Breaking the circuit at the 12VDC produced the instant on/off that I was hoping for. As to the pulsating-- gone.

[ArthurDent]

Leave the 5 VDC supply plugged in and disconnect the +5 lead from the 4.7K resistor. When you touch the +5 to the resistor lead, the motor will start and when you remove it, the motor will stop.

[Richard Crowley]

...discharge the transistor with my fingers...

Transistors do not "hold a charge" like that. Certainly not for low-voltage DC circuits such as yours.

Your power sources are "black boxes" that are not behaving like you imagine they should.

Even one of those $10 volt/amp panel meter things would show you what voltages and currents you are really dealing with.  For example...

https://www.amazon.com/ZRM-Digital-Voltmeter-Ammeter-Display/dp/B07DZTPSLS

[billbyrd1945]

"Leave the 5 VDC supply plugged in and disconnect the +5 lead from the 4.7K resistor."
Right. That's what I meant in my last sentence.

[billbyrd1945]

"Transistors do not "hold a charge" like that."
I didn't mean to imply that. I suspected the 5v wall wort (or is it "wart") was responsible. And even if it is, I'm fine with that. I'm sure the YouTuber didn't intend for anyone to think of unplugging a transformer as a switch in itself. I'm tickled that merely disconnecting the 12VDC (or the 5V wire) provides instant on and off. It's working perfectly and drawing 0.056 AMPS. Finished project photo attached. Thanks to everyone! This means a lot to me!

[alsetalokin4017]

Beautiful! I like the way you used the dowels to hold the connecting posts. All you need now is to find a little 3-prong socket connector so you can easily change transistors to see how different ones behave, and a 10k potentiometer to substitute for the fixed resistor. The idea is to find the greatest resistance that will still allow enough current through to turn on the transistor fully.

I found an ideal "socket" for a transistor on the end of a little cable I scavenged from a discarded CRT television that I found on the roadside. The blue screw blocks from Vellemann are ideal for mosfets in the TO-220 or TO-247 style packages. (The smaller ones need legs bent a bit, the larger ones are exact fit without bending.)

[alsetalokin4017]

...discharge the transistor with my fingers...

Transistors do not "hold a charge" like that. Certainly not for low-voltage DC circuits such as yours.

(snip)

Actually it would be more correct to say that "BJTs don't hold a charge like that."  Mosfets, which are charge-actuated devices, can and do hold a gate charge, staying 'on' (for Nch types) as long as that charge persists. It can indeed be charged or discharged with a finger!

  (the finger charge demonstration action happens after about 5 minutes in)

(Looks like the narrator was confused about lowside/highside switch arrangements, but never mind that.)

But of course bipolar junction transistors don't work like this.

[billbyrd1945]

Too cool! That (one with a pot and connectors) will be my next project. Still going to keep this one for posterity.

[billbyrd1945]

"Transistors do not "hold a charge" like that."

I misunderstood you. I thought you were referring to my thoughts about the wall wart retaining some energy. In reality, you were telling me that the finger shorting doesn't work on BJTs. Uh... yep, I stepped in it again. I had been watching vids about MOSFETS and forgot that I had changed my focus to the transistor project. So I don't know why there was some kind of state change when I did it. Hmm. I probably did something else at the same time and got confused.

[ArthurDent]

I would not replace the fixed 4.7K resistor with a 10K pot as suggested because when the pot is turned to one end there will be approximately zero ohms in series with the transistor base which will have a maximum drop to ground of about 0.7 volt and you could damage the pot.

A better way for speed control is to connect a 1K to 10K pot (it's not that critical) between +5 VDC and your common point and leave the 4.7K resistor in series with the pot wiper and the transistor base. You can now adjust the speed of the motor.

What you will now find is that instead of the transistor acting as a switch and dropping about .7 volts, it is acting as a resistor between the +12 VDC and the motor. When the motor is receiving a low voltage and turning slowly, the transistor will be dropping the majority of the voltage at whatever current the motor is drawing at that speed. This power must be dissipated as heat and the transistor may get quite warm.  That is why audio amps and your bench supply have large heatsinks and often fans to remove the heat from the transistors they use.

[billbyrd1945]

Thanks Arthur. Will do.