Electronics > Beginners

Need trouble shooting help (pics)

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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:

--- Quote from: billbyrd1945 on February 23, 2019, 09:31:13 pm ---...discharge the transistor with my fingers...
--- End quote ---
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.

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