Electronics > Beginners
Need trouble shooting help (pics)
billbyrd1945:
When everything is connected, the 12v motor runs only on the 5v. A YouTube video demonstrated the project as a way of understanding BJT transistors. I copied it as faithfully as possible, but can't get it to work. Any help will be appreciated.
Richard Crowley:
When you connect the motor directly to the 12V power supply, does it operate properly?
That means with no other part of the circuit connected to the motor.
Do you have a DMM? Can you measure the amount of current that the motor draws at 12V?
According to this data sheet: http://www.futurlec.com/Transistors/C9014.shtml
The C9014 transistor is only rated for 0.15 A of current.
It seems quite possible that your motor takes more than that.
You may have destroyed the transistor.
If you don't alreday have one, it would be good to have one of those $10 component testers.
Then you could at least check whether the transistor is good.
You didn't indicate the polarity of your 5V power source?
fourtytwo42:
Hello, well as Richard says you need a larger transistor BUT you have another problem and that is the circuit is an emitter follower, VBE of the transistor is about 600mV and can never be negative, so the emitter will not rise more than 600mV below the base, in this case around 4.4V. To allow the emitter to rise further you need a higher base voltage, in your circuit you could dispense with the 5V supply and instead connect the base resistor to your 12V supply.
rstofer:
Using that common collector configuration, the motor voltage will never be higher than the base voltage minus about 0.6V. So, about 4.4V, give or take.
That's a horrible circuit, junk it!
With only the 12V supply: Connect the emitter to PS (-) which we will call ground. Connect the motor between the PS (+) and the collector. Put the diode across the motor with the cathode (bar end) pointing back to the PS (+) (reverse biased).
Use the 4.7k (keep reading) resistor from the +12V (collector) and the base.
There will be limitiations. The maximum base current is (12-0.6) / 4700 or about 2 mA. If the transistor current gain is 100 then the maximum collector current is 200 mA. It's pretty obvious this isn't going to work. Try 1k... Then the maximum base current is (12-0.6)/1000 or 11 mA and if the gain is 100, the maximum collector current is 1.1 Amps.
All assuming of course that the transistor can handle 1.1 Amp of collector current while maintaining a current gain of 100.
For the 2N3055 (big power transistor), the gain at 1 Amps is about 90. So, maybe we have to kick the base current up a bit. Perhaps the resistor should be 470 Ohms.
Now, when it comes time to shut the transistor, we need to pull the base and the 470 Ohm resistor down to 0V. Well, that takes a current of 12V / 470 Ohm = 26 mA. We might use another transistor to do that. Perhaps a 2N3904. We would connect the emitter to ground, the collector to the base of the 2N3055 and then when we apply base voltage through a 1K resistor, a high voltage will turn the 2N3904 ON and this will turn the 2N3055 OFF. When we pull the base of the 2N3904 to 0V, the transistor is off and the 470 Ohm resistor turns the 2N3055 ON.
Something like that!
The base current issue (high base current for big power transistors) is one of the reasons we use MOSFETs.
https://www.onsemi.com/pub/Collateral/2N3055-D.PDF
https://www.onsemi.com/pub/Collateral/2N3903-D.PDF
Richard Crowley:
If we knew which YouTube video you got your information from, we could either:
a) Help you understand what the video was trying to say
or
b) Identify what was wrong with the information on the video(?)
If they really showed using a C9014 transistor then they must have been using a much smaller motor.
We wouldn't typically expect a small plastic TO-92 transistor to be able to control such a large motor.
Transistors have maximum voltage ratings, and maximum current ratings. Above those limits they self-destruct.
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