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Topic: Problems with Transistor TIP122 (Read 1 time) previous topic - next topic

silverxxx

#11
Jan 21, 2013, 01:58 am Last Edit: Jan 21, 2013, 04:17 am by silverxxx Reason: 1
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I can`t understand this currents. I was expecting 3mA ((5-2)/1000) on the 1st case and 3A (1000(gain)*3mA) on the 2nd and 3rd.



Where do you get 5- 2 from?
Edit: sorry i guess the arduino is powered from a 5v supply. So what I said is probably wrong, except the power dissipation.

the voltage across the darlington is , say, 1.4v, so 3v - 1.4 leaves 1.6mA which is close given i just guessed the emmiter-base voltage.
as for the collector current, I would guess the battery was not able to supply the current.

There is the third thing, say you succeed in drawing 3Amps from 3Volts, now the tip122 dissipates 3*3 =9Watts with no heatsink. Probably melt the breadboard.

I good idea is to add, say,a 10 ohms resistor in the collector circuit, then you can use a voltmeter to measure the current (1V = 100mA). It also functions as current limiter and fuse.


Docedison

Try to remember that measuring transistor current gain in that manner is not recommended.
If you had a current source of sufficient capacity... With NO CURRENT LIMITING.
All you would have succeeded in doing would have been to destroy the transistor...
To see the effects of current gain you must use a transducer of some sort, a resistor is a good beginning and a higher voltage power supply would be required as well.

Bob
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rafaellehmkuhl

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An odd justification. Do you also put an electric heater in the bath just to test?


Ok, I understand what you tryint to tell me. I will make another circuit and came here later. Thanks.

Grumpy_Mike

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the only thing that in annoying me is the currents that does not match.

That is because of two things:-
1) The internal impedance of the batteries is in effect putting a series resistor that limits the current.
2) You are not accounting for the fact that the data sheet gain is not one exact figure but can follow a range.

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I am just testing the transistor.

An odd justification. Do you also put an electric heater in the bath just to test?

rafaellehmkuhl

Yes. In fact, I was doing this circuit with a LED and a Resistor before.
I am not really worried about killing these batteries, the only thing that in annoying me is the currents that does not match.

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