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Topic: Driving 6 IR LEDs using a transistor (Read 15582 times) previous topic - next topic

adrian_h

It's connected just as it shows in the diagram.

fungus


It's connected just as it shows in the diagram.


The transistor needs to go between the LEDs and ground.
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dc42

822mV is within the expected range under these conditions. What matters more is the voltage between collector and emitter when the LEDs are on.
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fungus


822mV is within the expected range under these conditions.


According to my datasheet Vbe(sat) is 2V (or more). 822mV isn't enough to turn the transistor fully on.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

adrian_h



It's connected just as it shows in the diagram.


The transistor needs to go between the LEDs and ground.


Sorry, isn't that what the diagram shows?

adrian_h


What I think you are missing is the saturation voltage - Vce(sat) - of the PN2222A. The PN2222A (like the 2N2222A) is a rather poor transistor for medium-current switching because of its high Vce(sat). This is the voltage drop between collector and emitter when the transistor is turned on. More modern transistors such as BC327 or (even better) ZTX851 have lower Vce(sat).

You need to subtract the Vce(sat) from the supply voltage (along with the IR diode voltage drop) when calculating the current through the series resistor.

Also bear in mind that Vce(sat) is usually measured with the base current being 10% of the collector current, and with a 330 ohm base resistor, you are not driving it that hard.



822mV is within the expected range under these conditions. What matters more is the voltage between collector and emitter when the LEDs are on.


Sorry, I'm not that familiar with transistors.  I'm starting to read the book The Art of Electronics 2nd Ed, but am not there yet.  I'm not in school, this is just a hobby.

So the Vce(sat) is 1V, correct?  So I need to push the current on the base to be 10mA to drive 100mA thru the collector/emitter?

fungus


It's connected just as it shows in the diagram.


Oh, you changed it, sorry.

What current do you get if you short circuit the transistor's collector/emitter?
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

dc42



822mV is within the expected range under these conditions.


According to my datasheet Vbe(sat) is 2V (or more). 822mV isn't enough to turn the transistor fully on.


According to Fig. 3 of http://www.fairchildsemi.com/ds/PN/PN2222A.pdf, Vbe(sat) at 270ma (the current I believe he is aiming for) would typically be just over 0.9V, if he were supplying 27mA base current and drawing 270mA collector current. The maximum would be higher. But his base resistor and collector resistors are too high for those currents, so 822mV is reasonable. I agree with you that he isn't turning the transistor on hard enough, he needs a lower-value base resistor.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

adrian_h


What current do you get if you short circuit the transistor's collector/emitter?


34mV

fungus



What current do you get if you short circuit the transistor's collector/emitter?


34mV


That's not current. I mean amps, measured where the (A) is in your circuit.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

adrian_h




What current do you get if you short circuit the transistor's collector/emitter?


34mV


That's not current. I mean amps, measured where the (A) is in your circuit.


Oops, sorry, I had already gave that already so I misunderstood.  It is 40mA.

adrian_h

I'm going to the store to get more resistors.  Suggestions as to what values I should buy?

adrian_h





What current do you get if you short circuit the transistor's collector/emitter?


34mV


That's not current. I mean amps, measured where the (A) is in your circuit.


Oops, sorry, I had already gave that already so I misunderstood.  It is 40mA.

Oh, you mean remove the transistor and measure the current.  It goes to 50mA.

I don't understand, shouldn't it be around 90mA?  I thought USB2 was supposed to give out 100mA.  Nobody is challenging my math, so I'm assuming that that is correct.  What is it that I'm missing?

mjkzz

I think dc42 is right, the part you are missing is the Vce, according to you, Vce(sat) is 1V, you need to substract that from your math. In fact if you do, your 48ma measure is very close to calculation.

Had you used a MOSFET for a this type of load (100ma), you probably will not run into this. Right MOSFET has turn on resistance in milli-ohms range and it is voltage driven, not current. This is why I always advocate using MOSFET instead of "normal" usual transistor.
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runaway_pancake

adrian_h,
Are you sure you have the transistor leads right, looking at the flat/face with leads pointing down it's E-B-C.
330?: orange-orange-brown?
And I thought you were going to run this off Vin.
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

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