Re: Current flowing through transistor even when base is shunted to ground.
The V-I characteristics of the base-emitter junction are those of a diode. I.e., at low voltages (very close to 0), there is no current flowing. Once you get to about 0.6-0.7 volts, current flows. Pushing "extra" current through the diode will increase the voltage drop... but not very much! If you look at the curve, it heads ALMOST straight up beyond 0.7V.
So, imagine how high it will be (height = current) by the time the voltage has increased to equal that 5V that you are IMPOSING.
Ans: it will be so high that the diode will fry! :>
You're using the transistor to give you current gain (and increased voltage compliance). You can't source (or sink) the amount of current that the LEDs want/need so use the transistor for "muscle".
You can see what the ACTUAL current is by measuring the voltage drop across the 6R8 resistor and dividing by 6.8.
At the minimum hfe of ~30, that would give you ~75mA of collector current. If 75mA flowing through the LEDs and 6R8 leaves you with ~0.2V to spare, then you're saturating the transistor.
If, OTOH, the LEDs only dropped 2.2V each, you might see a different result; the "excess/residual voltage" appearing across the transistor, increasing the power dissipated in it.
Note that there are other consequences to using a reduced drive... it also reduces the amount of conducted and radiated "noise" from the circuit (assuming you are flashing them).
[In your case, you wouldn't likely be flashing them fast enough to make a noticeable difference. But, imagine you had to drive 100 sets of LEDs. You'd likely drive some subset of them with circuitry SHARED among tham all -- multiplexing. If you drove 10 sets at a time, then moved on to the next set of ten, etc. until every set had been driven... lather, rinse, repeat... and did this fast enough that a person wouldn't see the fact that the LEDs are actually "flashing very very quickly", you start to generate noise that can upset other bits of nearby electronics.
Similarly, if you tried to pulse-width-modulate the drive to give you an "intensity" control, you'd be switching the *single* set of LEDs on and off very quickly to give the illusion of being lit continuously -- albeit at a varying intensity.
Likely NOT getting 150mA -- simply because of the 6R8 (redo the math).
Originally posted by EasyGoing1
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So, imagine how high it will be (height = current) by the time the voltage has increased to equal that 5V that you are IMPOSING.
Ans: it will be so high that the diode will fry! :>
Interesting that simply adding the transistor can complicate the LED circuit that much.
It makes perfect sense of course. As it happens to be, when these LEDs turn on in this application, they are plenty bright enough to where normally, I would not look for any reason to change any of the parameters of the circuit. Based on what you're saying, I'm most likely not giving the LEDs their full 150ma, which is fine because that only means that I'm driving them at a safer current and hence they will last longer and the risk of any of them being damaged from overcurrent is a non-issue.
Based on what you're saying, since the output of pin 4 is roughly 5 volts (minus the .7 for the B-E junction), then when I divide that by 1.7k, I get a current of 2.5ma ... which shouldn't be enough to saturate the transistor.
If, OTOH, the LEDs only dropped 2.2V each, you might see a different result; the "excess/residual voltage" appearing across the transistor, increasing the power dissipated in it.
Note that there are other consequences to using a reduced drive... it also reduces the amount of conducted and radiated "noise" from the circuit (assuming you are flashing them).
[In your case, you wouldn't likely be flashing them fast enough to make a noticeable difference. But, imagine you had to drive 100 sets of LEDs. You'd likely drive some subset of them with circuitry SHARED among tham all -- multiplexing. If you drove 10 sets at a time, then moved on to the next set of ten, etc. until every set had been driven... lather, rinse, repeat... and did this fast enough that a person wouldn't see the fact that the LEDs are actually "flashing very very quickly", you start to generate noise that can upset other bits of nearby electronics.
Similarly, if you tried to pulse-width-modulate the drive to give you an "intensity" control, you'd be switching the *single* set of LEDs on and off very quickly to give the illusion of being lit continuously -- albeit at a varying intensity.
However, the LEDs are plenty bright enough (so much so, that you can't look at them directly without causing some discomfort - but they will be diffused in the final application of the circuit) ... so since I'm only driving the base at 2.5ma (the source is capable of 40ma), does that mean that the LEDs are not getting the full current that I originally intended?
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