Transistor Current Question

Hello

I was hoping someone could explain how to do this calculation for me. I've got 3 transistors and 5 RGB LEDs, and I need to use the transistor to drive them, as my microcontroller can't source enough current for all of them on one pin.

Because they're pins on RGB LEDs, and I need individual color control, I can't put them in series, so I was going to use 3 resistors per RGB LED, like so:

Vdd ---- | | | | \\ / \\ / V V --- --- | | / / \\ \\ / / \\ \\ | | ------ | |/ IN/\\/---| |\\>

| | ___ _

(Only two LEDs shown because I'm terrible at ascii art. There are actually 5.)

Since the green and blue have a Vf of 3.2, I'm guessing I'm going to have to use a 5V power supply instead of a 3.3V one, because I can't get a VCE small enough to drive them.

If I need to pass 20mA through each LED, with a voltage drop of 3.2, can someone please explain how I calculate the two different resistors (Base and on each LED) I need?

Thanks in advance!

Reply to
Elscimar
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Hello

I was hoping someone could explain how to do this calculation for me. I've got 3 transistors and 5 RGB LEDs, and I need to use the transistor to drive them, as my microcontroller can't source enough current for all of them on one pin.

Because they're pins on RGB LEDs, and I need individual color control, I can't put them in series, so I was going to use 3 resistors per RGB LED, like so:

Vdd ---- | | | | \\ / \\ / V V --- --- | | / / \\ \\ / / \\ \\ | | ------ | |/ IN/\\/---| |\\>

| | ___ _

(Only two LEDs shown because I'm terrible at ascii art. There are actually 5.)

Since the green and blue have a Vf of 3.2, I'm guessing I'm going to have to use a 5V power supply instead of a 3.3V one, because I can't get a VCE small enough to drive them.

If I need to pass 20mA through each LED, with a voltage drop of 3.2, can someone please explain how I calculate the two different resistors (Base and on each LED) I need?

Thanks in advance!

Reply to
Elscimar

E = IR. Or, voltage equals current times resistance.

So you do a bit of algebra and you find that I = E/R, and R = E/I.

If you start with 5V and use up 3.2V in the LED, then you're left with

1.8V. So divide 1.8V by 20mA, and there's your resistance.

Then you need to go back to the LED datasheet and note the variation in voltage over temperature, and any limits to current over temperature. Then you need to chose the resistor so the LED is never getting too much current, no matter what the part of your design temperature range you're going to operate it in.

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

I agree with everything Tim said.

However, one important point that may help clarify things is that the voltage across an LED is nearly constant. It goes up a tiny bit with current, but not nearly as much as resistors. So, you can pretend that the voltage is a constant, called Vf, or the forward voltage.

So, the entire voltage, (which in your case is 5V), must be the sum of the voltage across the resistor, Vr, and the voltage across the LED, Vf. The current is I. You want to find the resistance required for Vr. You know that R = Vr / I. Since Vf is 'constant', then Vr = (5 - Vf). So,

R = (5 - Vf) / I

If you have a few resistors that are 'near' your final value, you can see how much the Vf changes by measuring it as you swap out resistors.

Regards, Bob Monsen

Again, Vf really isn't constant, but the difference in voltage for a small increase in current is logarithmic, rather than linear. So, for a small difference in current dI, the voltage change across a resistor is R*dI, whereas the voltage change across an LED is C*ln(dI), where C is some constant. You never really know the constant, through, so all you can do is assume the change is small compared to the change across the resistor.

Reply to
Bob Monsen

It doesn't change much with current, but it _does_ change with temperature. If you're going to be using your device outdoors in all weathers, you want to check the variation in diode voltage over temperature.

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Saturate the transistor. Calculate the R for each LED for proper current load. etc..

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Reply to
Jamie

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