transistor based pwm generator

[ericgibbs]

Because the voltage across R2 is kept constant Ohm's law that the current through it remains constant so the current will vary between 0/33 and 0.7/33 or 0 to 21mA.

hi hero,
Are you sure about this, if the current thru the LED is to change from 0.7mA to 33mA, then the current thru R2 must also change by almost the same value.

I would also suggest you check the min/max currents you are expecting.

 

[Hero999]

Where did you get 0.7mA to 33mA from?

I think you misread my post, I said the the current will vary between 0/33 to 0.7/33 which is 0 to 21mA.

I know the current through R2 will vary as the LED current changes, I didn't complete the sentence, what I meant to say was:

"Because the voltage across R2 is kept constant, Ohm's law means the current through it remains constant, so the current will vary between 0/33 and 0.7/33 or 0 to 21mA, as the pot is adjusted from 0.59V to 1.4V."

In other words:

Vb = 0.59 to 1.4V

Ve = Vb - 0.7 = 0 to 0.7V

I(R2) = Ve/R2 = 0/33 to 0.7/33 = 0mA to 21mA

I(LED) approximately equals Ie.

 

[aruna1]

You can use a 30k pot (I've never seen such a beast) but you need to increase R3 to 22k if you want to use it.

I wouldn't suggest a higher value, I'd recommend going lower, do you have a 2k or 1k resistor?

You can use 30k if you like but it's bit high because of the loading created by the base current.

The regulator works because the base-emitter voltage is roughly equal to 0.7V and the diodes drop 0.7V each (1.4V in total).

The base voltage of Tr1 is varied between just under 0.59V to 1.4V as the pot is adjusted, the emitter voltage will always be 0.7V below the base so will vary between 0V and 0.7V.

Because the voltage across R2 is kept constant Ohm's law that the current through it remains constant so the current will vary between 0/33 and 0.7/33 or 0 to 21mA.

hmm,I'll try this