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transistor based pwm generator

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aruna1

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Hi guys I need a small PWM generator circuit to control brightness of two LEDs. well I'm looking for a transistor based circuit.any ideas? (i want to make it smaller thats why i didnt choose 555 timer)
thanks
 
You won't make it any smaller, because you need two or more transistors to do PWM, which will take up more room than a 555 timer.

Why not simply use surface mount components?

EDIT:

You do know that basic PWM can be done using a single 555 timer, a capacitor, a couple of diodes and a potentiometer.

Does it really need to be PWM?

A variable constant current source can be made using a single transistor.
 
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hi hero999,
can you tell me how to make this single transistor constant current source?

EDIT
i assume this is the one you are talking about

Current source - Wikipedia, the free encyclopedia



anyway what is the more efficient way of brightness control? using pwm or using constant current source?
i guess its PWM as constant current source burn energy through a resistor:)
 
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hi hero999,
can you tell me how to make this single transistor constant current source?
See the circuit with the opamp. Replace the resistor and zener in series with a potentiometer. That will make it possible to control the voltage over emitter resistors and therefore also the current through collector.
 
See the circuit with the opamp. Replace the resistor and zener in series with a potentiometer. That will make it possible to control the voltage over emitter resistors and therefore also the current through collector.

is op amp necessary?
what about this circuit? it doesnt use op amp
**broken link removed**
 
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That's close but not quite there.

The current through the LOAD will be regulated, not the LED.

Try this.
**broken link removed**

ok,what is the use of diode conected parellel to pot? (top diode)
"The current through the LOAD will be regulated, not the LED"

didn't get that.circuit seems ok led1 is the reference and load is the load
 
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The diodes used in my circuit are not critical, the 1N4148 will do.

The circuit you posted won't be any better than an LED with a resistor connected in series because the transistor is in series with the LOAD, not the LED. Read the Wikipedia article and you'll see why.
 
The diodes used in my circuit are not critical, the 1N4148 will do.

The circuit you posted won't be any better than an LED with a resistor connected in series because the transistor is in series with the LOAD, not the LED. Read the Wikipedia article and you'll see why.

ya ya thats what I'm telling.in above circuit LED work as a replacement for zener diode. current through the load is constant
 
hmm... is this good enough?
 

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hmm... pwm controller or simple transistor based driver????
 
Ok but if you need a real pwm crkt you need to have more components!
6 shmitt trgrs have a foot print of what 14 ????????
 
That will work, you can also buy tiny SMT ICs with only two Schmitt triggers.
 
SMTs are hard to find in local stores

@hero999
i simulated circuit i posted and circuit you posted.
both are working
but i cant control led (load) voltage using the pot. how much i adjust the pot voltage drop in led remain constant
 
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What simulator are you using?

Have you measured the current through the LED when the pot is at different settings?

My only concern was that it might not turn off fully.

This circuit should fix that.
LED Dim..PNG
 
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What simulator are you using?

Have you measured the current through the LED when the pot is at different settings?

My only concern was that it might not turn off fully.

This circuit should fix that.
View attachment 41514
I'm using multisim.

current varies as pot changes.but it does not change voltage drop on LED.I assume as pot changes voltage drop on LEDs should change becase as pot change Vb changes therefore Ve should change making voltage drop on LEDs to vary
 
Simulators tell lies.

The voltage across the LED should vary, but not by much.

The current through the LED should vary and the intensity will vary.

I suggest you build the circuit in real life rather than blindly trusting a simulator.
 
The voltage across the LED should vary, but not by much.

The current through the LED should vary and the intensity will vary.

I suggest you build the circuit in real life rather than blindly trusting a simulator.

ok I'll build it and try.
is it has to be 5k pot?

can i use 30k or 100k (i have 30 and 100k with spindles thats why)

and can you explain me how circuit works?
thanks
 
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.
 
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