Re: How does this circuit keep a constant 20ma going to the LED

On Wed, 28 Nov 2007 15:11:05 -0600, Chris W <1qazse4@xxxxxxx> wrote:

John Fields wrote:
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LTSPICE, free at:

http://www.linear.com/designtools/software/





Thanks for the schematic and software link. I found how I can set it up
to graph a voltage vs current for each component, is there a way I can
graph voltage and or current vs time to see how fast the capacitor
charges and switches between LEDs?

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Right click on the schematic, then left click "Edit Simulation Cmd."
Next, click the 'Transient' tab, set the run time for what you want
and click 'OK'. You'll then be back in the schematic and you'll
notice a cursor with a little box attached to it that moves with
your mouse. Place it where you want to and left click your mouse and
you'll notice that it changes to text.

Now, right click and then left click 'Run' and you'll see that as
you move the cursor around in the schematic pane a little voltag
probe will appear whenever the cursor touches a wire and a little
current probe will appear whenever the cursor touches a component.

Left click the voltage probe and a voltage VS time waveform will be
plotted, left click the current probe and current VS time will be
plotted.
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I plan on running this at a constant voltage and will probably want to
add more LEDs to get more light, how much current can I run through
those transistors and still be safe?

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http://www.fairchildsemi.com/ds/2N%2F2N3904.pdf
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Five blue LEDs would be the most I
would use. I could also run them in series and just up the voltage I
guess, but about 12V is the highest I have a handy power supply for.

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Run the simulator with various schenarios to see what works for you.


--
JF
.