Circuit design help... - basic_schematic.jpg (0/1)

[Fred Bloggs]

Martin wrote:

Now I am pretty poor with all this electronics mallarky...

If it's just a bunch of "mallarky" to you then go to some other NG.

but surely
even if you are providing a resistor to limit the current, this
current will not be constant if the voltage fluctuates?
So if my bike runs between say 11.7 and 13.7 volts and I am using 5
LEDs with I would need a variable resistance betwwen 320 and 520 ohms.
That's why I wanted a constant voltage.

Your LEDs have voltage drop- something like 2-3.5V depending on the type
and current- and these add to the IR drop to sum to the battery voltage.
If you don't know what you're doing then either 1) use the first
transistor current regulator circuit link *without modification*, or 2)
drop the project altogether. Your dumb questions belong on the basics
newsgroup anyway.

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[Tam/WB2TT]

"Martin" <me@me.com> wrote in message
news:tjcmb055bcvcqldg09ksi7m3uc977lllo0@4ax.com...

Hmmm... Let's think about that. *If* he wants that effect. Noemally,
one filament lights for brake or signal. If the brakes are on,
signaling what? turns it off? I can't recall. I think the tail light
is always a separate filament from the brake/signal, though.

Now OP has come forth with info that this is for his scooter. Don't
some bikes have a center tail and brake light and outrigger signals?

Hey, isn't management/customer supposed to figure all these
requirements out? Why do I find myself doing systems analysis before
I get to design anything?

Mike,

You are indeed correct, if you look into a stop / tail bulb you will
indeed see 2 seperate filaments. The tail filament is around 5watts
and the Stop filament is around 21watts.
My basic schematic has the tail section being current limitied by a
suitable resistor (to provide a current of say 0.01 amps) then when
the brake is applied another resistor is applied to increase the
current to 0.02.
I wanted a voltage regulator to limit the voltage to a stable 12v so
that the current would be constant.
Now I am pretty poor with all this electronics mallarky but surely
even if you are providing a resistor to limit the current, this
current will not be constant if the voltage fluctuates?
So if my bike runs between say 11.7 and 13.7 volts and I am using 5
LEDs with I would need a variable resistance betwwen 320 and 520 ohms.
That's why I wanted a constant voltage.

Martin,
In your original circuit, the current is independent of battery voltage, and
depends on the base emitter voltage of the bottom transistor. You should
measure the battery voltage with the engine running. I would not expect much
variation from 14V. I think your major variation would be due to Vbe changes
with temperature. I don't see that the diode does anything, that could not
be achieved by changing the value of R3.

Tam
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