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

[Fred Bloggs]

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.

 

[Tam/WB2TT]

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

 

[Martin]

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

That was said tounge in cheek, my apologies if it uspet you
I do have a very limited knowledge of electrical components and their
uses. I know the basics and find it get's me along

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.

Well please allow me to apologise to you once more, if, to you my
questions are beneath you then why lower yourself to reply.

1. The LED's I am using have a DC forward voltage of 1.7 volts
according to the spec I received with them.
I am using 5 in series which equates to a drop of 8.5 volts, now using
a stable voltage of 12volts I would expect to use a resistor of 350
ohms for a current of 0.01amps. But as I mentioned the voltage
fluctuates, with the engine not running the voltage is around 11.7
volts and with the engine running it will be around 14volts.
I have lots of 220 ohm resistors so I am going to use two of those to
give me 440ohms as I feel this is a good enough in between resistance
and will limit the current enough running at the higher voltages.

I do know what I am doing with stable voltages, after all a LED cct is
a pretty damn simple cct to build.
It's fluctuatings ones that I need help with.
And I believe that all the replies I have had have given me all the
help and assistance I require. For which I am eternally gratefull to
the great and the good in this newsgroup.

And no I don't want to drop the project all together, thanks for the
tip though !

If I remember correctly my initial query was mainly to do with the cct
URL I posted as I was unsure of the use of the diode and was wondering
if I needed to incorporate one into my cct, and also if I needed to
use a voltage regulator.

I shall be looking into the use of the LM317 as suggested by Mike
(Activ8)

I am grateful for any and all help received and am truly sorry if I
have offended any of you guys on here with my questions. I did not
know that there was a basics newsgroup, if I had been pointed in that
direction earlier then I would have asked my questions in there.

Once again aplogies if my questions were too dumb for this group.