Re: Powering LED's
- From: "Tom Biasi" <tombiasi@xxxxxxxxxxxxxxxxxxx>
- Date: Sun, 15 May 2005 05:48:16 -0400
<upgrdman@xxxxxxxxxxxxxx> wrote in message
news:1116148378.343595.316750@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> I'm working on making a very bright set of "headlights" for my
> remote-controlled car. I decided to go with white LED's because my RC
> car is power'd by a nitro engine that produces some good vibration
> which would probably make incandesent lights not last very long.
>
> I bought 20 white LED's which are spec'd for a forward voltage of 3.0V
> min, and 3.8V max. I read that I should wire them in series, and not
> wire them in parallel, but I'm planning to power them with 3 NiMH AA's
> (3x1.2v = 3.8V). Since that would be the same as the max forward
> current, can I get away with parallel wiring, and not using resistors?
>
> I am new to electronics, and from what I have read online, it seems
> that resistors are used when your power source produces more voltage
> than your electronics can handle. So if I am correct, 3 NiMH AA cells
> should produce a tolerable voltage at all times (perhaps too little
> when they get drained). So am I correct in assuming that I can wire all
> the LED's in parallel, without resistors, to the 3 NiMH AA's?
>
> For more information about the LED's I bought, the eBay page for them
> has tables of info on them:
>
> http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=66952&item=7514693502&rd=1
>
> Lastly, I'm curious about how long my 3 NiMH AA's will provide good
> power to the LED's. It looks like my LED's will be drawing 20mA each,
> and I plan to use all 20 LED's, so that will be 400mA total. My NiHM
> AA's are 2400mAh, so my first assumption is that they would last about
> 6 hours. But then I realized that would be about 6 hours to fully drain
> my batteries. So this leads me to the question of when my batteries
> will start to get to the minimum forward volage for my LED's: 3.0V. I
> have no idea how to calculate that... any tips?
>
> Thank you,
> --Farrell F.
Hi,
Your LEDs are speced 3.0 to 3.8 Fwd. Wiring them in parallel means they
would all have to drop the same voltage. There is no guarantee that this
will be the case, in fact they most likely wouldn't be.
So, no, do not wire them in parallel, and for the same reason as stated you
would still be better off with a resistor otherwise you are relying solely
on internal resistances.
Since you do not have a lot of 'extra' voltage a small value (39 ohms)
could be used on each LED then you could wire the combo's in parallel.
To know more accurately than you have calculated the expected useful time of
the lighting system, you would need to know two things.
1. The current at which the LEDs produce a light that is not acceptable to
you.
2. The discharge curve of your power cells at the current that you are
using.
BTW: You seem to imply that you believe that the 3.0 to 3.8 volts are
operating ranges. This is not the case. This is the range of the expected
voltage drop across the LED when it is fully conducting.
Good Luck,
Tom
>
.
- References:
- Powering LED's
- From: upgrdman
- Powering LED's
- Prev by Date: Powering LED's
- Next by Date: Re: Beginners question
- Previous by thread: Powering LED's
- Next by thread: Re: Powering LED's
- Index(es):
Relevant Pages
|
