I was trying to run 2x 3.xv LEDs off my 6v pack but the 6v is nominal, with the
rechargable batteries I'm probably only getting 5.2v and my alkaline batteries
are worn out and only reading 1v each so my problem was probably that I didn't
have enough voltage to run them in parallel.
The problem for aiming is that I didn't really pay attention when I cut the
piece of styrene the LEDs poke through so a couple of them just barely fit. The
original system used a metal bar down the center as a stiffener and ground
lead. I've got a couple LEDs leaning on the bar which makes them shoot off to
the side. On one hand this reduces the overall brightness for reading, on the
other hand it gives me a wider field of view so book placement is less
important.I'm also debating adding another 10 LEDs to the array as well. Need
to take it to camp and give it a shot before I make any decisions though.
3x 1.5v (or 1.2v in the case of rechargeable) cells like I'm using is pretty
much the way all LED flashlights are today...
-Curt
From: fmiser <fmi...@gmail.com>
To: mercedes@okiebenz.com
Cc: Curt Raymond <curtlud...@yahoo.com>
Sent: Monday, January 11, 2016 2:49 PM
Subject: Re: [MBZ] Using LEDs - was OT: Politics are angrier polls
> Curt wrote:
>
> Looking around I found a bag of through hole LEDs I'd
> bought but never used. Dropping one cell from my battery
> pack gives me 3.6v using rechargeable batteries which I
> tested and appeared to be acceptable with my 35ma LEDs so I
> rewired the thing with those.
Again, the LEDs don't really care what the voltage is - so
long as they only have 35 ma passing through. This is NOT
how most electrical devices work.
Using no resistor means you are depending on the source
impedance to limit the current. It so happens that NiCd and
NiMh cells have a much lower internal resistance than
alkaline. Therefore the LEDs will be brighter with
rechargeable than with primary cells - even though if they
are both the same voltage.
> I finished it last night and testing shows that it works
> pretty well for a reading light (its intended purpose)
> although it could stand a little work aiming the LEDs
> better.
Good! If it works, the math is academic. *smiles*
With the through hole LEDs, it _is_ possible to aim
them. If you don't want the focused spot, you may be able to
turn them into very wide angle. I have filed the rounded top
off of many of them to get rid of the lens. If you do that,
be careful to not take off so much you run into the wires
inside.
> I think you missed the point of my diode, I've got a single
> pole single throw 3 position switch. On low it activates
> one string of lights, on high it activates both.
You're right. I missed that. *weak smile*
> Also in my experiments LEDs must be used in parallel
> which matches my understanding of how they work. Because
> they are one way valves the electrical pixies won't pass
> through like they would a normal valve. Certainly when I
> strung them up in series they wouldn't do anything...
The problem with parallel LEDs has to do with the nature of a
diode. Below the voltage threshold, no current flows. Once
the threshold has been exceeded, the diode (LED) will flow
_huge_ amounts of current. It is practically a switch. To
survive, the current must be limited. Either from the
batteries internal resistance or from a resistor etc.
The threshold voltage is a "typical" value. Each device will
vary by a little bit. If there are two 3.6 V LEDs, and one
is actually 3.599 and the other is 3.601, the voltage may
never reach 3.061 because the 5.599 diode has conducted all
the current the limiting resistor has allowed through. If
you engineered the circuit expecting the diodes to share the
current, and one has "robbed" it all, it won't last very long.
That said, the 3.6 V white or blue LEDs seem to behave better
than the 0.7 V single color LEDs in a parallel circuit.
And using battery power without a separate current limiter
does warp the math a bit.
To run them in series, remember to add the voltages, so three
3.6 V LEDs will have a threshold voltage of 10.8 V.
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