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- John Riley
December 1, 2005, 11:18 am
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Hi folks. Sorry for asking what is a dumb newbie question here, but my Forte
Agent is not avialable to me at the moment, and I'm not too familiar with
OE to do much of a search.
I want to convert my granddaughter's 2 x AA alkaline cell torch from
incandescent to LED.
She goes through bulbs and batteries like they are jelly beans.
OK, so I see Dick Smith and others have this bewildering array of LEDs and I
see that many are above 3 volts.
Then I read that effectively, LEDs have no resistance, and so I wonder if I
chose a bright LED with a voltage drop of 3, could I run it straight off the
two alkaline cells? Or is there something else that I would be best doing?
What LED would folks recommend, and where best to get it? Dick, Altronics,
Jaycar? I'm close to all three. I've just got me a wee power supply and a
good DVM, and have a LOT to learn!!! Thanks for any help, I'm being pestered
to fix her fairy zapper light wand :) Parpy the Fixer :)
But that will cost me $18 and all the catalogue (paper and online) tells me
several times is what a great cheap and efficient kit it is. Not a number
to be found.
Anyways, what if I bought one of Dickie's high intensity/super bright LEDs
3V@20mA Z3984 for $4 and soldered it into this torch with two alkaline
This one puts out 16 candela (is that right? 16000mcd?). Will this be
better/worse than the kit? Or is my idea cockeyed and doomed to failure?
Have a look at:
SOme really bright LEDs at good prices. I used one of the brighter ones
from one of these sources in a 3V cycle lam p. VERY long life. I soldered
it into an old lamp socket, taking care to get polarity correct.
See my response to Matt. What do you think?
Did you add any resistance to this circuit?
Do these things (LEDs) have standard numbering?
Or are all these numbers just catalogue numbers
that the stores apply to their stock?
Is the $4 Dickies model reasonable?
I'm in Perth so not able to access the good
The subject of led torches is indeed a fascinating one. Bright white leds,
which are the ones you want, all have a forward voltage above 3V so you
could indeed run it straight off the two cells and not be overdriving it too
much. As the cells rapidly lose their as-new sparkle, the voltage under load
will drop and your led will be under-driven if anything. Use your good DVM
current range to measure the led current in a benchtop mock-up using cells
of various ages. One typical white led I saw on Jaycar had a max current of
30mA and a Vf of 3.2V. Trouble is that as cells age the led current drops
and you won't be getting full brilliance. Not that that bothers makers of el
cheapo led torches who are quite content to sell directly driven leds that
seem to glow forever, but not as brightly as when the cells are fresh.
That's why slightly upmarket 2XAA led torches incorporate a small power
supply board which takes the battery voltage and boosts it so that led is
correctly driven until individual cell voltage falls to practically flat.
The led torch kit from Dickies mentioned by matt2-amstereo below is like
that. (I've built two) Uses two transistors in a multivibrator format to
drive a third switching transistor which rapidly switches current thru a
small inductor. The constantly collapsing magnetic field produces a current
pulse that's stored in a capacitor and powers the led.
Commercially, the Ever-ready Hard Case 2XAA led torch is a beaut (around $22
in Woolies supermarkets) When I took one apart I found a tiny discrete
component circuit board (no chips, thru hole components not smd) that does
the battery volts boosting, though it appears to do job with just two
transistors. I replaced one AA cell with the Dickie Smith kit board
mentioned above and now have a 1XAA Hard case that performs just the same as
another Hard Case that I left intact. They are a durable torch with a thick
ABS plastic case and a focussing lens in front of the led that does lots to
form a nice long distance spot.
If your grand-daughters torch happens to be a genuine US made 2XAA Maglite,
Jaycar sells a Nite Ize 3 led drop-in bulb replacement, Part #ST3400,
$14.75. I can't swear that it has any active voltage booster technology
built into it. The www.niteize.com website does not mention any booster so I
The http://www.candlepowerforums.com/ is a website inhabited by obsessed
nerds constantly in search of the perfect led torch. Fascinating reading if
you want to look into it. But if you just want to get the g-daughter off
your back you could do worse than buy several sorts of white led at a couple
of dollars each, a battery holder for the benchtop mock-up and start
Thanks so much for that Peter, you've clarified a lot for me. What I
couldn't seem to find (prolly coz it's so obvious) was what happens when the
voltage supply to an LED is varied, and what the spec voltage (as when
Dickie only gives one, as opposed to Altronics who give several) actually
meant. If you increase the V over the single Dickie spec, you get more light
until you burn it out. If you supply a lower voltage, you just get less
light. Praps a bit like an incandescent in a way?
(except the curve of this will be considerably different?)
LEDs are not voltage operated devices, they are current operated.
Overvoltage will not kill a LED, overcurrent will. You do not change
the brightness of a LED by changing the supply voltage, you change it
by changing the current through the LED.
A LED has (essentially) a constant voltage drop across it for any given
current, and you must choose a series resistor value based on your
maximum supply voltage.
For example, a LED with a 3V drop powered by a 4.5V battery will need a
75ohm resistor to limit the current to 20mA maximum
This is why cannot (or more accurately, should not) simply connect a
LED to a battery or other voltage source, as there is no resistor to
limit the current.
I realise that it is the current that the diode uses to produce light, but
surely the voltage is what drives the current.
Are you saying that I can't hook up any LED to my power supply and set the
voltage as specified?
I wish I could get my head around this. Yes, if you only have a higher
voltage available, you can use a resistor to cut this voltage down to that
specified, but if you have 3 V available and the LED has a 3 V spec, I'm
having difficulty trying to see why this would not work. Please explain :)
Ok, I'll try!
A power supply (or a battery for that matter) has a low output
resistance, that means it can supply a LOT of current, more than enough
to blow any LED. A standard 5mm LED might be rated at say 20mA maximum,
any more than that and you risk blowing the LED. Now if you connect a
power supply or battery directly to it without a series resistor, you
cannot control how much current it draws. It migh work, or it might
blow, you just don't know. So you need a series resistor to limit the
current to safe maximum value, not just "drop the voltage". The LED
does not "need" 3V, it is not a requirement, it just happnes to "drop"
3V when you pass current though it. The real "requirement spec" for a
diode is how much current it needs.
LEDs are non-linear devices, that means it is not easy to predict how
much current they will draw when you connect a low resistance power
supply to them.
You can easily blow a "3V" LED by putting a low resistance 3V supply
Is that any more help?
Absolutely, thanks so much, Dave.
I see it all now. I came across a site where the author plotted V against I
for a bare diode. The curve was almost vertical at one point (rather too
close to the point where it should be running.) His calculator, when 3V
supply and 3V @ 20mA LED was put in did NOT show "Zero Ohms"
for the resistor, as so many simplistic ones had done, but showed I
should put in a One Ohm resistor. So I guess that the reason for a
resistor in series with the LED is to "flatten" the V against I curve to a
safer, more linear relationship. Your last sentence "You can easily
blow a "3V" LED by putting a low resistance 3V supply across it!"
says it all. Thanks a bunch, cheers, John
ps, I'm of to Dickies in Freo on Sunday for an LED,
resistor and a new soldering iron.
(My old one has had BAD things done to it! DAMHIKT :)
Ive learned a lot in the last few days, thanks to everyone.
Yep, that's exactly what a LED does, so if you have no series
resistance you can quickly go from a small safe current to a very high
current which will blow the LED.
That curve will change between LED/diodes too, and also varies with
The cheap small torches get away with it because they rely on the
internal resistance of small batteries (coin cell lithium types) to
provide the current limiting.
The V/I curve will actually remain the same, what the resistor does is
to limit the maximum current possible for a given supply voltage.
Calculated using ohms law like that example I gave in my first email.
The *best* way to drive a LED is to use a "constant current" source. It
will always provide a fixed current regardless of the supply voltage or
diode V/I curve. The high power luxeon LEDs recommend this method for
driving them. Your top quality torches will have a constant current
driver circuit which will provide a constant current and hence
brightness until the batteries die.
**There's no real mystery with LEDs and torches. Contrary to popular
opinion, the very best LEDs are only marginally more efficient that a good
torch bulb. Where LEDs score, is their tendency to deliver constant light
output, even when the battery Voltage has fallen considerably. Provided you
use a proper drive circuit, of course.
In summary: To get longer life from a torch, you can use an LED, but you
have less light output.
Seems to me after a bit of reading, that the advantages of converting this
toy torch to a 3V LED will be overwhelming. The original globe that
Energiser put in it was 2.2V Krypton, which are not cheap and seem doomed to
blow often. And the 20 mA drain of the LED will last the batteries a lot
longer than the 500 or 600 mA that the incandescent globe will draw. I don't
know how to get a reasonable light out of this thing with incandescents
that will last as long. Seems it's either half an amp or nothing. So I will
stick a white 3 V LED from Dickie into it and see how it pleases her
majesty. Thanks for your advice, BTW
I'd be asking if it's actually worth upgrading at all. You can buy a
LED torch for less than
one of those LED replacement globes. 8+ LED toches go on eBay for $10
or less. I just got a 12 LED one that runs off a single AA battery for
under $20 delivered.
What is the torch used for?
No point upgrading to LED if it's not bright enough or doesn't have the
light angle or throw required etc
You won't get a single white LED to run off 2x AA's without a DC-DC
Thanks Dave. Do you recall that Insurance agent on the telly trying to fake
a little girl's rabbit doll?
"That's not my Bunny!"
Yep, but I want to fix just one LED into the bulb holder.
Yep, but "That's not my Fairy Torch!" :)
Dunno what the specs for Fairy Torches are, sorry :)
Can you tell me why? I must still have a deal to understand about LEDs.
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