LEDs

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 :)

Reply to
John Riley
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dickies have an led torch kit, it uses 1 AA and the circuit board is the size of another AA so you use 1 battery and the circuit to fill a 2aa pen torch It then drives a single white LED at a high frequency to deliver battery friendly lighting.

Reply to
matt2-amstereo

Have a look at:

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and

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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.

Dave

Reply to
David

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

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website does not mention any booster so I imagine not.

The

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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 fiddling.

PH

Reply to
Peter Howard

Thanks Matt 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 cells (3V)? 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?
Reply to
John Riley

Thanks Dave 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 Sydney/Melbourne shops.

Reply to
John Riley

**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.

--
Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson

15 degree viewing angle - a fairly tight beam.

You could try a keychain LED torch first

Only $8. Dickies doubtless have a similar item.

-mark

Reply to
mark jb

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 converter.

Dave :)

Reply to
David L. Jones

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?)

Reply to
John Riley

Thanks Trevor.

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

Reply to
John Riley

Thanks Mark. How do you know it is 15 degrees angle of beam? Is that from the number, or is that the angle that these LEDs all have? Anyway, for a toy "wand", a narrow beam might be quite satisfactory.

Reply to
John Riley

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 ((4.5-3)/20mA)=75ohms. 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.

Dave :)

Reply to
David L. Jones

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.

Reply to
John Riley

Thanks Dave. 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 :)

Reply to
John Riley

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 across it!

Is that any more help?

Dave :)

Reply to
David L. Jones

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.

Reply to
John Riley

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 temperature. 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.

Have fun!

Dave :)

Reply to
David L. Jones

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