What is the Correct Amperage for Driving LEDs?

Go with the manufacturer's spec. The small, single-chip LED's have typically been spec'd at about 20mA since they first hit the market; newer devices can have all manner of stuff buried inside, and may have much higher current ratings. If the spec says 100mA (as long as it's continuously rated) then 100mA it is.

Word of warning: check all the details in the spec - you may need to provide a heatsink to run at the spec'd current.

Thanks for the help. I checked the spec's and as I read it, it shows 100mA continuous with a PD of 500mW. I'm hoping PD refers to pulse duration.

PD is Power Dissipation - it's going to get warm! Check through the application notes for what heatsinking is needed - I'll be surprised if there's none.

There are plenty of high current LEDs out there, but I'd be a little surprised to find one in a 10mm package. But, go with the datasheet. And yes, be sure the 100ma is the "continuous" rating, not the pulse rating.

Thanks for the answer, looking at the spec's it does seem that 100mA is continuous. So here's my next question, my formula to determine resistance in series (Vi-Vo/If=Rs) gives me a value of one ohm (3.6Vi-3.5Vo/.100=1). If my math is right (weak math skills) a one ohm resistor is right for the job.

The other option is to test each LED with 3.6V and vary my resistance until I am at 100mA. What do you think?

Best (easy) way to drive them is with a constant current source - simple to make with e.g. an LM317 regulator (see here for example).

With your supply voltage so near the LED forward voltage, it'll be hard to control the current with just a series resistor. If just using a resistor, I'd suggest working from, say, 5V. That means the resistor is R = (5-3.8)/0.1 = 12Ω. Resistor will dissipate 0.1 x (5-3.8) = 0.12W (120mW).

Great tip on the LM317 regulator. How do I adjust the voltage using the pin?

Once again I agree. I've used the LM317 in current source mode for LEDs, even in a PWM configuration. The problem with using a resistor is that the Vf of the LED, is not exactly 3.4V (or whatever) but varies over current and temperature. Now - you will need more than 3.5V to drive this with an LM317, probably ~5.5V due to the "headroom" requirements of the LM317. If you're truly limited to a lower voltage, you need to find the minimum Vf on the data sheet (for the current you're running at) and choose a resistor based on that voltage. That will protect your LED. The downside is that you may not get the full 100mA. If you'd like to give me the manufacturer and part number I'd be happy to look at the data sheet for you.

Terrific reply. I'm getting so many I'm not sure I can reply to all, but you've really helped me out and made it clear just what to do so I wanted to say thanks.

" ... ignore the voltage and watch the amperage" - that's good advice for any LED.

Grommet,

Does the spec for the led say 0.5 Watt (100,000mcd) white 8mm LED, viewing angle 140 degrees, 100mA at 3.4 to 3.8V?

Do you have a manufacturer's part number for the LEDs?

Wallace and Grommet

Jon

Smile. No that isn't the one. Here's the exact text:

Warm White LED contains 5 chips inside a crystal clear 10mm lens. This is a warm white output color 3000K-4000K (more like an incandescent light).

Specs VF 3.8V,
IV=200,000 to 220,000MCD, viewing angle 45º, PD=500mW
IF=100mA.

This beautiful high output LED glows almost a pale yellow at 3VDC and changes to bright warm white glow at 3.5VDC. Factory prime.

G16659

------------------------------

Cole (aka grommet)

btw I am a huge Wallace and Grommit fan but my name comes from elsewhere :-)

Grommet,

Thanks. It seems lately that it is like pulling teeth to get information to work with to answer questions from folks.

I did notice the difference between Grommit and Grommet.

I am a fan too.

Jon

There is no better way to apply correct current to leds than to have the power supply as used with new batts/cells of correct type to be used in final setup- then knowing max current recomended by led maker- use fully charged cells/batt with series trimpot to get that maximum current- then use fixed resistor for each led- this is the most economical way to run leds safely- forget about boost/oscillator circuits- in every case the power drawn from the supply is excessive- though the leds may be safely modulated to give far higher light outputs!- the bottom line is far more frequent r/p/r/charge p/s cells/batt.!. If a led torch comes with say 3x 1.5v dry cells, do not put in 3x 1.2v NiMh or NiCad cells- the led/s will blow due to reduced voltage drop(the r/c,s don't droop as much)- if using r/c/,s do as outlined above.

Neil,

"If a led torch comes with say 3x 1.5v dry cells, do not put in 3x 1.2v NiMh or NiCad cells- the led/s will blow due to reduced voltage drop(the r/c,s don't droop as much)- if using r/c/,s do as outlined above."

Could you clarify that? Especially the part about substituting NiMh for Alkaline cells and blowing LEDs. And "r/p/r/charge p/s cells/batt" and "r/c,s don't droop as much"

Jon

Simply: the r/c,s (rechargeables) though nominally lower voltage(1.2v to 1,5v) can supply higher current-if as most led torches you buy are set for alk or other dry cell, the series resistor is also- then if you put NiMh or NiCd in- the led/s WILL blow(I have seen this)- the remedy is as I said to insert a slightly higher resistor after test as also said. Droop refers to v olts drop under load. Note that led failure is usually 1 at a time- & also usually where a single resistor is employed to a parallel led combo.

Neil,

Ok, How does a lower voltage battery set (3 x 1.2V = 3.6V) cause an LED to blow when the lower voltage produces less current? New Alkalines 3 x 1.56V = 4.68V provide more current.

A set of LEDs in parallel is real dumb design. Inequality of Diodes causes hogging and kills on at a time.

My multi LED 3 cell torch has no components so I can assume that the LEDs have the current limiters built in. It just gets gradually dimmer as the batteries drain. NiMh cells hold their voltage up longer and then drop near the end og thier lives.

If you could fit Li-Ion cells in place of Alkalines I would believe you would kill the torch instantly. 3 x 3.6V = 10.8V. Lucky they are a different size and don't fit.

Jon

Dear Sir- I have been experimenting with white leds since they became available in 2000- I have made torches long before they became commercially available- it is an ever learning curve as new developments become available- trust me- I know what I am saying, the voltage is not the issue- it is the current that blows leds- this is easily provable by experiment- one of the 1st torches I made had a 9v alk batt- this lasted for many years with 1 led with a series resistor. You do not seem to understand that NiMH & NiCd cells/batts provide higher current than alk or carbon zinc dry cells- despite overall voltage being slightly lower- & again, this can vary between cell makers. It is so easy to get out your dmm on current & check!.

Neil,

So basically you are saying that the alkaline battery is your series resistance because it has a lower output current capability than the NiCad and NiMH cells.

Even if it has the same Amp-hour rating?

Can't recharge my Duracell copper top alkalines.

Let me see, 3 new Alkalines in series is about 4 and a half Volts. Hmmm this new Alkaline is 1.588 Volts. 3 of those would smack my 3.8 maximum Voltage rated LED with 4.74 Volts.

3 charged NiMHs in series is about 3.6 Volts. My NiMHs are all around 1.25 volts

My LED runs on 3.8 Volts.

Now let me see..... Which set of 3 batteries in series do I want to use to make the 3.8 Volt led light up without causing it to draw too much current when new batteries are connected.

Gee I hope I didn't connect one of my LEDs backward. They hate that.

Yes you can recharge your duracell cells- why not get wise & buy energisers from eveready- they will beat duracells always(experiments prove)- I or you or anyone can recharge any dry cell- time & again- but charge will not last more than 3 days usually- however if used in that frame- go for it!. The best recharger is a staggered ac 1/2 wave 2 step forward-1 step back.

Neil,

Recharging non-rechargeable cells - that is dumb. It can make them leak and serves no good purpose in the long run as the materials that made them work can not be restored.

Jon

Neil,

In the old days we could buy a charger designed to restore our Eveready cells.

We got a little more life out of them but..........Is this the answer to the OP's question?

Neil,

You seem to have avoided this and focused on my stupid "recharge my Duracell copper top alkalines." statement.

3 new Alkalines in series is about 4 and a half Volts. I have a new Alkaline that is 1.588 Volts. 3 of those in series would smack my 3.8 Voltage rated LED with 4.74 Volts.

3 charged NiMHs in series is about 3.6 Volts. My NiMHs are all around 1.25 volts. My LEDs run on 3.8 Volts with current limiting resistors.

Which set of 3 batteries in series do I want to use to make the 3.8 Volt led light up without causing it to draw too much current but give optimum light when batteries are connected. Either type, as they settle down to about the same voltage and do a fine job during most of their life.

My guess is that an LED that goes "phhht" when connecting the supply that is in the range of the design, be it 4.5 or 3.6 Volts, isn't caused by the battery type or current capacity.

Jon

Kudukdweller- whatever that is!- why don.t you use a little sense & try a simple experiment- fit your alk cells & use a dmm on dc current to see what the circuit is drawing- then substitute NiMH or NiCd cells (charged- of course!)- & see what the supply now is compared to draw previously- then(if you can remember) repeat down the track- I think you will be unpleasantly surprised! As to your nasty little comment about the futility of recharging dry cells- of course!- BUT- my young friend- if you or I or anyone was stranded out in the bush & we needed a fresh dry cell/s- this is it!. Now you will say"b-b-but we ain"t got ac power out in de wilderness"- well true- but all we need to do is put the dry cell to any source of dc power- eg 12v batt- like to like- for a few seconds- the cell will heat up so rapidly you will drop it! Now while ever that cell is warm- current & volts is as new- & can be repeated time & time again. This subject is closed - I will not respond to someone I perceive as a dummy or just out to liven up their boring existence.

Neil,

I used a single bright white LED with a 50 Ohm resister in series.

3 new alkaline cells in series totaling 4.84 Volts applied to the bright white LED produced 24.8 mA and a nice bright light.

3 fully charged NiMH cells in series totaling 3.83 Volts applied to the bright white LED produced 12.3 mA and the lite was adequate but not as bright as one might expect.

Nothing exciting happened.

According to the discharge characteristics both battery sets would settle to about the same voltage level of around 3.6 Volts for the majority of their useful lives.

You make a good point about emergency recharging of "non-rechargeable" cells. We used to do that in the mid and late 40s as it was not so easy to keep good batteries on hand in those days. We had no electricity so we took them to the neighbor to use their charger that was made for safe slow charging the carbon zinc cells. Then along came the rechargeable Alkaline.

Jon

May I respectfully suggest you use NEW NiMH cells fully charged to match the new alk cells?- they are cheap these days - with up to 2600mAh cap- as to charge NiMH/NiCd cells- the best way is to monitor the voltage- it will rise till fully charged then drop- delta v- the other way is to monitor the temp- as full charge is reached cell temp will rise. I can assure all that using NiMH or NiCd cells in commercial torches WILL burn out led/s- I have personal experience of this with several different torches with multiple leds- in every case removing the burnt out led/s- testing current draw with dmm on dc current after r/p leds- then using trimpot to get safe level- then substituting fixed resistor/s( a few had individual series res for each led- tested ind in these cases)- fixed the problem- there is info on the web I have read saying the same- putting the prob down to voltage droop- ie a set of dry cells can not supply the same current as when new for very long- whereas a set of new fully charged NiMH or NiCd can supply a high current for a far longer time- all my experiments have shown this. I have a meter which can measure cell internal resistance- in every case a dry cell ages far quicker than NiMH,NiCd- or even Li-Ion/Li-Po- the led does not blow straight away- but after a period- 1st comes flicker- then some time later a dull light- then blank. Kind regards from an irascible old Bassid.

Neil,

I used my freshly charged NiMH cells for the experiment. They are Duracell 2650mAh. (AA/HR6/DC1500 MiMH/1.2V)

The Alkalines are Duracell AA/MN1500/LR6/1.5Volts.

I have seen some LEDs go south the way you describe after being fed too much Voltage. Needless to say too much Voltage produces too much current.

I wouldn't use LEDS without protection from over current due to the high initial float Voltage that exceeds the spec of the LEDs.

Jon

They really hate it when you keep a soldering iron on it for like 10 seconds they burn out. Also some people do not know that LEDs are static sensitive see https://www.superbrightleds.com/led_prods.htm

"... forget about boost/oscillator circuits- in every case the power drawn from the supply is excessive-..."

Can you support that statement?

Here is a description of an LED driving PWM circuit giving 1A at 12.6V with an input range of 8V to 18V, claiming 94.8% efficiency at 18V.

That would take some beating with a series resistor.

I am so sick of true believers who believe everything purveyors of whatever say in effort to boost sales- experiment for yourself!. In EVERY case of inverters/converters/boosters there is a power debt- no circuit runs on nothing!. In EVERY case of boost as applied in led torches where 2 cells are used- the boost circuitry uses more power than the actual led where the led is powered by a circuit with a series resistor- try it!- check it!- I have- many times!- I don't know about you, but I believe in maximum return for my buck- & a series resistor circuit gives light far longer than any fancy boost circuit- and- whats more- at a higher light output- I have built boost circuits until I realised they were a dead end. To check light output from led torches I use a solar cell(12v nom) enclosed in shroud so torch tested is always at same distance- so voltage output is directly related to light output of torch- my 7 led torch of 200 mA gives as much light output as a commercial 3W with current draw of 1.4 AMPS! using 2 c cells- boost circuit uses 400mA!. The area I am lacking in at this time is focusing of the led beam(so the light is focused at a say 50m distance-as the 3W torch is)- I have tried magnifying glasses & just about everything else- I think what I am missing is that the led must be a diffused beam to enable to be focussed- as incandescent bulbs are.

Just a bit of history... In days of old, when knights were bold, LEDs were all red, and used for display only. If you wanted to turn one on, you simply put 20 ma through it. Of course today, there are all sorts of colors, using various chemistries (and different currents) and the ones used for illumination are quite a different creature.

So ignore the 20 ma. rule, and follow the manufacturers specifications.

HAVE FUN!!

Bill

grommet:

You should look up the datasheet for the specific LED you purchased for "Absolute Maximum Current", and stay below it. Some LEDs can run higher or lower than 20mA in steady-state (meaning with a constant DC), but, also, you can even run at higher currents if "pulsed" (modulated). Forward voltage is just a characteristic, just use it for the voltage dropping resistor calculation. R= (Vsupply - Vled) / (Iled)

Thanks for the tip. What's your suggestion for pulsing, a timer circuit? Something about an oscillator is buzzing around in my head.

grommet:

Yes, a 555 timer connected as an Astable Multivibrator circuit can be used. This will "strobe" the LED(s) very quickly (this trick was used when LED/Clock Displays were used to get higher brightness).

http://www.national.com/ds/LM/LM555.pdf

Thanks for explaining that. I have a bunch of 555's and 556's laying around. Finally have the chance to play with them :-)

The simple answer is 20ma for "constant current" and for most LEDs. But for LEDs which are sensitive devices you should NOT ignore the spec. The web sight https://www.superbrightleds.com/led_prods.htm has a LED calculator program and much information on LEDs. LEDs on this site are used for all kinds of apps check it out and they are the brightest highest quality LEDs I have used.

Thanks for the tip, I am checking out the website now. Cools stuff :-)