12 ohms - 6volts?

A question for experienced LED modders (or EEs):

I peeked inside the lamp assembly of my streamlight 4aa 7LED and saw 12 ohm resistors! I assume 7 leds are in parallel because 6v would not drive a series circuit with 7 leds (3.6v drop each.) Could it be a combo circuit?

I don't want to take this assembly apart - this is one of my favorite lights.
Can anyone shed some light on this?

Is there a formula that takes into consideration the maximum current that can be delivered by 4AA batteries? (I'm sure that this must have something to do with the 12 ohm resistors)

I would guess that each LED has one 12 ohm res. in series with it, and that all 7 of those are in parallel getting 6v each.

The problem here is this:
according to ohm's law, to drive a Bright-White LED properly under 6v, you'd use a 130ohm resistor (or as close as you can come with a 5% resistor) assuming a 3.4v drop on the LED, and 20ma of current.

Now if you drive a LED with 6volts and use a 12ohm resistor, the current draw would be MUCH bigger than 20milliamps!
Actually- it would be enough to fry the little fellow.

Naww. Don't assume 20mA of current. Some of those 5mm LEDs can handle 90-100mA w/o too much trouble. Overdrives of 60-70mA are actually pretty common. A better assumption is probably 50 - 60mA, however they appear to be driving these pretty hard. Also, remember that the 6v under load will more likely be about 4.8v in reality. It looks like they may be driving these at about 100 - 120mA.

The negative lead of most 5mm LEDs acts as a heatsink, so as long as there's enough thermal mass to dissipate the heat, ODing the LEDs to a certain point won't really hurt them much, if at all. Now if you go too far, their life expectancy goes down dramatically.

I had read here somewhere that the Streamlight 4 AA LED had different revisions.
Mine has 22 ohm resistors. I didn't know the earlier models had 12 ohm.

Mine drives the LEDs at ~83 mA each on fresh alkalines.

83ma..... that's overdriving them quite a bit... unless they use LEDs that aren't available to the general public....


Everything I've read out on the web says 30ma is overdriving the LED.....

One way to test this is to get a few LEDs and push them until they pop.... /ubbthreads/images/graemlins/smile.gif

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LEDmodMan said:
Naww. Don't assume 20mA of current. Some of those 5mm LEDs can handle 90-100mA w/o too much trouble. Overdrives of 60-70mA are actually pretty common. A better assumption is probably 50 - 60mA, however they appear to be driving these pretty hard. Also, remember that the 6v under load will more likely be about 4.8v in reality. It looks like they may be driving these at about 100 - 120mA.

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Just a thought...... perhaps they use the smaller resistor so they can drain the batteries to a lower voltage before the LEDs go out?

I put a dummy AA in place of one of the cells in mine, so overdriving isn't an issue with the one I have. Still plenty bright and useful.

One thing you'll learn here is that a lot of us like to crazy overdrive LEDs!!! /ubbthreads/images/graemlins/cool.gif

Everything else you've read out on the web is about to change! If anyone out there knows how to overdrive lighting products, it's the people here!

To see how, check out posts about Mr. Bulks's Space Needle II's or the MR-X Hotlips. They're driving 5W Luxeons at 9v upwards of 2A!!! /ubbthreads/images/graemlins/ooo.gif

Actually, most good 5mm LEDs (like Nichias) are rated up to 30mA. Driving them at 100 - 120mA is not too unheard of.

Well, I've got 10 Nichias on the way for some experimenting- let's see what we can do with them. /ubbthreads/images/graemlins/wink.gif

12 ohms is the original value as I recall. One for each LED. There is an interum fix that adds a one ohm (?) in series with the lot on the back of the lamp assembly (one lead to the brass rivets is broken to insert it). Current ones (no pun intended) use 22 ohms (still a serious overdrive with fresh alkalines).

For sure you're in the shadow land of diminshiing returns, that is you don't get twice the light at 60 you do at 30, but it is brighter.

I run mine on NiMH (a fairly flat 4.8 Volts) and am very happy.

I believe PT will upgrade you for free if you care to send it in.

Doug Owen

I have NiMH AAs around.... perhaps I'll switch over- it's one of the "Most reached for" flashlights in my house. /ubbthreads/images/graemlins/smile.gif

What I'm wondering is- will the lower value resistors make the light work longer on a set of batteries?

I'm not sure how 'overdriven' it is, but I recently put 10 Nichia LEDs into one of those 6 AA Double Barrel Energizer flashlights [it is two sets of 3 AA's running in parallel] running with no resistors. Overdriving? Probably so. Do I care? Probably not. /ubbthreads/images/graemlins/grin.gif

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Orion said:
I'm not sure how 'overdriven' it is, but I recently put 10 Nichia LEDs into one of those 6 AA Double Barrel Energizer flashlights [it is two sets of 3 AA's running in parallel] running with no resistors. Overdriving? Probably so. Do I care? Probably not. /ubbthreads/images/graemlins/grin.gif

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Great idea for a Mod! Thanks- I'll have to look for one of those next time I'm out and about.

/ubbthreads/images/graemlins/grin.gif

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charliek said:
I have NiMH AAs around.... perhaps I'll switch over- it's one of the "Most reached for" flashlights in my house. /ubbthreads/images/graemlins/smile.gif

What I'm wondering is- will the lower value resistors make the light work longer on a set of batteries?

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Just the opposite.

Lower resistors will allow more current, the LEDs will be brighter. They will be hotter and use the battery sooner. Shorter run time.

More resistance (like 12 to 22 ohms) lowers the current, power and light, raising the run time.

It's also not linear. Doubling the current gives less than twice the light (higher LED losses) and less than half the run time (lower total energy from the battery at higher rates). More over your eyes see on a log scale, it takes four times the light to seem twice as bright. Kind of a loosing deal, really.

Doug Owen

So, given a 4cell battery pack and four LEDs to drive, what resistors would you choose? 20Ohm or 80Ohm?

Correction to my post above. I have 12 Nichia LEDs in that light, not 10. /ubbthreads/images/graemlins/twakfl.gif

White LEDs? 4 in series? What kind of batteries? You need 3.4v per white LED in series. If they are in parallel, I would use 4.8v (niCad or NiMH) - 3.4v = 1.4v . Then 1.4v divided by 30ma (.030) = 46.67 Ohm. For alkaline or other batteries do the same calc with the proper battery voltage.
I wouldn't worry about the alkalines voltage myself, IMO, 50 or 60 ma is not going to zap the LED anytime soon. If it 50ma affects the life at all, it hasn't shown up on diodes I've been using - some of which are over 10 years old.

I don't think the 12 or 22 ohm LEDs are intended to limit the current UNLESS there is thermal runaway. A small resistance (I use 10 ohm) is usually enough to prevent thermal runaway. If you stay under 100 ma., you probably won't see thermal runaway anyway. I think I had one out of the last several hundred diodes I used flash at less than 100ma. (right around 100ma., actually) which is what caused me to occasionally use a 10 Ohm resister........