How to calculate direct drive (need info for P7)

Hey everybody. Need info on how to calculate direct drive for a P7 (M*g host). Do I need resistors? What batteries are best/least expensive? I'm still very new to this and would like some help. As always, thank you all in advance for your help.

https://www.candlepowerforums.com/threads/195358

I hate to be that guy, but this has all been covered very well lately. You should try searching.

Short answer: No, you can DD off a single lithium ion or 3 NiMH batteries. No resistors required.

Thanks Drewfus2101, yeah, I've read that thread before, it talks about drivers, but I didn't see on where it talks about direct drive. I know there's more that just hooking up some batteries, like volts, mAh, I just don't want to blow up a P7 when trying this. Thanks.

I think from memory the P7 has about 4.2 volt? so if you drive direct with 18650 bat it should be ok.

Thats how i mode my Wf 600 and it work fine until now.

Edit: try this handy tool if it helps . Founded on the net and thanks to the developer:naughty:
http://www.megaupload.com/?d=4RKGWS6X

LED Pro by Jtice

Link to John's Site

chitoz28 said:

Thanks Drewfus2101, yeah, I've read that thread before, it talks about drivers, but I didn't see on where it talks about direct drive. I know there's more that just hooking up some batteries, like volts, mAh, I just don't want to blow up a P7 when trying this. Thanks.

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There's not really much else to it... 3 Ni-MH batteries, 1 P7 and you have light.

chitoz28 said:

Thanks Drewfus2101, yeah, I've read that thread before, it talks about drivers, but I didn't see on where it talks about direct drive. I know there's more that just hooking up some batteries, like volts, mAh, I just don't want to blow up a P7 when trying this. Thanks.

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I did some direct drive Mags and my direct driven Tiablo A8 before I started using 'drivers':
https://www.candlepowerforums.com/threads/192740
https://www.candlepowerforums.com/threads/194753

As long as you limit yourself to three NiMH cells or one LiION cell, max 18650 or C-size, you are ok without resistors. The current from three NiMH can vary widely, depending on internal resistance, quality and overall condition I guess. Even if you hook up D size NiMH that initially might provide up to ~4A it should go down quick with direct drive. I recommend those drivers for getting flatter regulation etc.
Stefan

This might be of some help to you. I bought a CSXPI-P7 from 4sevens. I also bought a D cell MagLite heat sink from him. The "I" voltage range of this LED is from 3.25-3.5 volts. At work I am lucky enough to have access to a power supply with adjustable voltage and current, that has digital readouts for both. I turned the current all the way up and the voltage all the way down. It was a simple matter of dialing up the voltage until I hit 2.8 amps. My particular LED hit the magic number at 3.289 volts. So that's my forward voltage.

From another post by StefanFS, 2.5 amps has as much output to the eye but with a lot less heat, seemed like a good idea to me. LED's are logorhythmic about the current they draw. The difference between 2.8 and 2.5 amps on my LED was only 0.078 volts! That is to say they are very touchy.

I went the resistor route like you're thinking of doing. I put two 0.47 ohm 5 watt Radio Shack power resistors in parallel to make a single 0.235 ohm resistor. I don't have anything to measure that low, that's just math. Three brand new D cell alkalines in my shinny new MagLite read 3.3 volts. That's close enough for me. I figure by the time the Sandwich Shoppe comes up with a 2.8 amp constant current regulator, I'll be able to afford 75 bucks for a set of D cell NiMH's and a charger.

I only have an old analog volt meter at home with a 250 ma current scale. I had to infer current from the voltage readings I got at work. Just to test direct drive mode, I put the clip leads from the resistor directly to the LED and it read 3.5 volts. Way too much for my LED. Guys that use direct drive I think are using KaiDomains CSWOJ that has a 3.5-3.75 volt range.

Well at long last the short answer is if you want to go direct drive, you need a good DMM with a 10 amp current scale. Amazon has one for less than 10 bucks with free shipping. Much cheaper than a second attempt with another LED.

My direct drive was with a PhotonFanatic CSWOI, hence the 4.1A draw on fresh batteries and then quickly dropping to mid 3A range. No heat issues or apparent colour shift, it was still perfectly white.

Al Combs said:

The difference between 2.8 and 2.5 amps on my LED was only 0.078 volts! That is to say they are very touchy.

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And for this reason, the burden voltage of any ammeter in series with an LED will make an enormous difference to the measured current.

I would love to be wrong, but I have never heard of any meter, digital or analog, which has a burden voltage of less than 0.1V on its current scale***.

Most of them are around the 0.15 - 0.25V mark at FSD.

Since LEDs are non linear, this burden voltage makes a HUGE difference in the current reading - the current when the meter is not in circuit will be a lot higher than the reading. Even if your meter is perfectly accurate.

The only way to tackle this problem is by doing exactly what Al Combs has done:

1) Measure (or predict from published data) the Vf at your desired current.

2) Measure the voltage your batteries will deliver into a dummy load at your desired current. This can be done with perfect accuracy, provided your curent meter stays in-circuit while you measure the output voltage with a high-resistance voltmeter (eg any DMM).

3) Calculate the difference between the voltages, divide this difference by your desired current, and there's your required resistance.

4) Subtract, from this calculated value, your estimate of what you think is the internal resistance already built into the batteries/connections/wiring/etc. A figure of 0.2 ohms is typical for a Maglite, I read on this forum.

5) If you now calculate zero, or less than zero, you are just doing direct drive. Otherwise, mutiply your reduced resistance (in ohms) by the square of your desired curent (in amps) to get the required wattage (in watts) for your dropper resistor. Small resistors can be parallelled to get the value or wattage right. This resistor then is spliced into the positive lead.


***The 0.1V champion? You might not believe me, but it's ye olde 500-ohms-per-volt Avo Model 7. Yes, really. But even this will give erroneous readings, as even a tiny burden voltage will affect the reading hugely.

lctorana said:

And for this reason, the burden voltage of any ammeter in series with an LED will make an enormous difference to the measured current.

I would love to be wrong, but I have never heard of any meter, digital or analog, which has a burden voltage of less than 0.1V on its current scale***.

Most of them are around the 0.15 - 0.25V mark at FSD.

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An excellent point about the amp meter shunt changing the results. Kind of a low tech uncertainty principle. Actually I had two meters when I did my testing with resistors at home. I was able to measure about a 0.1 volt rise after the removal of the meter I was using to check current. I interpolated my results based on a voltage reading. But now that I think of it, the $500 power supply I used at work also had an amp meter in it. All this seemed like too much detail for my point to get lost in.

Really what I was trying to say is failing a proper constant current regulator, the 0.25 volt range these emitters fall within is more than enough at the very least to greatly shorten the life of the LED. If not all together fry it. Direct drive is always a little risky without know the particulars of the LED you are using.

There was a whole thread I saw here a while back about DMM's for under 10 bucks. A excellent investment for any experimenter.

WOW, sounds like direct drive isn't as simple as hooking up some batteries to an LED, but knowing what that particular LED needs. Sounds like it would be easier to just use some drivers. Since I'm still very new, what is a DMM? As always, thanks to everybody for their input on this subject, Richard.

DMM = Digital MultiMeter

Direct drive can be as simple as batteries + LED = light, it just depends on how long you want the light to last. I think the P7 is rated at 50,000 hours when run at 2.8A, so running it harder is going to reduce the life of the LED... How much it reduces it by, you can't say, but how many hours do you expect to use the light before you upgrade or some super new LED comes out?

I know i'll only use mine maybe 100hrs, if that... So i'm happy to give my little P7 some punishment

chitoz28 said:

Sounds like it would be easier to just use some drivers.

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I disagree.

Battery, switch, resistor, LED. Perhaps without the resistor.

darkzero said:

LED Pro by Jtice

Link to John's Site

Click to expand...

Thanks for the tip. A lot easier than using a calculator.

KrisP said:

DMM = Digital MultiMeter

Direct drive can be as simple as batteries + LED = light, it just depends on how long you want the light to last. I think the P7 is rated at 50,000 hours when run at 2.8A, so running it harder is going to reduce the life of the LED... How much it reduces it by, you can't say, but how many hours do you expect to use the light before you upgrade or some super new LED comes out?

I know i'll only use mine maybe 100hrs, if that... So i'm happy to give my little P7 some punishment

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You're probably right about the 100 hours. I can see myself popping this LED off as soon as they come out with whatever the next big one is. More likely, it'll be a whole new light. Maybe it'll be a 1,200 lumen F Bin. Sorry, just daydreaming. I have no idea what they have in the pipe.

You mentioned getting 4.1 amps with your CSWOI direct drive setup. What kind of batteries did you use? Were they Alkaline or NiMH? If you tried both I was just curious about the difference.

One of the tests I did when I first took the D cell alkalines out of the package was to stick a 1 ohm resistor across them just to see how much they could put out. Brand new Duracells dropped from 4.75 volts no load, down to 3.35 volts with my resistor. I'm guessing those 10 Amp hour NiMH's hold their voltage a lot better than alkalines do.

I've only used Ni-MH cells. They are the cheap Tenergy 10,000mAh model. I've been looking at replacing them with Accupower LSD cells but the price is ridiculous to get them over here.

KrisP said:

I've only used Ni-MH cells. They are the cheap Tenergy 10,000mAh model. I've been looking at replacing them with Accupower LSD cells but the price is ridiculous to get them over here.

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Thanks for the info. Yeah those Accupower LSD's are $24 a pair. You can buy a set of 4xAA eneloops for 10 bucks. I didn't realize D cells were 5 times as expensive as the AA's until I built this P7 MagLite.

I saw a set of 8 Tenergy or was it Powerex, plus a generic version of the Cross charger for $75. Think I'm going to buy that next. But then again, I could buy 2 pairs of the LSD's plus the Chinese generic Cross for the same price.:thinking:

If I was to buy Accupower locally (from Australia), they're $41USD a pair plus postage. If I was to buy from Thomas Distributing, they're $24USD plus $40USD postage

KrisP said:

If I was to buy Accupower locally (from Australia), they're $41USD a pair plus postage. If I was to buy from Thomas Distributing, they're $24USD plus $40USD postage

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Wow, $40 for shipping, that's outrageous. I guess Tenergy is OK at those prices.