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  1. #1
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    Getting 5 amps when running a p7 direct drive

    After recent discussion on running a p7 direct drive with a 3x18650 battery pack I wired a p7 and heat sink directly to a 3x 18650 battery pack and got a 5 amps reading using my clamp meter.

    With my ammeter in series I then got 3 amps and proved this was correct using the clamp meter around one of the leads.

    Looks like the resistance of the ammeter and leads is dropping the current significantly.

    Maybe we need to rethink running a p7 direct drive with a 3x18650 battery pack??????

  2. #2
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    Each 18650 gives you around 1800ma's depending on the make, although this drops off the more you have wired up, you'll need a driver in short.

  3. #3
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    What are you looking for MrAb I can build you a 3.75V 7.8Ah with a 5A protection circuit fitted.

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    I am not looking for a battery pack, its just a note to anyone thinking of building a direct drive p7 that the current will be higher than the reccomended 2.8 amps.

    Do you do battery packs that will limit the current to 2.8 amps?

  5. #5
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    Were you using the high amp setting? The 10A (or 20A) setting should have very little resistance and shouldn't affect the current that much. It could also be the meter leads are introducing too much resistance. Many meters come with thin gauge leads which could easily add a few tenths of ohms and affect the current.

    Anyway, the measurement error caused by the series ammeter is besides the point. Turveyd is right, you'll need a driver, or at least a resistor, to limit the current.

  6. #6
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    I am using the 20A setting, its a fluke rms multimeter and the leads are not that thin.
    I agree 5A is too high direct drive so beware if you are planning on building a direct drive p7.

  7. #7
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    After recent discussion on running a p7 direct drive with a 3x18650 battery pack I wired a p7 and heat sink directly to a 3x 18650 battery pack and got a 5 amps reading using my clamp meter.

    With my ammeter in series I then got 3 amps and proved this was correct using the clamp meter around one of the leads.

    Looks like the resistance of the ammeter and leads is dropping the current significantly.

    Maybe we need to rethink running a p7 direct drive with a 3x18650 battery pack??????
    Correct. The resistance of the ammeter and leads can significantly drop the current in a single P7 circuit. This was all discussed in the thread from 4 weeks ago, entitled "Running a P7 direct drive". To quote from that thread :

    Your easiest and best solution is probably to use a series resistor to limit and control the current ...
    I strongly do not recommend connecting your P7 to a fully charged cell, and even less to several parallel connected cells. You will almost certainly over-current the led, perhaps with fatal consequences. Attempting to measure the current when you do this is likely to give erroneously low readings, because the lead resistance and voltage burden of the multimeter will significantly reduce the current.
    Calculate resistor value as follows ...
    I own a digital clamp meter, will this give an accurate current measurement?
    Yes, that should be fine.
    Now let's look at how much error could be introduced by insering a typical multimeter into the circuit in a misguided attempt to "accurately" measure the current ...
    This is a classic measurment error. The insertion of the multimeter has dramatically altered the very current that we were trying to measure ....
    My advice to AB remains unchanged. Use a clamp on ammeter, wrapping several turns through the pickup if necessary to get a reading near full scale, or use the DVM on "volts" to measure the voltage across the resistor that was added to the circuit.
    I suggest you re-read the entire earlier thread. Direct drive can be sort-of OK, but you really do need to add a series resistor, and measure the current correctly as described above.

  8. #8
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    I have just built an external battery pack (4P 18650's) for my MTE P7-C 2-mode, which is a direct drive flashlight as far as I know.
    I assumed the emitter would just draw as much current as it needs, but now reading this post it would appear that my assumption was wrong??

    Strange though, I did a runtime test last night with my external battery. I simply turned it on and left it (on max) for about 3 hours. The brightness seemed about the same as the P7 with a single cell, and it didn't get any hotter than normal.

  9. #9
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    Quote Originally Posted by Mr AB
    After recent discussion on running a p7 direct drive with a 3x18650 battery pack I wired a p7 and heat sink directly to a 3x 18650 battery pack and got a 5 amps reading using my clamp meter.

    With my ammeter in series I then got 3 amps and proved this was correct using the clamp meter around one of the leads.

    Looks like the resistance of the ammeter and leads is dropping the current significantly.

    Maybe we need to rethink running a p7 direct drive with a 3x18650 battery pack??????
    Are you sure you didn't accidentally hook up your batteries in series?
    That would triple the voltage, but what would it do to the current?

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    Thanks for the reply cdcdcd, it was your initial posts that got me wondering about the accuracy of measurements when using the ammeter in series.
    Thats why I have made this post to backup your original comments and warn against direct drive. Unless you can accurately measure the current going through the led then you obviously are risking burning out/shortening the life of the p7.

    Maybe the airlight battery pack has different batteries that can supply a higher current?????????????????

  11. #11
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    The cells are not in series they measure 4.2 volts when fully charged, they are an airlight battery pack of which the light was stolen leaving me with just the battery pack and charger. As above maybe these batteries work different to standard 18650's.

    I suggest you work out a way to quickly switch between 1 and 4 batteries and see if you see a difference in brightness.

  12. #12
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    Quote Originally Posted by bradjackson
    I assumed the emitter would just draw as much current as it needs, but now reading this post it would appear that my assumption was wrong??
    YIKES!!! That's completely incorrect and can lead to burned out LED's. An LED (just like any diode) has an exponential current to voltage relationship. Tiny changes in voltage can lead to massive changes in voltage and power. This is why you never direct drive an LED unless you are absolutely sure you've run though the calcs to make sure the LED will not draw too much current and burn out.

    A lot of P7 lights are direct driven with 18650's due to the coincidence of the Li battery having a similar voltage, and the internal resistance of the battery, switches, and wiring is usually enough to keep the current below the LED's maximum rating. However, it is much safer to use a current limiting driver, or at the very least, run through the calculations to determine the proper size resistor to keep the current below the maximum allowable amount.

  13. #13
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    Not a thread hijack but on the same lines.

    I am being given a Phlatlight SST50 as payment for work done and wondered how to get
    it running from my existing 14.8 volt packs .

    it wants 3.6 volts and 5 amps for a lot of lumens . is there a driver that will do the biz

    I am thinking of using it with a ledil iris as the dome will fit and I have one in stock.

  14. #14
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    Woohoo!

    I think paralleling a couple of 2.8A drivers will be the way to go Trout.
    Cheap Dx stuff or better drop George a note and see what he can do for you.
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  15. #15
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    Quote Originally Posted by desolder
    YIKES!!! That's completely incorrect and can lead to burned out LED's. An LED (just like any diode) has an exponential current to voltage relationship. Tiny changes in voltage can lead to massive changes in voltage and power. This is why you never direct drive an LED unless you are absolutely sure you've run though the calcs to make sure the LED will not draw too much current and burn out.

    A lot of P7 lights are direct driven with 18650's due to the coincidence of the Li battery having a similar voltage, and the internal resistance of the battery, switches, and wiring is usually enough to keep the current below the LED's maximum rating. However, it is much safer to use a current limiting driver, or at the very least, run through the calculations to determine the proper size resistor to keep the current below the maximum allowable amount.
    OK so I guess its probably time I invest in a multimeter to replace my little dinky toy meter which I have been using up till now (it doesn't measure current).
    I want to spend as little as I can, and was looking at this one: http://www.dealextreme.com/details.dx/sku.619
    Any comments?

    Maybe I got lucky. I think my cable from the battery was long enough, and poor enough quality to introduce enough resistance. I was gonna replace it with a better one, but I think I will wait until I have done the measurements and added the resistor if needed.

  16. #16
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    Maybe you could use your meter to measure the voltage and calculate the current using the methods described in a previous thread by cdcdcd. My point is my meter didn't give an accurate measurement as was giving enough resistance to drop from 5 to 3 amps.

  17. #17
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    bradjackson - that meter should have a low enough series resistance at the 20A setting BUT you have to use short (or thick) wires and stout low-resistance connectors, like Anderson Powerpoles, in order to minimize the measurement error. The contact resistance of probes is surprisingly high and unpredictable. This is especially important when only a few millivolts of voltage drop can dramatically change the current draw of a non-linear device such as a diode.

  18. #18
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    I dont think is a good idea to increace recomended current TO much. If P7 can go ok with more current I see no reasons why vender can write down this in spec's. Even if this led working on double current I think it will have not much life time, double current slovly and may be fast destroy the crystal. Not a pro at this point but have some knowlage ang experience. Ha, I whish to se cooling system.

  19. #19
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    Quote Originally Posted by jetigm
    I dont think is a good idea to increace recomended current TO much. If P7 can go ok with more current I see no reasons why vender can write down this in spec's.
    The LEDs are usually spec'd for something like 50000hrs runtime (at which point they might be 70% as bright as a fresh one). Upping the drive current reduces this time. I'm guessing 1000 hrs would cover 99% of our lights. So overdriving a little is ok. Same as overvolting halogens.

    See the thermal management guide linked on this page
    http://www.seoulsemicon.co.kr/en/pro...powerLEDp7.asp

    The thing is the LED efficiency goes down substantially at these high currents, so you get little extra light out when you put a lot more power in. The opposite to overdriving halogens.
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  20. #20
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    Quote Originally Posted by znomit
    The LEDs are usually spec'd for something like 50000hrs runtime (at which point they might be 70% as bright as a fresh one). Upping the drive current reduces this time. I'm guessing 1000 hrs would cover 99% of our lights. So overdriving a little is ok. Same as overvolting halogens.
    Though I never dealed with P7 and use MC-E in my headlamps, they are pretty the same.
    So from my experience I can see that the best results one can reach while underpowering the LED. While increasing MC-E power cosumption from 60% of maximum to 100% I have to use three times more effective heatsink to keep the same temperature. Not to mention cyanotic tint and loss of light effectiveness.
    If anyone interested it happened that I discovered experimentally the power limit for MC-E. It's only about 20 sec at 24W repetitive LED consumption, even with a huge heatsink. With more time or power it occured that four out of 25 LEDs got at least one quadrant burnt to short cirquit.

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