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Thread: AMC7135, is a 1uF capacitor necessary?

  1. #1

    Default AMC7135, is a 1uF capacitor necessary?

    I bought one of those 10 packs of 1,400ma regulators from DX that is populated with 4*AMC7135. I'm planning on wiring 2 in parallel like many people do for a basic regulator in a P7 upgrade to a 2C MagLite.

    I downloaded a pdf from ADDtek on the AMC7135. I'm not sure if there's more than one manufacturer of these. On page 1 of the manual there is a diagram called, "TYPICAL APPLICATION CIRCUIT". In the diagram is an asterisk saying, "CO is strongly recommended". CsubO is a capacitor wired from the negative output of the 7135 to ground. Here is another quote from page 4, "If LED and AMC7135 is located in separate PCBs, or the length of the routing path L1>10cm or L2>3cm, the output capacitor CO should be added. Typically, capacitance of 0.1uF ~ 1uF is recommended and 1uF is needed when L2 is much longer than 3cm". For clarification, L1 is the length of the wire from the positive side of the battery to the positive side of the LED. That could easily be under 4 inches. L2 on the other hand is the length of the run from the 7135's negative output to the negative side of the LED. That's less than 1¼ inches from chip to LED. I think it's safe to say that every Mag mod I've seen fails at least 2 of the 3 conditions to their test for the capacitor's necessity. Namely the negative lead to LED being longer than 30mm and the 7135 chip not being mounted on the same PC board as the LED.

    So my question to the electronics guys is what is this capacitor for and why is it necessary? If nobody uses it and that without incedent, how necessary can it be?

    TIA


  2. #2
    Flashaholic jeffosborne's Avatar
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    Default Re: AMC7135, is a 1uF capacitor necessary?

    Hi AL Combs! One thing struck me about your plan - the 2C mag light will give 3 volts or so to the circuit with alkalines, and 2.4 volts (nominal) with NiMH batteries. The 7135 is a buck regulator only, so your input voltage need to be more than the LED's Vf. Maybe you are going to use C-sized Li-ion? Also, when in doubt, use the manufacturer's suggestion about the cap. A .1ufd capacitor can be very small, and cheap too. Cheers, Jeff O.

  3. #3

    Default Re: AMC7135, is a 1uF capacitor necessary?

    Quote Originally Posted by jeffosborne View Post
    Hi AL Combs! One thing struck me about your plan - the 2C mag light will give 3 volts or so to the circuit with alkalines, and 2.4 volts (nominal) with NiMH batteries. The 7135 is a buck regulator only, so your input voltage need to be more than the LED's Vf. Maybe you are going to use C-sized Li-ion? Also, when in doubt, use the manufacturer's suggestion about the cap. A .1ufd capacitor can be very small, and cheap too. Cheers, Jeff O.
    Funny thing, I just got a pair of these in the mail today. A couple of people mentioned these on CPF. I only bookmarked the source of the battery, not the find. They claim to be Manganese Dioxide chemistry like a Molicell and AW's new IMR batteries. I couldn't afford to buy one of those $150, 28 volt Milwaukee drill packs. I wanted one battery for a spare. What was I going to do with 8 more. I suppose I could sell them in B/S/T, but I didn't want to bother. Since LuxLuthor dropped out of the battery business, I wouldn't know where to buy just 2 of them loose.

    I'm not sure I believe the part about these batteries having an extra 100 mAh capacity over a Molicell while being 5 mm shorter. But hey, they were a lot cheaper. They have a paper sleeve like the Moli IMR's. When the paper is removed, the steel casing is only 26.03 mm wide. Since I'm only using 1 battery at a time, I figure it's perfectly safe to use that way in a Mag-2C. For a spacer I though I'd get a 1 inch dowel, drill a hole in and put a brass nut and bolt through it. That and a big washer for tailcap spring contact. A piece of 3/4 inch dowel, some wire and magnets in my Pila IBC should be a slow if effective charger.

    I bought a pair of AW's new IMR18650's to use with a direct drive P7 Surefire P60 drop-in I got from Gene Malkoff. He told me it was a J voltage bin P7. The batteries arrived with 3.99 & 3.98 volts respectively. I figured that level of rested voltage would make a great test before I charged them. I stuck in in my Surefire with the DMM on the 10 amp scale. It only took about 5 or 10 seconds to reach 3.5 amps and was accelerating. I had to buy a regular pair of Cobalt Li-Ion's. Even fully charged they are only draw about 3 amps. Originally I was going to buy 3 of these Elite 2200 4/5 SubC's. It was the experience with AW's IMR batteries that made me think the lower internal resistance of the Spinel batteries would be better than the high drain NiMH's. That's as far as maintaining a level of voltage to drive my new DSVNI P7 from PhotonFanatic before the AMC7135's drop out of regulation.

    I'd still like to how if the manufacturer says you need a cap that nobody noticed that and nobody has had a problem from not using it? Do you have any idea what it's supposed to do in the circuit? You're right of course about a 1uF being small enough to stick between the negative output and the ground. I'd just like to know why.

    BTW, I loved that Hydra light in your sig line. I never bothered to say so because I didn't have anything useful to say besides, that's cool.


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    *Flashaholic* Mr Happy's Avatar
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    Default Re: AMC7135, is a 1uF capacitor necessary?

    Quote Originally Posted by Al Combs View Post
    Since I'm only using 1 battery at a time, I figure it's perfectly safe to use that way in a Mag-2C.
    Just be careful the polarity of the outer shell is the same as the polarity (negative) of the terminal that contacts the tail spring. Some batteries are constructed so that the outer shell is positive.

    I'd still like to how if the manufacturer says you need a cap that nobody noticed that and nobody has had a problem from not using it? Do you have any idea what it's supposed to do in the circuit? You're right of course about a 1uF being small enough to stick between the negative output and the ground. I'd just like to know why.
    Most likely this is what is known as a bypass capacitor. Regulators usually have a high gain feedback circuit to keep the regulation on spec, and when there are long wires in the system this can sometimes lead to unwanted oscillations due to stray inductance and a consequent loss of efficiency. The bypass capacitor shorts out these oscillations and prevents them from getting into the regulator where they will be amplified and create a problem. (Unless you put an oscilloscope on the output you may never know anything is wrong.)
    Last edited by Mr Happy; 06-24-2009 at 08:23 PM.

  5. #5

    Default Re: AMC7135, is a 1uF capacitor necessary?

    Quote Originally Posted by Mr Happy View Post
    Just be careful the polarity of the outer shell is the same as the polarity (negative) of the terminal that contacts the tail spring. Some batteries are constructed so that the outer shell is positive.
    It is a normal negative polarity case.

    Quote Originally Posted by Mr Happy View Post
    Most likely this is what is known as a bypass capacitor. Regulators usually have a high gain feedback circuit to keep the regulation on spec, and when there are long wires in the system this can sometimes lead to unwanted oscillations due to stray inductance and a consequent loss of efficiency. The bypass capacitor shorts out these oscillations and prevents them from getting into the regulator where they will be amplified and create a problem. (Unless you put an oscilloscope on the output you may never know anything is wrong.)
    Thank you for the lucid explanation. The diagram in the ADDtek pdf shows the capacitor as 2 parallel straight lines. If I remember correctly, that means non-polarized? I have a 1uF Tantalum cap on hand is the reason I'm asking. Would that work just as well in this instance?


  6. #6
    *Flashaholic* Mr Happy's Avatar
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    Default Re: AMC7135, is a 1uF capacitor necessary?

    Quote Originally Posted by Al Combs View Post
    Thank you for the lucid explanation. The diagram in the ADDtek pdf shows the capacitor as 2 parallel straight lines. If I remember correctly, that means non-polarized? I have a 1uF Tantalum cap on hand is the reason I'm asking. Would that work just as well in this instance?
    I think so, as long as you keep the polarity correct.

  7. #7

    Default Re: AMC7135, is a 1uF capacitor necessary?

    Thanks.


  8. #8

    Default Re: AMC7135, is a 1uF capacitor necessary?

    Quote Originally Posted by Al Combs View Post
    It is a normal negative polarity case.

    Thank you for the lucid explanation. The diagram in the ADDtek pdf shows the capacitor as 2 parallel straight lines. If I remember correctly, that means non-polarized? I have a 1uF Tantalum cap on hand is the reason I'm asking. Would that work just as well in this instance?
    A tantalum cap should be perfect for this LDO regulator output capacitor function. Should have the right ESR for output stability.

  9. #9

    Default Re: AMC7135, is a 1uF capacitor necessary?

    I have one more question guys. I'm getting ready to put this 1uF Tantalum cap on my piggy-backed boards and it occurs to me. If one end goes to ground, and the other goes to the 7135's negative output, where do I put the positive end of this polarized cap if both connections are negative?


  10. #10
    *Flashaholic* Mr Happy's Avatar
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    Default Re: AMC7135, is a 1uF capacitor necessary?

    Quote Originally Posted by Al Combs View Post
    I have one more question guys. I'm getting ready to put this 1uF Tantalum cap on my piggy-backed boards and it occurs to me. If one end goes to ground, and the other goes to the 7135's negative output, where do I put the positive end of this polarized cap if both connections are negative?
    You need to connect the positive lead of the capacitor to OUT and the negative lead to GND. That's because OUT is sinking the current flowing through the LED and is thus more positive than the GND (battery negative) potential.

  11. #11

    Default Re: AMC7135, is a 1uF capacitor necessary?

    I actually asked myself, which way is it going? And then I thought, let me ask before I blow it up. Thanks again.


  12. #12
    Flashaholic* bshanahan14rulz's Avatar
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    Default Re: AMC7135, is a 1uF capacitor necessary?

    I know, old thread, but still pertinent. In fact, better than if I had made my own thread. So I understand that the suggested cap is there to ensure proper regulation. I was wondering if this cap, since it smooths out the spikes that would otherwise be amplified by the regulator, is enough to protect a current load that is very sensitive to overvoltage, or if I need to put a capacitor parallel with the load. Also, how do you pick capacity of the cap, and are tants the best type to use for this application?

  13. #13
    Flashaholic brted's Avatar
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    Default Re: AMC7135, is a 1uF capacitor necessary?

    Prevents oscillation too, which might reduce PWM whine that some lights have?

  14. #14

    Default Re: AMC7135, is a 1uF capacitor necessary?

    Hello, I have just a few somewhat related questions:

    - Is the heat-plate on the 7135 ground? I ask because I don't see the GND pin on any pre-made board ever soldered.

    - If I want to use a 555 timer for PWM, do I put it before, or after the 7135?

    - I'm sorta new, but why is the Vin running to both the 7135 and the LED? I'm somewhat confused at how a linear regulator works. It's kinda acting like a shunt in this setup. Isn't that really inefficient? Does the 7135 work in series if I just apply Vin to Vdd and OUT to the LED's + terminal (just like a regular resistor)?

    Thank you!
    Last edited by eatkabab; 12-26-2011 at 08:25 PM.

  15. #15

    Default Re: AMC7135, is a 1uF capacitor necessary?

    ohhhh, is the Vdd just for reference? As in, the 7135 is placed in series, it's just placed AFTER the LED. Right?

  16. #16

    Default Re: AMC7135, is a 1uF capacitor necessary?

    Quote Originally Posted by Mr Happy View Post
    Most likely this is what is known as a bypass capacitor. Regulators usually have a high gain feedback circuit to keep the regulation on spec, and when there are long wires in the system this can sometimes lead to unwanted oscillations due to stray inductance and a consequent loss of efficiency. The bypass capacitor shorts out these oscillations and prevents them from getting into the regulator where they will be amplified and create a problem. (Unless you put an oscilloscope on the output you may never know anything is wrong.)
    Cin I can understand, but if the LED and ACM7135 is on the same board (L2<3cm but L1>10cm>1m), Co should still be added? Is it really doing anything to connect the Vout and GND pins of the AMC7135? If it does in fact help, it can increase efficiency? Could it at all decrease efficiency?

    My understanding of stray electrical oscillations is very limited.

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