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Finally Got A Cob Lit Up! Multi-meter Questions

Discussion in 'LED and other Lighting' started by CCCmints, Nov 14, 2017.

  1.  
    CCCmints

    CCCmints Well-Known Member

    So I finally ordered a couple COBs and wired one of them up. Honestly, I was a little shocked it actually lit up lol. Even the dimming works great.

    So about the multi-meter...I'm thinking I just didn't order a high enough quality one. I got this: https://www.amazon.com/gp/product/B00NWGZ4XC/ref=oh_aui_detailpage_o00_s03?ie=UTF8&psc=1

    On the 2/20A setting the maximum reading reached is .012a, while my driver is 1.4a. I want to use the multi-meter to bring the current down to what's showcased in the COB's datasheet so I can determine the measurement distance used in that datasheet.

    What do you guys think? Get a better multi-meter? Or I'm overlooking something simple? Hopefully the latter. But hey, at-least the COB actually lit up! And damn this thing is bright.
     

    Attached Files:

    Abiqua likes this.
  2.  
    Abiqua

    Abiqua Well-Known Member

    That is micro [funky shape greek U] and milli amps by the way...not Amps

    Measuring a lot of AC current is a pain in the ass ...even a $400 Fluke meter will handle only an Amp or two..

    sorry only could take a screen shot of Amazon ad specs....
    Screenshot 2017-11-14 at 3.40.19 PM.png
     
  3.  
    psychedelicdaddi

    psychedelicdaddi Well-Known Member

    did you break the circuit with the meter to measure current or are you trying to measure it by just holding the leads across the circuit. That would be parallel and you would not get the correct value displayed.

    You must disconnect a lead from the cobs to the power supply and rejoin them through the multimeter.
    [​IMG]
     
  4.  
    Abiqua

    Abiqua Well-Known Member

    I edited my response and took out describing a shunt, regardless that meter will never measure 1400ma's of current, that is the bigger problem.
     
    CCCmints and psychedelicdaddi like this.
  5.  
    psychedelicdaddi

    psychedelicdaddi Well-Known Member

    oh wow yea this meter has got to go
     
    CCCmints likes this.
  6.  
    a mongo frog

    a mongo frog Well-Known Member

    Nice counter tops!! Where did you get?
     
    CCCmints likes this.
  7.  
    GBAUTO

    GBAUTO Active Member

    After reading the manual for that meter a major limitation is obvious-it only measure AC amperage. Good luck getting it to measure driver current.
     
    CCCmints likes this.
  8.  
    CCCmints

    CCCmints Well-Known Member

    I was just placing the positive needle onto the positive terminal of the potentiometer and the negative to the negative.
    Do you have a meter you'd suggest? I'm new to LED hence why I started out with a single COB.
     
  9.  
    nfhiggs

    nfhiggs Well-Known Member

    Your meter only measure AC current. But I can tell you how you can do it. Go a get a 0.1 ohm 1W resister. Wire the resister in series with your cob. Power it up and measure the DC VOLTAGE across the resistor. Multiply times ten and you'll have the current in amps.
     
    CCCmints and psychedelicdaddi like this.
  10.  
    nfhiggs

    nfhiggs Well-Known Member

    Not hard to do at all. Just need a shunt resistor.
     
    Abiqua likes this.
  11.  
    nfhiggs

    nfhiggs Well-Known Member

    Yes, it
    Yes, it actually can. You just need a 0.1 ohm series shunt resistor. DC voltage on the shunt will equal current divided by ten.
     
    Abiqua likes this.
  12.  
    Abiqua

    Abiqua Well-Known Member

    Your correct, I meant that the shunt was needed. Apologies.
     
  13.  
    nfhiggs

    nfhiggs Well-Known Member

    Its all good.
     
  14.  
    VegasWinner

    VegasWinner Well-Known Member

  15.  
    xX_BHMC_Xx

    xX_BHMC_Xx Well-Known Member

    There are many reasons this isn't working... first being that the inductive clamp on top is what is used to determine AC current... supposedly to a very high rating according to the listing. Second being you are measuring the current in the wrong location. You want to measure current at the wall plug (for AC) or across your load (for DC). What you need to do is switch to DCA (EDIT: on closer look, seems like you can't switch to DCA, I would suggest a new mulitmeter, you won't get much use out of that one on many of our projects), and measure current across the load, that way you know exactly how many amps your LEDs are getting, no guesswork involved and no resistors.
     
    CCCmints likes this.
  16.  
    CCCmints

    CCCmints Well-Known Member

    Thank-you all for the helpful replies. This forum has always been an invaluable source of knowledge.
    This brings up another point I'm a bit foggy on. Ultimately, I'd like to wire 3 drivers to 1 potentiometer. I've read that to achieve this I need a 25k pot and 10k resistor. Problem is, I don't see how I would include the resistor in my series. I've attached a couple pictures of the 10k resistor I purchased.
    resistor1.jpg resistor2.jpg
    It has 3 prongs. How would I wire that in series with my COBs? I tried researching a little bit, but I've been really busy unfortunately, so I haven't got as much time as I'd like for my LED project. But that's another reason I love collaborating with you guys on these forums.
    Would this remove the need for a 0.1ohm 1w resistor in series with the ammeter/voltmeter like @nfhiggs suggested? That is a great deal though. I'll definitely be ordering this. Thanks!
    I'll definitely be returning the multi-meter and likely replacing it with the ammeter/voltmeter @VegasWinner suggested.

    So to recap my understanding after reading all of your replies... If I wire the ammeter/voltmeter @VegasWinner suggested in series with my COB, then I should be set? Or do I also need a 0.1ohm 1w resistor like this: https://www.amazon.com/Axial-Colore...pID=51b6gKEGFOL&preST=_SX342_QL70_&dpSrc=srch in the series?

    Also, with this ammeter/voltmeter, what can I expect accuracy wise? I'm ok with buying a proper multi-meter even if its a bit expensive. I really want to have an accurate reading so I can optimally test single COBs in order to design a full fixture.

    Thanks again to all of you for the help. Hopefully in the near future I'll have a full fixture assembled. Sure am glad I chose to test out a couple single COBs before taking the big leap though.
     
  17.  
    GBAUTO

    GBAUTO Active Member

    The panel VW has shown is measuring the AC side of the driver-not the DC output side. The only downside that some units have is the backlighting may stay on.
    Getting back to the original question...To measure the DC output current of the driver you will need a DVM that measures DC current-almost any cheap DVM is capable of this measurement.
     
    Last edited: Nov 16, 2017
  18.  
    VegasWinner

    VegasWinner Well-Known Member

    The panel is for ac equipment. As you adjust the dimming you see the change in amps drawn ac amps are the same as dc amps. Dimming reduces amp draw.
     
  19.  
    VegasWinner

    VegasWinner Well-Known Member

  20.  
    CCCmints

    CCCmints Well-Known Member

    Like this? https://www.amazon.com/Topone-Digital-Meter-Voltmeter-Multimeter/dp/B005EK3NRS#Ask

    So I can measure either A/C (with the device you mentioned) or D/C (with a DVM) and achieve the same results? What is the difference between measuring A/C vs. D/C? Is one more accurate than the other?


    I’m probably just overthinking this to an extreme degree, but I want to be sure I understand all of this before I commit to assembling a full fixture.
     

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