Using a Multimeter to Check COB Voltage and Current

Hey all,

Time for another quick guide. In this one, I’ll go over how to use a digital multimeter to check 2 important characteristics of your LED COB circuit: voltage and current. If you haven’t worked much with electricity, I’d recommend brushing up on the very basics, so you can minimize your risk of zapping yourself. These circuits can be very powerful and you need to exercise caution when working with the voltages and currents inherent to high-powered LED systems. Be careful!

Edit 09/07/17: Check out the video version of this tutorial here!

Getting Started

Before getting into the specifics, a quick description of how I’ve set up my lights is in order. As you can see below, I’ve wired the positive and negative terminal of each of my 3 COBs to a screw terminal strip. Note that the left and right screw of each row of terminals share the same piece of metal and are interconnected on the terminal strip.

Wiring to a terminal strip wouldn’t be ideal in a normal environment where you’re using these lights to grow, as it’s dangerous since each connection is exposed by the bare screw. The only reason I’ve wired it like this for this tutorial is to make it easier to contact each point along the circuit with the DMM leads (the red and black needle-like probes). If you’re interested in checking voltage in your circuit, you’ll need to find a way to expose the point in your circuit you’d like to measure. This can be done other ways too (e.g. – use wagos to make your connections).

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Below is a picture of my digital multimeter; it’s pretty beat up since I use it for my day job, but it’s still going strong! Chances are, yours will look very similar. There really isn’t much to these things – there’s a screen, a dial in the middle to select what you want to measure, and a pair of positive/common leads or probes:

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Measuring Voltage

Using the classic water analogy, voltage in an electrical circuit is like water pressure in a water circuit – it is a representation of potential difference between 2 points. In order to measure voltage, you’re going to want to do 2 things before you start probing:

  1. Plug your black lead into the common input, and plug your red lead into the input marked with a “V” (voltage), “Ω” (resistance), or ANYTHING but an “A”. The input marked “A” is for measuring amperage, or current. Trying to measure voltage while the red lead is plugged into the “A” input provides a short between the 2 leads and this will wreck your shit.
  2. Switch your dial to DC Voltage, which is what LED drivers put out. The symbol will look like a V with a solid and dashed line above it. See the picture below. Note – If your meter does not automatically set the measurement range, you may need to set it yourself. Your number will likely be somewhere between 0 and 200 or so, depending on how many COBs you have.

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Now, use the red probe to touch a positive part of the circuit, and the black probe to touch a negative part of the circuit. If you are testing a series circuit, placing the 2 probes at different parts of the circuit will show different voltages between them. If you are testing a parallel circuit, voltage will be the same between the 2 probes no matter where they are. Note that if you touch the same point with both probes, there is zero voltage, as no potential difference can be present from the same point in the circuit. Below are some examples of measuring different voltages in a series circuit (I will describe how to wire in series and parallel in more detail in an upcoming post):

LED COB Voltage

Voltage drop across the first LED in the series circuit.

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Voltage across the first two COBs in the circuit.

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Total voltage of the entire circuit.

Measuring Current

Referring back to the water analogy, current is like the amount of water flowing through the pipe. Measuring current is considerably different than measuring voltage. When you measure current, you are interrupting your circuit and inserting your probes in place of the wires that used to connect it. Think of it as current flowing through your LED circuit, into one of your leads, through your DMM, then out through your other lead and back into the circuit.  Make absolutely sure that you are interrupting the circuit, but not shorting it out. If you tried to measure current in your circuit by touching the positive and negative leads from your LED driver, it would simply short it out! As mentioned before, do not try to measure the same way you would measure voltage or resistance or anything other than current when your leads are in this configuration, as they are essentially a short circuit. Before you measure current, do these 2 things:

  1. Plug your black lead into the common input, and plug your red lead into the input marked with an A  for amperage. Note that there will be a limit to how much current you can measure with your multimeter – mine is 10 amps – more than enough for little LED circuits that draw 1-2 Amps.
  2. Switch your dial to DC current. The symbol will look like an A with a solid and dashed line above it. See the picture below:

Now, the circuit will not work until you connect it with your 2 probes. In the picture above, I am connecting the negative terminal of COB 1 to the positive terminal of COB 2, just like the chunk of wire that I removed from the circuit to test this used to do. My leads are interrupting this circuit by replacing that chunk of wire and forcing the current that used to run through the wire to instead run through the leads. As soon as I touched these 2 screws and completed the circuit, it lit up like the sun and it gave me a reading of 1379 milliamps (1.379 Amps).

Measuring Current with a Clamp Ammeter

Now if you have one of these handy-dandy tools, it makes this process a billion times easier. Clamp ammeters measure magnetic flux produced by current flowing in its jaws and let’s just say they’re mostly magic because it’d take me a lot more research to begin to really learn and explain how they work. Here’s what you need to do to measure current with a clamp ammeter:

  1. Set your ammeter to measure DC current
  2. Zero it
  3. Open the clamp and clamp one of your wires
  4. Read the measurement and laugh at how easy that was

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What Now?

Now that you have your voltage and current measured, you can figure out even more about your circuit by applying Ohm’s Law to it. For example, let’s say your total circuit voltage is 105.3 Volts like mine read above, and your current is measured at 1.3 Amps. From these figures, you can determine how much power the system is producing, in watts. Using the formula P=IV, where P is power, I is current, and V is Voltage, the equation would be P= (1.3)(105.3). P= 136.89 Watts.

Any techniques you’d like to share? Leave a comment!

 

3 Comments

  1. hi, thanx for your guide!!
    1.379A * 136.89V= 189W ish …but regard to clamp multimeter measure 1.3*136.89=178W ish…6% of differece…really easy and fast measure but not so good..am i wrong?

    • LEDGardener

      January 29, 2017 at 10:58 am

      Yeah you’re spot on. There are probably more accurate clamp meters out there, but if you want a really accurate measurement and not just a quick reading, use your multimeter!

  2. Hello Admin,
    Thank you for your valuable post. It is true that we should know the measurement of electric voltage exactly once we repair any accessories. Unless having proper voltage measurement we might fail to repair and detect problems.

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