What am i doing wrong here? - Page 1
Forum section: electronics / beginners

author: ElectricPower
author_rank: Frequent Contributor
timestamp: November 24, 2019, 05:08:49 pm
content: This may be a very stupid question from an amateur, but how do you measure how many amps the strip in the picture draws with a multimeter?I set the multimeter wheel on mA and connect the red contact to rhe mA port on the meter. When i do this the led strip light turns of and turns on when i remove the probes.I just want to measure how many amps (mA) this led strip uses.

author: ArthurDent
author_rank: Super Contributor
timestamp: November 24, 2019, 05:15:28 pm
content: If you have the meter set to ma and connect it between plus and minus for the LEDs, you are shorting the supply output. If you put the meter in series with the positive lead to the LEDs, you will measure current.https://www.sciencebuddies.org/Files/4808/13/multimeter-series-measure-current.png

author: ElectricPower
author_rank: Frequent Contributor
timestamp: November 24, 2019, 05:25:36 pm
content: Do you mean like that?

author: Docara
author_rank: Regular Contributor
timestamp: November 24, 2019, 06:56:34 pm
content: HiYou measure Voltage ACROSS 'things' and you measure current IN-LINE with 'things'unlessthe expectant current is reasonably high or you are measuring ac then you could use a clamp meter.Estimate the magnitude of the current you are measuring to see if your meter is capable and not to blow it's fuse. Always set your meter on the highest range (usually 10 or 20A) and switch down the ranges as necessary.With power off remember, break the circuit. Assuming you have connected your probes correctly (if required), connect the positive (usually red) meter lead to the most positive point of the broken circuit, and the negative (usually black) lead to the other part of the broken circuit.Switch on

author: Gyro
author_rank: Super Contributor
timestamp: November 24, 2019, 07:40:22 pm
content: Please edit your thread title to something meaningful.

author: james_s
author_rank: Super Contributor
timestamp: November 24, 2019, 07:41:45 pm
content: Disconnect one of the wires that goes from the regulator to the LED strip and connect the meter between the two formerly connected points. You have to break the circuit and measure the current flowing through it.

author: radiolistener
author_rank: Super Contributor
timestamp: November 24, 2019, 07:56:06 pm
content: OMGEven don't try to measure mains in such way.You will destroy your multimeter and there is a high risk for your life...

author: Etesla
author_rank: Regular Contributor
timestamp: November 24, 2019, 08:24:24 pm
content: Some basic knowledge you may or may not already have:With a multimeter, if its set to measure voltage, the two probes have an open circuit between them.If it is set to measure current (10A, mA, uA modes), the two probes have more or less a short circuit between them.Side note: If you have another multimeter, call it multimeter 2, you can test the above claims by measuring the resistance between the probes of multimeter 1 when multimeter 1 is set to different settings, with multimeter 2.Here's how you measure the DC current going into the LED strip:Start by setting the multimeter to 10A mode, and putting the red probe in the appropriate hole on the multimeter.You see the blue wire that connects the output of the buck converter to the led strip? You need to 'break' that blue wire and put the multi meter in series with the break. Because the multimeter acts like a short circuit when in a current measuring mode, it will not impact the function of the circuit because its as if the two blue wires were simply connected by a wire.You will also notice that flipping the two probes, putting the red one where the black one was and the black one where the red one was, will flip the sign of the current your multimeter reads. (.12 amps will read -.12 amps). Don't be deterred by this, that's normal. The current measured is the current going into the red probe and out of the black probe. If you get a negative reading, it means the current in real life is going into the black probe and out of the red probe instead of the other way around.Here's a really bad image of what it should look like if this isn't clear:

author: Brumby
author_rank: Unknown
timestamp: November 24, 2019, 09:00:00 pm
content: Quote from: radiolistener on November 24, 2019, 07:56:06 pmWhile your advice is something any experimenter should heed, the OP is not measuring mains here. They are measuring on the secondary side of a power brick - which is clearly stated as being 24V.

author: radiolistener
author_rank: Super Contributor
timestamp: November 24, 2019, 10:47:15 pm
content: Quote from: Brumby on November 24, 2019, 09:00:00 pmYes, but you can see mains wires near multimeter on the photo, so there is a high risk that he may decide to measure current on mains in the same way. It will leads to mains short circuit and is unsafe for his life. This is why I warned him to NOT do that. Because this is dangerous.Once in the past I had such experienceI just wanted to measure mains voltage, but forgot to switch probes from 10 Amps shunt to voltage socket. As result, I got a big bada-boom

author: james_s
author_rank: Super Contributor
timestamp: November 25, 2019, 12:17:06 am
content: Thus the importance of a proper CAT rated meter with suitable fusing to protect it and the user.And it's also important to take the time to learn the fundamentals and learn how to use the tools properly. If people bothered to learn the basics they wouldn't be having these issues.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: November 25, 2019, 03:47:05 am
content: Thanks alot! That WorkedI'm understand now. Probes becomes a part of the sercuitAnd yes, i'm not measuring AC from the wall here. On the right side of the Buck Converter it's 12V and it's 24v on the left side (input).

author: avogadro
author_rank: Contributor
timestamp: November 25, 2019, 11:28:16 am
content: Quote from: radiolistener on November 24, 2019, 10:47:15 pmDid that with a 10 buck chinese meter. Melted he tip of the probe. Still works though just wont measure current anymore

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 26, 2019, 08:45:39 am
content: what is wrong?- 1)meaningless subject(it should be something "how can I measure current with a multimeter?")- 2) breaking the circuit to measure current is not granted to work as there is a voltage drop (depends on the multimeter and on the range) that may decrease the current value (unless the load is a constant current sink) or prevent it to work properly if the voltage drops too much- 3) uploaded images are too largewhat would be more appropriate in this case ?- 1) acurrent clamp- 2) as a poorer alternative a low value accurate resistor to ensure a really lowburden voltagecalculating the current via the Ohm law

author: james_s
author_rank: Super Contributor
timestamp: November 27, 2019, 02:38:32 am
content: Breaking the circuit is absolutely the correct way to measure the current in this situation, unless you happen to have an expensive DC current clamp that is suitably sensitive for the relatively small load being measured here. That is a pretty specialized tool, I don't personally know anyone who owns one and I would not expect anyone who needs to ask how to measure current to have one. Burden voltage is not going to be any issue at all in this application, use the 10A range of a typical multimeter and it is no problem at all.

author: Rick Law
author_rank: Super Contributor
timestamp: November 27, 2019, 06:31:22 am
content: Quote from: ElectricPower on November 25, 2019, 03:47:05 amA picture is worth a thousand words... See attached picture --- but, it requires explanation still. There is something missing that may affect your measurement.To measure voltage, it is done in parallel (seetop left picture). To measure current, it is done in series (seetop right picture). ButNOT SO FAST. There is one very important thing calledburden voltage.Your 12 Volt first hit the DMM, when it comes out of the DMM, it is no longer 12 Volt , it dropped a bit. The amount it drops is calledburden voltage. Say the burden voltage is 1 Volt for easy math, 12Volt dropped by 1Volt meaning your LED is going to powered by only 11 Volt. The Current at 11 Voltwill not be the sameas the Current at 12 Volt. It doesn't matter whether it hit the LED first or the DMM first, it can "eat" the 1V at the top (DMM first) or the 1V at the bottom (DMM after), you are still left with only 11 Volt for the LED.Burden voltage varies from DMM to DMM, and it varies from DMM setting as well as from amount of Current. The larger the Current, the larger the burden voltage drop. On some DMM and when set to mA,it may drop so much that the number becomes meaningless.Typically, when your DMM is set to the highest current setting, there is minimal stuff in between (lowest value shunt) and the burden voltage drop is the lowest. The highest current setting is of course DMM dependent. On mine, the max is 10A max and the burden voltage drop is 1 Volt at 10A (measurement done via 0.1ohm shunt). So, if it is running 10A, the 12V input is really only 11V to the LED. I don't know what mine is at 220mA-Max setting, but I know the drop is so big that the mA reading is fairly meaningless.If youonly have one DMM, try using your DMM's max current setting (note: it looks like from the picture of your DMM thatyour probe needs to be plugged to a different socketin your DMM), that will typically minimize your burden voltage drop and gets a number that would be closer than otherwise.For better measurement, you need to use two DMMs. (seebottom picture). The second DMM will show you exactly what voltage is fed into the LED - at least you know what the damage is, so to speak. If you are reading say for example something like 9Volt powering your LED, you know the Current reading isprobablynot very meaningful.Since you are using a buck convertor, may be it is an adjustable buck. If you canadjust the output carefully increasingthe voltage until your second DMM (measuring voltage) is reading12Voltafterthe burden voltage drop, now you know for sure what your Current will be at 12 Volt. (Don't forget to adjust it back after measurement - otherwise, once DMM1 is removed, you no longer have the burden drop and you would be over 12V now if you don't change it back).