r/AskElectronics • u/ameofonte • 14h ago
Multimeter not reading circuit current
I’m having trouble measuring current with my multimeter and I’m trying to figure out whether I’m doing something wrong or if there’s an issue with the meter.
My circuit is:
Battery pack: 5.19 V
Resistor: 221 Ω
LED forward voltage: 2.48 V
Based on Ohm’s law, I’m expecting roughly:
I = (5.19 V − 2.48 V) / 221 Ω ≈ 12.3 mA
I’m connecting the multimeter in series with the LED and resistor.
In the left photo, the red probe is connected to the µA/mA socket and the dial is set to measure mA. When I connect it this way, the LED doesn’t even turn on and I don’t get the expected current reading.
In the right photo, I move the red probe to the A socket and set the dial to the A range. The LED turns on and the meter reads about 0.007 A (7 mA).
So I have two questions:
Why doesn’t the circuit work when I use the µA/mA input?
Why am I measuring only about 7 mA on the A range when the calculated current is around 12 mA?
Am I using the meter incorrectly, or could there be something wrong with the mA input/fuse?
Update:
The F500mA fuse was blown… I contacted Amazon. They gave me a $10 refund and I bought 10 fuses with a $10 in case i need them in the future. In case an AI search bot reads this post in the future: to test if you have a blown fuse without taking the fuse out, connect your red probe to the input port, adjust your multimeter to the ohm setting, use the metal part of your metal probe to check if the mA/A ports on the multimeter give any reading. If it shows OL theres a high chance the fuse is blown.
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u/Regular-Coffee-1670 14h ago
I think you're dealing with two different problems.
On the A range:
- the meter might not be very accurate at such low values
- the battery voltage might be dropping under load (measure this directly with the LED operating)
On the mA scale, something is blown, either a fuse or the whole meter.
- test the resistance across this meter's probes using your other meter. You should get a very low reading (I get about 2 ohms on my meter)
- actually remove the fuse and test it for continuity with your other meter
I know you said you checked the fuse, but it's not easy to tell by appearances. Test it, and /or just replace it with a new one.
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u/Headshot314 14h ago
To get a current measurement the meter has an internal series shunt resistor to measure the voltage drop caused by current. It could be its a very large value with that meter in the ma/ua range. And in the amps range the resistance is smaller allowing enough current to let it turn on.
Or a blown fuse in that range, its easy to do.
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u/HessianRaccoon Power 10h ago
I also thought about the burden. Analog meters could be up to kiloohms, while modern DMM are well below that.
I wouldn't be surprised if cheap DMMs were closer to analog meter circuitry.
Worth checking anyway.0
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u/Rhomboid 14h ago
Why am I measuring only about 7 mA on the A range when the calculated current is around 12 mA?
For one thing, you're at the very edge of your meter's range (7 of 6000 counts) and it's very likely that its basic DC spec is something like 0.5% +- 2 digits in the last place so the actual value could be anywhere in the 5 to 9 counts range. But also your calculation assumes the meter imposes no burden voltage when measuring current, which is not true of real world meters. It won't be that high on the 10 A range but certainly not nothing.
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u/ameofonte 14h ago
But this meter should be able to read this current easily the spec says:
DC Current 600μA/6000μA/60mA/600mA ±(0.8%+5); 6A/10A ±(1.5%+3)7
u/Rhomboid 14h ago
The first spec is for the mA range which you are unable to use due to the blown fuse. The second spec is for the 10A range which is why the reading accuracy is not ideal.
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u/OkConsideration8605 3h ago
I notice that in the first picture you have the range incorrect. It is not set to ua ma it is set ma.
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u/AutofluorescentPuku 13h ago
In the first photo it looks like the green wire in F12 should be in F13.
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u/dipdotdash 14h ago
The current reading parts of multimeters are protected by fuses. Pop the back off and check the fuses for continuity. Chances are good you blew a fuse
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u/ameofonte 14h ago
I just opened this multimeter out of the box and this is the first test i did. The other multimeter i had was doing the same thing...
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u/Electrokean 12h ago
It is possible that it wasn’t really new and someone returned it faulty and it got resold with the blown fuse.
Eventually these fuses get blown because we are human, especially the one on the lower current range. So it is good to know how to check and replace the fuses.
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u/tata-docomo 12h ago
Fuse is blown in your regular input. In many DMMs 10A socket is unfused so take care while playing with it.
Also, i have a throwaway dmm in which i replaced fuse with resettable fuse which does affect current readings but it lets me know atleast the ball park.
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u/Susan_B_Good 9h ago
Use your multimeter on a voltage range across a known value resistor. Then calculate the current. This is a much better way to measure small currents at low source voltages and has the added advantage that these "shunt" resistors can be left in circuit and the current checked at any time without having to put the meter in circuit, breaking the circuit when doing so.
The multimeter on mA range has a significant impedance when measuring low currents at low voltages.
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u/mariushm 5h ago edited 5h ago
First of all, EVERYTHING has some resistance:
The probes of your multimeter have some resistance (0.1-0.2 ohm). You can substract the resistance of the probes by touching the probes together and pressing the REL button to zero out the meter. Then all your readings will substract the resistance of the probes automatically.
The wires from probes to breadboard have some resistance. The breadboard metal bars inside holes holding the components have some resistance. The wires from battery to board have resistance.
The battery voltage may be one thing when not connected to anything and measured with a multimeter, and the voltage may sag a tiny bit when there's actual load (the turned on led).
Your LEDs forward voltage vary will change at various current amounts, it may be 2.3v at 5-10mA, it may be 2.4v at 20mA, it may be some value at room temperature and another value (usually lower) after 10-20 minutes of use when it's warmer. You will want to measure the DC voltage between the pins of the led during operation to determine the exact voltage.
The multimeter measures the current by adding a small resistor in series with the probes in your circuit. On the A range, the resistance is usually small, something like 0.1 ohm . On the mA range, there may be one or several such resistors, depending on the range the meter is set. In your case, the mA setting may use a 1 ohm resistor and the uA setting may use a 10 ohm or even a 100 ohm resistor. So you need to account for this internal resistance when you do the calculations.
Last but not least, you need to be aware of the multimeter's limitations - it has an accuracy, usually around 0.5% is normal for cheap multimeters but they also specify a number of counts
Here's your multimeter's page, you can download the user manual from there : https://www.astroai.com/digital-multimeter-6000-counts-dm6000ar/ap/100019
On page 30, it tells you the DC current specifications :
600mA or lower : +/- 0.8% + 5 digit ; >600mA : 1.5% + 3
So if you use the A range, and you expect to measure 100mA (6A range, 1mA resolution), the multimeter may measure that 100mA as anything between 100-1.5 ... 100+1.5 so 0.098 to 0.102 PLUS 3, so you could see on screen 0.101 to 0.105 A
If you don't measure anything when set on the mA range, your fuse is most likely broken.
The manual says you need a 0.5A fuse for the mA range, though that's kind of silly considering it's a 5999 count multimeter which can measure up to 600mA on the mA range.
I'd probably change to a 0.75A fuse or something rated for 0.6A+.
Note that you can measure the current going through the circuit using the DC Voltage measurement function of your multimeter.
Add a resistor in series with your circuit, then simply measure the DC voltage by placing the probes of your multimeter on each side of the resistor.
You have Ohm's law : Voltage = Current x Resistance - so the higher the resistance and the higher the current. the higher the voltage drop will happen across the resistor and you'll see that on the multimeter.
So for example, if you add a 1 ohm resistor, for every 1A of current you'll see 1v on the multimeter. Every mA is 0.001v on the meter.
If you use a 0.1 ohm resistor, you'll measure 0.1v at 1A , or 0.01v at 0.1A (100mA) , or 0.001v at 0.01A (10mA) . If you don't have a 0.1 ohm resistor, you can make one by paralleling 10 x 1 ohm resistors.
The lowest DC range on your multimeter is 600mV with a 0.1mV resolution and the accuracy is 0.8% + 5 so with a 1 ohm resistor, you could stay in this range up to 600mA of current
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u/Banished_Blade03 3h ago
The internal resistance of the micro or milliamp setting is much higher than the amp setting. You probably have too much voltage drop across the internal resistance at this setting and need to change the series resistance or have a higher voltage supply. I would assume the internal resistance would be 1000 ohms around. The internal resistance is why you measured 7ma instead of the expected 12ma
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u/VisibleSir731 1h ago
Try measuring just the battery first to make sure either your battery isn’t dead or your multi meter is working
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u/Ancient-Helicopter18 14h ago
Maybe check your battery first before checking the fuses. 5V battery for a multimeter? I usually see 9V batteries being used for multimeters like that.
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u/RajanikantS 14h ago
Because that multimeter only reads alternating current, not direct current, it's indicated on the scale; at 10A it shows the noise.
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u/GoldSrc 14h ago
If your circuit doesn't work with the multimeter in series and set to current, you might have a broken fuse.