Multimeter current input has high resistance - Page 1
Forum section: electronics / beginners

author: Hamrbooo
author_rank: Newbie
timestamp: December 21, 2021, 08:37:01 pm
content: Hey all,To put first, I'm new here, so I hope I post this on the right page.ContextI was trying to measure the current through a simple circuit connected to my arduino. The circuit basically consists out of a resistor and 28 LEDs.First, I measured the resistance of the resistor (disconnected from the circuit, of course), and then I measured the voltage across the resistor while the circuit was running.Then it was an easy calculation to get to the current, but I wanted to double check, with my multimeter in mA mode, which gave a different result, with the current being lower on the multimeter than the current calculated.After some thinking what I could have done wrong, I decided I could test the current going into the arduino with a usb meter. I discovered that the current flowing through the circuit was indeed the calculated current,unlessI connected my multimeter again, then it showed the same current on the multimeter as on the usb meter.Trying to factor out options, I tried measuring the current on A mode instead of the mA mode and put the probe in the A terminal (the mA and A have different terminals on my multimeter). Now the current shown on both the multimeter and the usb meter matched the earlier calculated current.My multimeterMy multimeter is a UNI-T UT161Ehttps://www.uni-trend.com/meters/html/product/General_Meters/Digital_Multimeters/UT61%20161%20Series/UT161E.htmlI bought it fromhttps://www.eleshop.nl/uni-t-ut161e.htmlWhat else I triedSince I have opened my multimeter before to disconnect the buzzer, I opened it again to make sure I screwed in the terminals all the way, which they were.I checked the resistance of both current terminals by plugging the probe into thesocket, putting the multimeter onmode and putting the other end of the probe into the current terminals. The A terminal reads 0.00, while the uA and mA terminal reads 0.9379 M.I checked the fuse, by putting the probe that's still connected to theterminal on the upper side of the fuse and then on the lower side of the fuse. Both side give me the same 0.9379 Mresistance.My conclusionsSomething is wrong inside my multimeters uA and mA terminal or what's behind that terminal. The part of the multimeter that actually reads the current, is good. It's just that the mA function gives a lower current because connecting the multimeter adds resistance.My questionDoes anyone have an idea what could be wrong with my meter? I hope it will be a simply fix.

author: Hamrbooo
author_rank: Newbie
timestamp: December 21, 2021, 08:41:18 pm
content: Don't know how to edit the post, so in a reply.The 'calculated current' is about 130 mA.The measured current with faulty terminal is about 80 mA.The measured current on the good, A terminal is about 130 mA.The resistor in the circuit is 6.8.This makes the high resistance from the meter kind off odd to me, since I would expect a lower current measurement when adding like a million ohms to a circuit with a 6.8 ohm resistor.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2021, 08:42:25 pm
content: The symptoms look like a blown fuse. Most meters have fuses for the current range. With considratbly to high a current the fuse may blow to avoid further damage to the meter.

author: Hamrbooo
author_rank: Newbie
timestamp: December 21, 2021, 08:48:25 pm
content: Just double checked the fuse, it measures 0.83 ohms..The lower connector gives me O.L. on the meter in resistance mode.The upper connector of the fuse gives me the same 0.9379 M.

author: IanB
author_rank: Super Contributor
timestamp: December 21, 2021, 08:52:39 pm
content: Quote from: Hamrbooo on December 21, 2021, 08:37:01 pmThis is absolutely normal, and there is probably nothing wrong with your meter at all.You are seeing what is called the "burden voltage".Any mA or µA meter will add resistance into the circuit, which will change the current reading. The meter needs the resistance to measure the current, since it works by passing the current through a resistor and measuring the voltage difference.Even the A range will insert some resistance, but it will usually be much smaller.

author: IanB
author_rank: Super Contributor
timestamp: December 21, 2021, 08:57:22 pm
content: Note that some meters have a lower resistance when measuring current than others, but with most regular meters it is significant. If you want to get a much lower burden voltage, Dave made a device called the µCurrent, which uses an internal amplifier to permit the use of a much lower value current sense resistor.But for most purposes, the best way to measure current is to measure the voltage across an existing resistor in the circuit, as you did. That way you do not disturb the circuit under test.

author: Gyro
author_rank: Super Contributor
timestamp: December 21, 2021, 09:21:39 pm
content: The UT61E has a particularly high burden voltage on some current ranges, I'm pretty sure the UT161E is the same. Joe Smith characterised it, and did some work on improving it in one of his videos...He is clear that he does not recommend duplicating his mods for safety reasons. I achieved a useful reduction by substituting Schottkys for both pairs of 'bypass' diodes.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 21, 2021, 10:06:35 pm
content: Did you check both fuses...Fuse: F1 Fuse 600mA 1000V Φ6x32mm (mA/μA input terminal)F2 Fuse 11A 1000V Φ10x38mm (A input terminal)