Measure current with multimeter question - Page 1
Forum section: electronics / beginners

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 05, 2017, 05:02:56 am
content: I have a simple circuit, which I suspect draws about 2 Amps.However, when I connect my multimeter in series with the simple circuit, it only shows about 15 milliAmps.It this possibly due to the multimeter has some internal resistance (and therefore not very reliable for reading current flow)?

author: Monkeh
author_rank: Super Contributor
timestamp: February 05, 2017, 05:21:54 am
content: Quote from: mikeinfodoc on February 05, 2017, 05:02:56 amWhy do you suspect that?QuoteIt of course has internal resistance, that's how current is measured - voltage drop over known resistance. In the mA and especially uA ranges, this is a fairly large value. In higher ranges, it is substantially lower.

author: sleemanj
author_rank: Super Contributor
timestamp: February 05, 2017, 05:26:15 am
content: Quote from: mikeinfodoc on February 05, 2017, 05:02:56 amYes (inside the meter it puts the current through known resistance, measures voltage, calculate current by Ohm's law), however it's very small.Make sure you have the probes in the appropriate positions for measuring 2A and that the range switch is in an appropriate range if not auto-ranging.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 05, 2017, 05:42:33 am
content: Thank you, it is auto-ranging.I had the black common in the common port (of course)I had the red in the DC 10A port, and it measured .015I then tried the red in the other/normal voltage port (which also does small currents), and it had the same reading of .015AWhen I have done tests in the past, such as trying to measure current through an LED strip, the multimeter would not even allow the strip to turn on. So I wondered if the internal resistance was pretty high...

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 05, 2017, 05:49:58 am
content: Please post the exact circuit you are trying to measure, including power supply and load. Please tell us the exact make and model of the multimeter you are using. Please make sure that _all_ fuses of the DMM are intact and of the proper type and value, and that it has a fresh battery installed.

author: Dragonfly
author_rank: Contributor
timestamp: February 05, 2017, 06:36:47 am
content: Quote from: mikeinfodocQuick question... do you realize that to make a current measurement you need to insert the meter in series between your power supply and the device under test? The statement above implies to me that you may be trying to put your meter probes in parallel across your power supply instead like you do when making a voltage measurement.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 05, 2017, 11:59:09 pm
content: I do realize that Sir.I had my multimeter in series both times.Once using the ground and voltage port (which also handles small current)and Once using the ground and current port (which handles up to 10Amps for a short time).In both cases, the circuit flow went "THROUGH" the multimeter.When I was testing the LED strip, I was surprised the multimeter prevented the LED lights from coming on.In this case, should I raise my voltage a bit to overcome the internal resistance of the meter?

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 06, 2017, 12:02:58 am
content: Quote from: alsetalokin4017 on February 05, 2017, 05:49:58 amWell in this particular case, I was simply testing various lengths of resistance wire (nichrome) with a 12V battery. I have 22 guage and 32 guage (2 coils I am testing).So ONLY the 12v battery and various lengths of wire. Nothing else. (...well, except the meter to measure current).I was hoping to get a feel for how much current is passing through the resistance wire.I suppose it is possible that it IS TRULY only about 15 mAmps, as much of the power is dissipated as heat in the wire (which is the purpose).However, I really expected MUCH more current to be flowing.FYI the Multimeter is an Innova 3320 (so a cheapy)https://www.amazon.com/INNOVA-3320-Auto-Ranging-Digital-Multimeter/dp/B000EVYGZA

author: sleemanj
author_rank: Super Contributor
timestamp: February 06, 2017, 12:22:11 am
content: Quote from: mikeinfodoc on February 05, 2017, 11:59:09 pmI would suggest that you check the fuses.A blown fuse would likely result in the symptoms you report.Fuses are typically used on current measurement modes only, so other modes would not be affected.

author: xrunner
author_rank: Super Contributor
timestamp: February 06, 2017, 12:44:13 am
content: Do you have an external power supply? Try a simple test circuit with a resistor and your lab supply - like 10V and a 100 R which would draw 100 mA. Test it with your DMM and see if that read 100 mA on your meter. Then you will have confidence in your meter ....

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 06, 2017, 01:46:12 am
content: Quote from: xrunner on February 06, 2017, 12:44:13 amI tried a 330K resistor with 10 Volts (from my benchtop power supply).I expected 30 mAmps, but the meter showed 11 mAmps.I also adjusted my bench power voltage, and the meter did in fact, adjust also.So this make me think this meter just has more resistance than I expected.Could this just be the "cheapness" of my meter model?

author: tautech
author_rank: Super Contributor
timestamp: February 06, 2017, 01:52:30 am
content: Did you read and understand this:Quote from: alsetalokin4017 on February 05, 2017, 05:49:58 amFuse type and value impact on the measurement as they have resistance and are part of the shunt circuit that a DMM uses in current measurement mode.

author: Muttley Snickers
author_rank: Unknown
timestamp: February 06, 2017, 01:53:15 am
content: As sleemanj has suggested I suspect you have possibly blown a track on the unfused 10 amp input and probably popped the 200 mA fuse as well, you should be able to do a resistance test between the meters general input (far right) and the 10 amp current input (far left) to verify this and it should display almost zero ohms otherwise a track or the current shunt are open circuit, you might also need another multimeter at hand for testing the 200 mA fuse or alternatively to test the resistance between the general input (far right) and the common input (centre) with the faulty meter set to measure mA and the second meter set to resistance measurement.

author: xrunner
author_rank: Super Contributor
timestamp: February 06, 2017, 02:01:01 am
content: Quote from: mikeinfodoc on February 06, 2017, 01:46:12 amNaw that ain't right. You shouldexpect.03 mA.(see above posts ...)

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 06, 2017, 02:11:41 am
content: Quote from: xrunner on February 06, 2017, 02:01:01 amSorry, decimal error. You are exactly correct.Also, after taking off the cover, it does appear that the fuse is blown. (if my eyes are seeing correctly)I expected to see 2 fuses, but I just see one.The fuse appears to be marked: f315mAL 250VSo I assume the fuse is a "fast blow" fusedesigned to handle up to 250Vand only 315mAmpsI am not sure what the trailing "L" means however.Does that sounds right?Anyways, I will replace that, AND the 2 double AA batteries.Then I will plan to post a follow up.

author: sleemanj
author_rank: Super Contributor
timestamp: February 06, 2017, 02:15:54 am
content: Quote from: mikeinfodoc on February 06, 2017, 02:11:41 amCheap meters often only have one replacable fuse, the 10A socket is typically one-shot fused by a trace on the PCB vaporising open.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 06, 2017, 02:26:01 am
content: Ok, now that I have replaced the fuse (and batteries), the meter is measuring properly with expected values.Thank you all fore the feedback.