mA meter says 0.04mA = 4.0uA - Page 1
Forum section: electronics / beginners

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 30, 2014, 03:57:44 pm
content: Subject says it all. I'm using a UEI DM 383. Granted it isn't the world's best, but geesh.I want to use one of those cheap little buzzers for an alarm on an Arduino project I have. Since there's no datasheet, I connected it to a mA meter to ensure it wouldn't burn out a pin. I set the mA selector scale to 200 mA. I got 0.04 on the display. Switched to 20 mA scale. Same. Same at 2 mA scale. 200 uA scale however, read 4.0.I find that a little disturbing. While both readings are far below the pin limit on the Arduino boards and renders my concern moot on that point, the greater concern is that I have an unreliable meter, or I simply don't know how to use it. What gives? Any ideas?

author: Balaur
author_rank: Unknown
timestamp: April 30, 2014, 04:16:04 pm
content: Did you use the AC scale (and the buzzer was ringing?) or DC? The standard approach is to output a tone and use the ACA scale to get the actual current when driven.The 0.04 could be the residual reading. Is the multimeter able to go down to 000.0/00.00/0.000 when not connected?

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 30, 2014, 04:24:01 pm
content: Quote from: Balaur on April 30, 2014, 04:16:04 pmDC. It's one of those 5vdc buzzers with the oscillator built in. ACA reads nothing. Yes, to the zeroes. However, on the uA scale is 00.0. It's only a 3 digit lcd display plus sign.

author: Monkeh
author_rank: Super Contributor
timestamp: April 30, 2014, 04:34:31 pm
content: Quote from: chipwitch on April 30, 2014, 03:57:44 pmPossibly urden voltage. Check the resistance in mA and uA ranges.

author: w2aew
author_rank: Super Contributor
timestamp: April 30, 2014, 04:55:15 pm
content: Quote from: Monkeh on April 30, 2014, 04:34:31 pmThis is likely what is happening. In the 2mA range, the burden voltage is higher, thus dropping more voltage across the meter, making less voltage available across the load, so under those conditions, the load (buzzer) is drawing only 4uA.

author: dog80
author_rank: Regular Contributor
timestamp: April 30, 2014, 05:27:14 pm
content: Also make sure that the red test lead is in the mA?A socket of the multimeter, not the A one.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 30, 2014, 05:30:37 pm
content: Quote from: dog80 on April 30, 2014, 05:27:14 pmIt is.

author: electr_peter
author_rank: Unknown
timestamp: April 30, 2014, 06:25:00 pm
content: Quote from: w2aew on April 30, 2014, 04:55:15 pmI agree with w2aew that issue is likely connected to DMM burden voltage (or current shunt resistance in other words).I measured few meters input resistances - 2A-10A range has <0.02 Ohm, 40-400 mA range has ~1-2 Ohm, 4000 µA and less range has ~100 Ohm. Resistances go up in factor of 100 (likely to reduce complexity and cost of DMM construction), so it may affect readings when choosing certain current ranges.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 30, 2014, 06:54:23 pm
content: Quote from: w2aew on April 30, 2014, 04:55:15 pmThat seems pretty unlikely to me. Burden voltage is usually less than 1 volt at full scale even on cheap meters. That would mean the burden voltage for 4 microamp on the 200 microamp scale would probably be ~20 mV out of a 5 V supply. That is extremely unlikely to make any difference. Also, 4 microwatt @ 5 volt seems like too little power. Even if your piezo buzzer were 100% efficient, that would produce a sound pressure level of only 60 dBA @ 1 meter. That is a pretty quiet buzzer. Also, the oscillator is going to take some power.My guess is that you are connecting the meter wrong, and in such a way that you are measuring some small voltage (20 mV) from a low impedance source -- possibly you are measuring the voltage drop across a supply wire. When you change range you change the shunt resistor and that changes your reading.

author: jlmoon
author_rank: Unknown
timestamp: April 30, 2014, 07:10:11 pm
content: so... wouldn't it be best to resort back to the basics and measure the voltage drop across a known value resistor to establish the actual current flow using the same potential as the micro controller pin of this unknown buzzer. That's what I would do .. wink winkJLM

author: idpromnut
author_rank: Unknown
timestamp: April 30, 2014, 09:35:23 pm
content: Another idea is to test the buzzer directly with a current limiting power supply, or with a 5v supply and a current limiting resistor: 5/(max current for buzzer) = minimum resistor value to use. Then measure the voltages, insert your meter to measure current etc.

author: Wytnucls
author_rank: Super Contributor
timestamp: April 30, 2014, 10:58:13 pm
content: You didn't say what voltage was supplied to the pin. If it was really 5V, the burden voltage wouldn't account for the uA difference and either your meter is faulty or your measurement setup is flawed.

author: han
author_rank: Frequent Contributor
timestamp: May 01, 2014, 04:15:07 am
content: Maybe noise from the buzzer effecting the measurement.You can put a small capacitor across the buzzer to fix the problem.

author: sleemanj
author_rank: Super Contributor
timestamp: May 01, 2014, 06:11:00 am
content: Buy or make yourself (many in parallel) a very small value resistor, put it in series, measure voltage across it. Then Ohm's law will tell you you what is what :-) Preferably use your second meter (everybody needs at least 2, even if one is a $5 job it's good enough) to measure the drop across the resistor at the same time the suspect one is in series showing you what it thinks the current is.

author: jancumps
author_rank: Unknown
timestamp: May 01, 2014, 06:54:18 am
content: ... Or a µCurrent?

author: robrenz
author_rank: Super Contributor
timestamp: May 01, 2014, 11:45:10 am
content: Or a SMU with 100µV burden voltage.