Multimeters and microamps range - Page 1
Forum section: electronics / testgear

author: 001
author_rank: Super Contributor
timestamp: November 02, 2018, 10:23:20 am
content: Hi!How often do You use DMMs in microamps range? What cases?I see what common multimeters has only three current ranges: 10A, 200mA and 2mAWhy? Burden voltage became so high or something else?

author: Fungus
author_rank: Super Contributor
timestamp: November 02, 2018, 11:39:50 am
content: And so it begins...

author: coromonadalix
author_rank: Super Contributor
timestamp: November 03, 2018, 02:21:04 am
content: @001I dont want to be rude, but I have to ask : you seem to start some never ending threadsis there a reason ?You should be able to have your own opinion ? without creating a fuss / endless arguing in the threads.I think the "So it begins" maybe appropriate ?

author: 001
author_rank: Super Contributor
timestamp: November 03, 2018, 09:03:20 am
content: Quote from: coromonadalix on November 03, 2018, 02:21:04 amI`m tech only I`m not sertified engineerAnd my opinion may be wrongI never use DMM in a mA mode but maybe someone use it

author: Fungus
author_rank: Super Contributor
timestamp: November 03, 2018, 09:16:43 am
content: Quote from: 001 on November 03, 2018, 09:03:20 amIf it's on there then somebody uses it.Quote from: 001 on November 02, 2018, 10:23:20 amNope. Even these have microamps.

author: 001
author_rank: Super Contributor
timestamp: November 03, 2018, 09:35:07 am
content: Quote from: Fungus on November 03, 2018, 09:16:43 amWOW! I never seen it. My 838 has no microsWhen do You use it? What is typical application?

author: Martin.M
author_rank: Frequent Contributor
timestamp: November 03, 2018, 10:16:36 am
content: nanoA.. and it like resistors < 2 Gigaohms.

author: HKJ
author_rank: Super Contributor
timestamp: November 03, 2018, 11:27:09 am
content: Quote from: 001 on November 03, 2018, 09:35:07 amAnything that can run for more than a few weeks on batteries need uA in power consumption, it is rather practical to be able to measure it.When you get into electronic stuff there is often small currents.

author: Performa01
author_rank: Super Contributor
timestamp: November 03, 2018, 11:40:49 am
content: Some people use the µA range in a DMM to check the current consumption of low power circuits.Most of these designs make use of a modern MCU that has several energy saving / sleep modes, where the current consumption actually can be as low as just a couple microamps. Then the MCU wakes up e.g. every 10 seconds for just a few milliseconds and does something, where the current consumption is much higher of course. But the average power consumption of such a circuit is still very low.A DMM with microamp range would be able to measure the sleep mode current, but not the average current (unless the circuit wakes up many times within a second) and the sleep mode of the circuit has to be bypassed in order to measure the true operating current.Long story short: A DMM is not the best tool for this anyway, we rather use a DSO with external shunt resistor (often together with a differential voltage probe) or a dedicated low current probe for such measurements.We can also use an external shunt resistor with reasonable accuracy to measure microamps and even lower with just about any DMM. My >30 years old Metex M-4650 meters (20000 counts, 0.05% base accuracy) can measure currents down to single digit picoamps in the 200mV range, thanks to their very low input current and high stability.EDIT: For this, I need not even have an external shunt resistor, the internal 10MOhm resistor is way accurate enough for this kind of measurements.I have often used them for leakage current measurements and did not need to power up my electrometer.

author: CDaniel
author_rank: Frequent Contributor
timestamp: November 03, 2018, 12:12:10 pm
content: Usually is not very practical to have very low current capability since in circuits you measure the voltage on some resistors to figure out the current and you can't cut every trace to insert a microampmeter . But they put this ranges because it is very easy ... not because people use them often .

author: drussell
author_rank: Super Contributor
timestamp: November 03, 2018, 12:14:26 pm
content: Quote from: 001 on November 03, 2018, 09:35:07 amThere are many, many different versions of the838, made across many decades.QuoteUhhh, you would use it for measuring currents less than a few mA?Low power circuits where you need the resolution of a small current range to measure properly are quite common in electronics. Low power micros, small or long-term battery operated circuits, sleep modes, standby, etc. you often want to know what the current consumption is, even if it is very small. Consider a batter-backed SRAM setup to hold settings and machine state when power is not applied. You want to be able to see how much current is actually being drawn so you can tell if it will meet your requirements for battery life, etc, so you need to know how much power it draws...What do you use your A and mA ranges for? uA are also used to measure stuff, just at lower currents...

author: HKJ
author_rank: Super Contributor
timestamp: November 03, 2018, 12:18:07 pm
content: Quote from: Performa01 on November 03, 2018, 11:40:49 amYou may be able to measure average current by placing a large film/ceramic capacitor after the ammeter.Another solution is a fast 6½ digit meter, my Keithley's are very fast and with the number of digits they can handle a decent dynamic range, But not everything.

author: Gyro
author_rank: Super Contributor
timestamp: November 03, 2018, 12:54:27 pm
content: I rarely (if ever) bother to use current ranges on DMMs (my bench meters don't even have them!).There's usually a resistor somewhere in the circuit that you can measure the voltage drop across to derive current with minimal circuit disruption. If not, I stick in a series resistor with low enough burden voltage not to disrupt things.For higher currents I use an external shunt (I've established a collection) or current clamp... or use the current display on the bench PSU (normally close enough).This approach means minimum circuit disruption and burden voltage in all cases, leaves the DMM free for other voltage measurements, and minimizes the amount of blown fuses and magic smoke from leaving the leads in the wrong jacks!

author: Gyro
author_rank: Super Contributor
timestamp: November 03, 2018, 01:00:44 pm
content: Quote from: 001 on November 02, 2018, 10:23:20 amWith your 2mA range, everything after the decimal point is microamps anyway!

author: Fungus
author_rank: Super Contributor
timestamp: November 03, 2018, 01:33:27 pm
content: Quote from: CDaniel on November 03, 2018, 12:12:10 pmNot true when you're prototyping on a breadboard.Quote from: CDaniel on November 03, 2018, 12:12:10 pmRubbish, it's an essential feature for a lot of people.I bet most people here wouldn't even consider owning a "primary" multimeter without a uA range.

author: nctnico
author_rank: Super Contributor
timestamp: November 03, 2018, 06:05:16 pm
content: Quote from: Fungus on November 03, 2018, 01:33:27 pmThe burden voltage often kills the uA range in regular DMMs. For measuring uA supply currents I have power supplies with uA range in the current metering.

author: 0culus
author_rank: Super Contributor
timestamp: November 03, 2018, 06:13:07 pm
content: My Fluke 87 has a uA range, but haven't really had much opportunity to use it.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 03, 2018, 06:18:17 pm
content: Quote from: Performa01 on November 03, 2018, 11:40:49 amA DSO is not really accurate enough for long measurements though. If a DMM doesn't have the dynamic range to do it, how on earth will a DSO be any more usefull? This what you describe is an example of why people use 6.5/7.5 digit DMM's!If we really need to measure very very small currents we tend to use pico/femtoampmeters, which use a TIA feedback network instead of a current shunt, giving much lower (pretty much 0) burden voltage, have worse AC performance.As an IC researcher, I almost never goabovea few milliamps... if I am sending milliamps through a circuit I'm often already dealing with driver stages.

author: HKJ
author_rank: Super Contributor
timestamp: November 03, 2018, 06:58:36 pm
content: Quote from: nctnico on November 03, 2018, 06:05:16 pmNo, for uA the burden voltage is very low on almost all multimeters, the problem is the high uA range that goes into mA. At a few mA the 100ohm resistor (Standard uA shunt for most meters) will have a maybe significant voltage drop.

author: Gyro
author_rank: Super Contributor
timestamp: November 03, 2018, 07:29:34 pm
content: That's why it's better to pick a resistor value with an appropriate burden voltage for the specific application and measure the voltage across it, rather than use the current range.A nice low mV range is far more useful.

author: nctnico
author_rank: Super Contributor
timestamp: November 03, 2018, 07:33:25 pm
content: Quote from: HKJ on November 03, 2018, 06:58:36 pmNot no. YES. That is exactly the problem! A lot of circuits don't like a 100 Ohm resistor in series with the power supply. IIRC Dave's uCurrent has been designed specifically to overcome problems due to the relatively large current shunt values in the uA range of a typical multimeter.

author: Fungus
author_rank: Super Contributor
timestamp: November 03, 2018, 07:33:48 pm
content: Quote from: Gyro on November 03, 2018, 07:29:34 pmI often use the one built into my multimeter.

author: Gyro
author_rank: Super Contributor
timestamp: November 03, 2018, 07:36:47 pm
content: Quote from: nctnico on November 03, 2018, 07:33:25 pmUnfortunately he didn't includeenoughshunt resistors, only one for every 3 decades.

author: HKJ
author_rank: Super Contributor
timestamp: November 03, 2018, 07:49:27 pm
content: Quote from: nctnico on November 03, 2018, 07:33:25 pmTry doing a bit of math, at 100uA the voltage drop is 0.01V, I doubt that will be significant for any circuit.When you get into mA it is often significant, the best solution is usual to use the mA range on multimeters. That has a 1ohm resistor and a 1ohm fuse (Roughly speaking), but it requires some resolution on the meter.Where it really goes bad is above 100mA, hopefully your meter has a A range with enough resolution.

author: CDaniel
author_rank: Frequent Contributor
timestamp: November 03, 2018, 10:23:15 pm
content: Many active circuits consume current in impulses ... so your calculation can be very wrong and the burden voltage inacceptable .It is allways better to use an external shunt resistor and measure the voltage .

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 04, 2018, 06:18:14 am
content: Quote from: HKJ on November 03, 2018, 07:49:27 pmIf you are dealing with a modern CMOS chip that has a supply voltage of say 800 mV or less, 10 mV drop is very significant. And that doesn't even start to get into the problem of you building an RC filter with your input.Even when you are using an external shunt you still have a burden voltage. If you really want to measure small currents, the best solution is to use a feedback topology like a TIA, similar to what electrometers/picoammeters do.

author: Fungus
author_rank: Super Contributor
timestamp: November 04, 2018, 11:30:54 am
content: Quote from: TheUnnamedNewbie on November 04, 2018, 06:18:14 amYes, but if you're not...

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2018, 12:54:37 am
content: The problem with the lowest currant ranges is that if the circuit intermittently draws high current, it will either fail due to excessive burden voltage or blow the fuse in the multimeter. The later can be avoided by using a current limited power supply as shown below where the right most meter is measuring 2mA with a resolution of 0.1uA.