Multimeters and microamps range

[001]

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 2mA
Why? Burden voltage became so high or something  else?   

[Fungus]

And so it begins...

[coromonadalix]

@001   

I dont want to be rude, but I have to ask : you seem to start some never ending threads    is 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 ?

[001]

You should be able to have your own opinion ? without creating a fuss / endless arguing in the threads.

I`m tech only I`m not sertified engineer
And my opinion may be wrong
I never use DMM in a mA mode but maybe someone use it

[Fungus]

I never use DMM in a mA mode but maybe someone use it

If it's on there then somebody uses it.

I see what common multimeters has only three current ranges: 10A, 200mA and 2mA

Nope. Even these have microamps.

[001]

Nope. Even these have microamps.

WOW! I never seen it. My 838 has no micros 

When do You use it? What is typical application?

[Martin.M]

nanoA 
.. and it like resistors < 2 Gigaohms.

[HKJ]

When do You use it? What is typical application?

Anything 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.

[Performa01]

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.

[CDaniel]

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 .

[drussell]

WOW! I never seen it. My 838 has no micros 

There are many, many different versions of the 838, made across many decades.

When do You use it? What is typical application?

Uhhh, 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...   

[HKJ]

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.

You 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.

[Gyro]

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!

[Gyro]

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 2mA
Why? Burden voltage became so high or something  else?   

With your 2mA range, everything after the decimal point is microamps anyway! 

[Fungus]

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 .

Not true when you're prototyping on a breadboard.

But they put this ranges because it is very easy ... not because people use them often .

Rubbish, 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.

[nctnico]

I bet most people here wouldn't even consider owning a "primary" multimeter without a uA range.

The 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.

[0culus]

My Fluke 87 has a uA range, but haven't really had much opportunity to use it.

[TheUnnamedNewbie]

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.

A 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 go above a few milliamps... if I am sending milliamps through a circuit I'm often already dealing with driver stages.

[HKJ]

The 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.

No, 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.

[Gyro]

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.

[nctnico]

The 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.

No, 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.

Not 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.

[Fungus]

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.

I often use the one built into my multimeter.

[Gyro]

The 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.

No, 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.

Not 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.

Unfortunately he didn't include enough shunt resistors, only one for every 3 decades.

[HKJ]

The 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.

No, 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.

Not 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.

Try 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.

[CDaniel]

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 .

[TheUnnamedNewbie]

Try 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

If 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.

[Fungus]

If 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.

Yes, but if you're not...

[David Hess]

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.