Simple active/standby current measurement

Intro

Analog multimeters are measuring current, anything else needs to be converted to current. Digital ones measure voltage, any other measurement needs to be converted to voltage. Need to measure a resistance: inject a current into it and read the voltage across. Need to measure a current? Add a series shunt resistor and measure the voltage across it. And here lies the problem: the voltage dropped on that resistor is usually lost from your power supply and it can sometimes be a significant fraction. Things get complicated when you need to measure a current from a low voltage supply, as most multimeters will drop a significant amount on the current shunt, degrading your supply. You really cannot afford to lose a few hundred mV of your 2V supply. There is another case where things are complicated: a circuit that consumes power over a high dynamic range: mA while active, uA while on standby.

Sensor nodes

I am working on some sensor nodes that will be battery powered. While active and transmitting they may consume some 20-50mA, but while sleeping the current goes down to a few µA. Current measurement is a very important task to make sure the circuit is working properly, especially in the stand by sate. My main multimeter is a Brymen BM857a which provides a good 50.000 count on the current measurement range, even though it wins the contest for the worst back light you will ever see. If I use it on the mA scale which allows 1µA resolution, it has a shunt of 5Ω, dropping 250mV at peak current. Since everything is powered from 2x Ni-MH batteries of 2.4V supply, the drop can push the circuit to go outside the operating range whenever the batteries are not full. Switching to the µA scale is a total no go, since the shunt resistance here is 100Ω, dropping even more at peak current.

Micro current measurement

The typical way to solve this problem is to use a dedicated circuit with a smaller shunt and a precision amplifier connected to your multimeter. The precision amplifier will take care of amplifying the drop on a smaller resistor and feed that to a normal multimeter to be read. There are quite a few designs out there, but Dave’s µCurrent  is very popular recently. I remember having some precision op-amps from the Led logger v3, but while those where not as good as the one used in the µCurrent design, they could do the job for me.

Simple design

The uCurrent has more features that I could need, so I was looking into simplifying the design…. when it hit me. If my meter can count up to 50.000 then on a 50mA scale it could give me the perfect range: read both the peak and the sleep current on one scale. But how to get there? Now, on the 500mV scale my multimeter has a special 500.000 count mode, which could resolve 1uV, but the truth is that the added extra digit seems to change randomly so it is useless. Using just 50.000 counts and a 10Ω shunt brings me to a worse case scenario than the above mentioned mA scale. But then I found the perfect compromise: 500uA scale has a resolution of 0.01uA and a 100Ω shunt. Changing the shunt to 1Ω would yield 50mA with a 1uA resolution, exactly what i need for my tests. The maximum current of 50mA would drop just 50mV which is a lot more acceptable in the circuit.

Conversion

The conversion is very simple and I did not need to change anything inside the multimeter: the shunt resistor can be added outside, in parallel with the internal one located between the COM and uA connectors, which is precisely the thing that I did. I have used a 1Ω 1% 0.6W resistor which is good enough for the application. I really don’t care about the 1% systematic error. No, I will not draw a schematic for this. 4 banana connectors and a piece of board later and the nice adapter is born. I really cannot remember when was the last time the connectors outnumbered the parts in the circuit.

Here is the multimeter with the extremely complicated circuit installed, before and after photos. No weight loss or muscle gain occurred.

Tests

Now, let’s run some tests. I used another, less precise multimeter, with a high burden voltage in series with the modified one, a 5V supply and a few resistors to check if everything is fine. First test at 5uA:

Then 50uA

Then 5mA

and finally 33mA. Everything looks fine.

Conclusion

I you are working with circuits spending time in active and standby and need to measure current in the 10s of mA and uA range, then this is the simpler method to use, provided you have a 50.000 count multimeter. With the external shunt my multimeter reads 50mA and 500mA on the uA scale, with enough resolution to count the uA going away in standby mode. With a 1Ω shunt I would say it is safe to measure up to about 100mA without worrying on the dropped voltage.