Best way to measure current

[BuffaloEngineer]

Our most frequent need for measuring current is on battery powered devices that sleep 99% of the time, then have brief current spikes. What measurement options are available to measure the current, either by averaging the current or accurately capturing the current spikes?

 

[KeepItSimpleStupid]

Before you ask that question, what range of battery voltage? Where you measure it turns out to be a BIG problem if a large cap is in the way.
What range of currents are you interested in?
What bandwidth are you interested in?

You should be able to use a current to voltage converter (e.g. Zero resistance ammeter) and possibly the math functions on the scope.
Multiply V(t)*i(t) and average.

At 1 mA it's easy to design at 10 mA, you really have to worry about your error sources and at 100 mA it's more difficult. I successfully built a 4-terminal I-V converter that was bias able from -10 to 10 V with 4 ranges from 100 m and lower by decades. It had an offset of about 40 pA for DC, but AC was right on. In the system, the AC/DC gains could be calibrated out. The DC provision was never debugged/developed. A glitch on may part where 0V from a D/A converter wasn't zero and the offset voltage was not divided down.
The system was used in-house as as front-end for a Lock-in amplifier. 2 terminal and 4 terminal mode was one of the options. V(open circuit)/measure was another), Suppress was another and so was Zero Check/Zero correct (which was never debugged/developed). I also added a clipping indicator.

 

[Ian Rogers]

The way I read your question is:- You have a battery powered device and you want to find out how long the battery will last??

Run the circuit in NON sleep mode and measure the current on the power supply... Then run the circuit in sleep and measure the current... When your circuit is running normally you can work out the current in 1 second...

Take the battery data... ie 1200mA/H I'm assuming a a voltage of 3.7V... If your circuit runs at 20mA for 1omS and .01mA for 990mS you can average that out and divide... The battery data will also give you the curve for battery drain...

Allow for the battery drain ie... 4.2V to 3.0v and you can work out how long the device will function..

 

[Diver300]

The current spikes may not be constant current. I would suggest that you run the device with fast repetition rate and see how long the battery lasts, and then factor in the sleep current.

So if your device runs for 1 second every two minutes, get it to run for 1 second every 2 seconds and record the battery voltage. That way you will capture the turning on and off phases that will happen every time it runs. The battery data will tell you what effect the sleep current will have on the battery capacity.

 

[BuffaloEngineer]

Run the circuit in NON sleep mode and measure the current on the power supply... Then run the circuit in sleep and measure the current... When your circuit is running normally you can work out the current in 1 second...

I like that idea, but one thing that's important for us is changing register values and seeing how that affects current - so if we wanted to check 8 different register values on a 200mAh battery and our active current is still less than 20mA, it's a long test.

 

[BuffaloEngineer]

You should be able to use a current to voltage converter (e.g. Zero resistance ammeter) and possibly the math functions on the scope.
Multiply V(t)*i(t) and average.

This has come up a few times - we'll put a circuit together every once in a while, but I haven't take the time to build and validate it's performance. The question for me is whether to make something or buy something - what will make these tasks simplest in the long term. Do you have a schematic of the circuit you built?

Our batteries are all rechargeable LiPo or Li primary, so low current, low voltage.

 

[Ian Rogers]

If you have different things causing "current Spikes" You place a resistor on the PSU and measure the voltage on a scope.... I used a PC scope to store several minutes, and I was able to watch the voltage... The current fluctuations are shown on the scope.... "Where there is a will!!!" as they say...

 

[ronsimpson]

I would not count on the discharge of a battery as a current meter. If the battery was charged slightly differently then it will last differently. Temperature changes will have too much effect. Too may sources of error.

I would put a meter in line with the battery. That simple. Analog meter average the current over a period of time. Digital meters also have that function.

 

[BuffaloEngineer]

What PC scope do you use? We need to upgrade our scope, but haven't yet - a PC scope

You place a resistor on the PSU and measure the voltage on a scope.... I used a PC scope to store several minutes, and I was able to watch the voltage... The current fluctuations are shown on the scope....

What PC scope do you use? We need to upgrade our scope, but haven't yet - a PC scope with decent logging ability might be a good start!

 

[Ian Rogers]

I have several I made myself... But a storage scope would do the job.... I have a decent one but it was easier to use my own... I have kinda ported the EPE one to a pic18f4550 and blitzed it out the USB port to a PC running a bit of VB6 code... The pic18f4550 has quite a decent amount of RAM, so it can store quite a bit!! The USB has two 64 byte transfer buffers so I could shift 128 bytes at a time...

 

[KeepItSimpleStupid]

Schematic: I probably have a hard copy somewhere. Later, I'll post some stuff buried in my head.

For now, look at: http://www.kerrywong.com/2013/01/17/current-adapter-for-low-current-measurement-using-ts1001/

and

http://www.eevblog.com/projects/ucurrent/

And watch "leakage current" effects: http://www.farnell.com/datasheets/1558295.pdf Max 3 uA could swamp measurements.