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Measuring wide range current draw of micropower applications

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Amper:
Hi!

I just had a little bit of time continuing my geiger counter power supply project (https://www.eevblog.com/forum/projects/low-quiescent-standby-boost-regulator-3-3-500v/), which now is working pretty much as i want it to but i ran into a new problem.

To summarize: The current state of the project is a Attiny13 controlling a boost converter feeding a cocroft walton multiplier boosting the voltage up from 3.3V to 500V. It charges up a 1uF storage cap so the multiplier, diodes and voltage divider go to sleep after a charge cycle. A new cycle is triggered either by watchdog timer or a certain number of pulses detected corresponding to a known discharge of the storage cap.

This results in a current draw of roughly 6uA while sleeping and 20-50mA for a few milliseconds recharging which happens at least every half hour by watchdog. The amount of energy pumped will depend on the depletion of the storage cap by detection, moisture or just loss in parts.

Until now i measured the average current draw by supplying the entire thing with a set of two 25F 2.5V capacitors in series and measuring the drop over a longer period of time. This worked nicely but has a set of annoying drawbacks, its labor intensive, the voltage is not constant, the capacitors have a voltage dependent capacity, a lot of tolerance and unknown self discharge which is also dependent on charge state, time, settling time and so on....

I now reached a current of 50uA using this method but i feel like this is the end of what is reasonable this way.

Now my question is, how do i o it better without having the dedicated professional instruments?

Sadly i dont have a bench meter with a modern logging function and the hp3457a only supports gpib which i dont have any hardware for.

How do i get the huge dynamic range required and the sampling rate needed for including millisecond pulses every few hours?


cheers!

iMo:
Sense a shunt with a logarithmic amplifier and ADC it with say 12bit adc at 10k samples per second. You may be logging the data on an SPI sdcard.. $10 hw solution.. :)

Amper:
mhoh, log amp is a nice idea, this way i could even do the math on the uC and record just averages and peaks.

OM222O:
you can use a universal shunt that has different ranges (I have a general purpose one that goes from 1ohm to 100kohm in 6 steps, where each step increases resistance by 10x) (you can look up ayrton shunt as well) and combine that with an ADC that measures the voltage across the shunt resistor. then you can dynamically switch the shunt resistance when the voltage goes too high / too low. a simple ADC + micro sd card should be easy enough to finish the task. I can draw a full schematic if you want but parts count is less than 20 and it's a fairly trivial task , so you should be able to do it yourself.

edit: I have a made a milliohm meter using that exact same technique, you can copy the universal shunt and the ADC sections directly and use them. just add a SD card reader to the MCU for logging over long periods. see the attachment for the schematic.

Amper:
Thats actually also something thought about but i didnt know how to bridge the resistors fast enough to catch millisecond pulses. Just having one shunt and multiple amplifiers and DACs could work too i guess just doing the switching over purely in software as the ranges are overrun.

One thing i just noticed is that basically i could just have the logger measure the high pulses and times between them as the low state is pretty much 6uA unchanged and i can measure that using the 3457a. Then its just a question of sampling rate and enough resolution to integrate the pulses, a cheap stm32f103 could probably do that (they have pretty fast adcs iirc). Of cause its only suitable for this particular case but it may e the smallest effort.

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