Measuring amps changes circuit behaviour

Other Hardware General Electronics
1

Hi there!

There is something that is driving me nuts:

I got a circuit thats running fine when it’s directly connected to my bench power supply. Also programming the mini pro is no problem.

As soon as Im trying to measure the current consumption with my multimeter its beginning to behave oddly:

The PIR sensor fires exactly every 6 seconds which does not happen when connected directly.

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When I connect the programmer and trying to upload a sketch I get a programmer out of sync error.

Also the mini resets when entering deep sleep.

Connected supply -> multimeter uA -> multimeter com -> main power rail of the breadboard. Supply GND connected to GND directly to breadboard powerrail. I tested 2 multimeters. Both result in the same behavior.

Since I develop a battery powered device I like to optimize the power consumption but its nearly impossible with this behavior.

What could be the problem?!

Thanks in advance!

2

Check your voltage at the load, your meter is putting a resistor in series with the power lead. It measures the voltage drop across that resistor and reports it as current. After you are measuring the voltage at your load adjust the voltage up a bit. An annotated schematic would help answer the question more accurately. Your load is indicating the Arduino input voltage.

3

Which current range are you using? Higher current ranges have smaller burden voltages. I suspect the burden voltage is too high.

As an alternative try putting a 10 ohm resistor in series with the supply and measure the voltage across this with your meter. Then you know the burden resistance is small.

Auto-ranging meters are a nuisance for this kind of reading BTW.

4

Thanks for your replies!

Im using an autorange multimeter set to uA.

I suspect that could be the problem then :confused:

Okay so classical ohms law calculation with the value measured over the small resistor?

5

How accurate is your meter?
100uA x 10 ohms = 1mV.

6

Hm, thats true.

I have this one at hand:

7

It's a nice meter.
On the 500mV range, the accuracy is +/-0.1% + 5 digits, that will be +/-550uV (when using REL Q mode)
So if the meter reads 1mV, then the actual value could be anywhere between 450uV and 1550uV

If you are measuring current with a 10Ω resistor then it's somewhere between 45.0uA and 155.0uA

8

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