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I am using a BK Precision 2709B multimeter and attempting to measure the current of a device that runs off of a ~3.7V battery. The device boots up, initializes and then goes into its lower power state.

If I use the 10A setting, my device boots up just fine but I need to measure uA or mA. When I attempt to use the mA setting, the device attempts to boot and then continuously reboots. Its not able to finish its initialization process before it cuts off. The current reading on the meter is ~30mA before the device cuts off and attempts to boot up again. The meter says 400mA max, so it doesn't seem like it is over the maximum allowed current for the mA setting.

Is there anything I can do to get past the initialization so I can measure the lower power state?

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    \$\begingroup\$ The internal resistance of the ammeter is probably interfering with the device. Maybe the startup current is limited and the device detects a voltage drop, which causes the protection to kick in. \$\endgroup\$ – a concerned citizen Feb 4 at 14:39
  • \$\begingroup\$ Is there anything I can do to get past the initialization so I can measure the lower power state? Either use 10A jack, or force the scale to 400mA range...and accept the reduced accuracy of the reading. Do not allow auto-range! - these sometimes cause great problems when close to full-scale when they cannot decide between two ranges. Your current meter reduces the battery voltage passed on to your device, especially on the lower-current scales. \$\endgroup\$ – glen_geek Feb 4 at 15:13
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The problem will be in the shunt used to measure the current. The resistance of the low current shunt is high enough that the voltage to the circuit drops when it tries to draw a (somewhat) higher current.

If you are only interested in the current after the circuit is started, then you could connect the meter as before - but short the meter leads. This effectively takes the meter out of the circuit.

Once the start up phase is passed, remove the short. The current will now go through the meter.

If the current draw is low enough, then the circuit should keep on going.

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  • \$\begingroup\$ This worked perfectly! Thank you so much for the quick response and helpful info. \$\endgroup\$ – Sean Feb 4 at 14:51
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    \$\begingroup\$ You can also try this "hack" - place a large-value, low-ESR capacitor across the current meter. This will behave like a dead short for a small amount of time. It will also filter any fast transients from measurement. \$\endgroup\$ – rdtsc Feb 4 at 16:26
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As mentioned in answers and comments: your multimeter has some internal resistance that is needs to measure the current. At a high current range (10A) this resistance is quite low so there is no issue. But at lower current ranges (30 mA) the meter needs a higher value resistor in order to measure the current with enough resolution.

The voltage across this measurement resistor is called the burden voltage. During startup your device draws a lot of current and that means a very high burden voltage will develop across the multimeter when it is in the 30 mA range.

Is there anything I can do to get past the initialization so I can measure the lower power state?

Yes, we have to avoid that voltage drop during startup, for that I would try this setup:

schematic

simulate this circuit – Schematic created using CircuitLab

First close the switch and connect everything.

Let the device boot up, it will work now as the switch is closed so the device can take all the current it needs without a voltage drop across the multimeter.

When the device has booted, open the switch, you can now measure the current.

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You could use a low burden current meter from Altonovus called a NanoRanger. It will only drop the voltage by 50mV on all current ranges - barely noticeable to your device.

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    \$\begingroup\$ This would be a good answer if you used could, instead of should. \$\endgroup\$ – a concerned citizen Feb 5 at 9:24

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