# Why would a multimeter (ammeter) connected in series prevent the instrumented device from powering on?

I'm trying to measure the current used by a 3.5" hard drive in an external enclosure. I connected my multimeter in series with the positive 12V power supply, but in doing so, the hard drive fails to startup. It tries to startup (I can hear mechanical movement) but quickly aborts and retries cyclically without success. During this process I can measure about 350mA of current (on the 10A meter setting) and a stable 12V -- of course, the device works fine without the multimeter in series.

I'm wondering what might cause this phenomenon and if its common or well-understood in general. I've tried with two multimeters, both of different grades and observe the same results. Could there be an in-rush current limiting device in the multimeter which is impeding a high-current demand of the hard-drive? The hard drive would draw the most current during the acceleration of its motor, which typically happens during startup.

I don't have a clamp meter, but I suspect using one would be best for this application as it removes the physical connection with the device.

• Assuming the ammeter's fuse is still OK ... try putting a 1000uF (or larger) capacitor across the HDD's supply pins, downstream of the ammeter. That may smooth out the current demand pulses enough to work. Or, ... get that clamp meter. You know you want it... Feb 14, 2021 at 18:51
• Read this question: electronics.stackexchange.com/questions/546569/… and its answers, it describes the same issue. Feb 14, 2021 at 19:26
• Does this answer your question? Why is my multimeter stopping device from booting when measuring current? Feb 14, 2021 at 21:02
• Yes, that is a very similar situation, but I'm experiencing the issue on the 10A setting, not in the mA range. It does sound like the exact same problem though -- if the resistance of the of the 10A metering rail is very low, then the current demand must be compensating for a detectable voltage drop (causing the device not to boot). Feb 14, 2021 at 22:35
• @BrianDrummond - I do want one, and I will probably get one soon :) Feb 14, 2021 at 23:14

Meters of the 1.999 type are designed with a 199.9 mV full-scale DC meter internally. Every reading is scaled to give 200 mV at full scale on the selected range. That means that a 20 A range will have a resistance of $$\ R = \frac {200m}{20} = 10\,\mathrm {m\Omega} \$$ so it will hardly affect your drive.