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I'm using an arduino due (3.3V) which its power regulator can deliver 800mA of current. Can I connect a multimeter in series and get a reading of the current my circuit draws, and can i leave it connected so to measure the current in real time?

(I regularly re-program the Due with the circuit connected is there a chance the multimeter can damage the device in this situation?)

Will such a config affect the circuit?

At last should I break the circuit and connect the multimeter to the positive or to the ground connection?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ You have current probes? And the multimeter typically has internal protection for current, voltage, and surges. \$\endgroup\$ – 12Lappie Apr 13 '17 at 19:19
  • \$\begingroup\$ A multimeter in ammeter mode is just a shunt resistor. So your question becomes "Will adding a very small resistor (less than 1Ω) in series with a power regulator cause damage?" The answer is almost always "no". \$\endgroup\$ – Bort Apr 13 '17 at 19:24
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    \$\begingroup\$ @JohnAm - Depends on your circuit, which you didn't provide a schematic of. Most likely, the positive side would be the best option as "all" current should be flowing through it. \$\endgroup\$ – Bort Apr 13 '17 at 19:26
  • \$\begingroup\$ @Bort Can you explain why this is the case, isn't the same to measure current go -in and current go -out? The circuit is "expanding" and i connect more op-ams pots, etc. All the circuit is powered by the V+ and returns to the ground of my arduino. \$\endgroup\$ – John Am Apr 13 '17 at 19:28
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    \$\begingroup\$ @JohnAm The information in Measuring current drops voltage? may be of interest to you. \$\endgroup\$ – Andrew Morton Apr 13 '17 at 20:07
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If your DMM is of auto-ranging type, never use it for measurement of current in a low-voltage environment. The auto-ranging DMM will change the measuring resistor (shunt) value in-flight, which will add a varying voltage drop affecting your circuit power supply.

Instead, use a 1% 0.1 Ohm resistor in-line with your power rail (between the regulator and Arduino) as a starting value, and use the milli-voltmeter mode of your DMM to measure the current, I (mA) = V(mV)*10 for a 0.1 Ohm resistor.

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    \$\begingroup\$ It's ok if it does not change on the fly @JohnAm. \$\endgroup\$ – Trevor_G Apr 13 '17 at 19:40
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    \$\begingroup\$ I would seriously advise against the use of DMM as current meter in digital electronics. It will depend where the power rail is cut, and where bypass caps are residing. Insertion of 1-meter- long leads might disrupt stability of voltage regulator, so all things might happen. \$\endgroup\$ – Ale..chenski Apr 13 '17 at 19:50
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    \$\begingroup\$ @JohnAm if there is only one supply, and that's the only return path, it theoretically makes no difference. Like someone else said though, the long leads can cause issues. If you are going to do it you are better to actually add banana plugs on the existing line where you cut it and plug them into the meter so you do not add any wire. \$\endgroup\$ – Trevor_G Apr 13 '17 at 20:02
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    \$\begingroup\$ @JohnAm its not the resistance its the extra inductance. Yes, when in doubt use a current sensor... even just a simple micro-resistor is fine. \$\endgroup\$ – Trevor_G Apr 13 '17 at 20:07
  • \$\begingroup\$ Ok, this seems to go nowhere. In general, what goes in (V+), same goes out on GND. Could you please describe what is your REAL PROBLEM with measuring the consumption current in your particular device? What kind of "damage" did you suffer? What is the "danger" you perceive? \$\endgroup\$ – Ale..chenski Apr 13 '17 at 20:26
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Meters in current mode and voltage mode do not stimulate the circuit, so, when used appropriately should do no damage.

Using it in current mode between the wrong two points can however create a short so care must be taken not to have it in current mode when you are trying to measure a voltage to ground. Also, as others have mentioned, avoid auto-ranging ammeter scales.

When in resistance or diode mode the meter actually generates a current in the probes. Although the levels are normally small, use these modes with great care when testing in-circuit.

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    \$\begingroup\$ Do NOT place a current meter between a positive source and ground, as you will be shorting it to ground! That is to say: You don't use an ammeter in parallel, you must use it in series. \$\endgroup\$ – Bort Apr 13 '17 at 19:28
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The "Output Short Circuit Current" of an MCP6002 operating at 5.5 V is in the region of 23 mA. Ref: Microchip datasheet. It will be less at 3.3 V.

For a TL081, Output Current (Typ) is 10 mA. Ref: TI datasheet.

So your circuit as presented is not going to approach 100 mA let alone 800 mA.

On an Arduino Due (circuit diagram), the 3V3 power output line comes through an LM2734Y with a max. output current of 1 A to get 5 V and then an NCP1117ST33T3G, also with a maximum output current of 1 A. Both devices have thermal overload and over-current protection.

It appears that you do not need to worry about drawing too much current from the power supply circuitry both because your circuit won't draw much current and because the supply is protected.

If you expect to be drawing a high current, maybe more than 300 mA, (through the power supply ICs) for a sustained time, it would be prudent to either use a separate power supply instead or add small heatsinks to IC2 and IC4 on the Due.

It is my understanding that op amps may have better performance characteristics when their voltage supplies are in the higher part of their allowed range. Adding a resistance to measure the current draw will have the effect of reducing the voltages on the power rails of the op amps, which is probably not desirable. You have 5V available so it might be better to supply the op amps with that and translate the voltages, if appropriate, for a lower overall power draw and possibly improved op amp performance.

In summary, measuring the current with a multimeter will not physically damage your circuit, but it may affect its operation and does not appear to be necessary.

If the current measurement is important then you could power your circuit from a bench PSU which has a voltage sense input. Models with a voltage sense input are going to have an output current reading. Connect the sense wire(s) appropriately and connect the PSU ground to the Due ground. If you determine that your circuit is never going to draw too much current for the Due, your job is done.

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