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I am just learning electronics and trying to reason some figures I measure in simple circuits.

I need a bit of a hand with this one.

I have a 3.2V power source, and 2, 1 ohm resistors wired in series.

I want to determine the current in the circuit. I know that

I=V/R

So, I know the voltage and the resistance, so assume that after I use my multimeter over the start and end points of the circuit I should see

I=3.2/2 I=1.6amps

However, my multimeter reports 102mA!

So how does this make sense? What am I overlooking. It sounds like I may be plugging in the wrong figures somewhere.....

Help please!

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    \$\begingroup\$ You'll want to present a schematic if you want anyone to answer this. Edit your question and use the schematic editing tool therein. \$\endgroup\$ – user65586 Oct 1 '16 at 22:12
  • \$\begingroup\$ Hey jdv, I would, but I'm using a phone and its not presenting that option. Sorry! \$\endgroup\$ – RenegadeAndy Oct 1 '16 at 22:14
  • \$\begingroup\$ Does your power source feature high output resistance or a constant current limit? \$\endgroup\$ – winny Oct 1 '16 at 22:21
  • \$\begingroup\$ What they said + some meters have high resistance on current measurement which can affect results. \$\endgroup\$ – Russell McMahon Oct 2 '16 at 0:01
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I assume you have checked and double checked the value of your resistors and voltage source, and that you are are correctly handling your multimeter.

Without more details it is difficult to guess. If it was sightly less that 1.6A I would have said it is because of the output impedance of your source and wires.

But I am quite sure it is because you have hit the maximum power rating of the 3.2V source.

You see, if you wanted to put 1.6A at 3.2V on your load, that would be 5.12W. But if your power source can not deliver that much power (a small power source of a battery), it will saturate and reduce its output voltage, and consequently, output current.

But you should test it yourself. Connect you load to the source and measure the voltage as well as the current. You will probably see that once the load is connected, the voltage should drop. Also check the maximum power rating of your voltage source.

Nevertheless, if you could provide more details we will do our best to pinpoint what is going wrong on your circuit :)

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I use my multimeter over the start and end points of the circuit

This is not how to measure current with a multimeter. Current flows through a circuit, so to measure the current you need to break the loop and insert your multimeter in series with the current you want to measure.

This explains how to do it right.

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Did you make sure the resistors are 1 ohm? Did you measure them with your multimeter?

Moreover, check if your power source is actually capable of sourcing 1.6A of current. Measure the voltage across the source with the resistors still connected. Chances are that your load is too heavy for the source and the voltage it provides drops because of its internal resistance.

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  • \$\begingroup\$ Aha this sounds possible. The power source is just 2, 1.5V cells in series. \$\endgroup\$ – RenegadeAndy Oct 1 '16 at 22:26

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