Measuring voltage and current with a multimeter

While making these measurements likely seem like an obvious task to most – it wasn’t immediately apparent to me how to take these measurements.  Voltage and current are measured in two very different ways.  One of the big stumbling blocks for me was understanding that voltage is always constant in a circuit.  That is – if you have a 5 volt power supply, and a simple circuit with two LEDs in it, each LED will get 2.5 volts.  We can see this by doing a quick example in the lab…

Here’s a simple series circuit with two LEDs.  It might be hard to se here, but I have both LEDs in series. and the leads hooking up to the multimeter are in parallel with the blue LED.  In this configuration, we are measuring the voltage drop across the LED which happens to be 2.46 volts.  If we move the leads to be in parallel with the white LED…

We can see that the voltage drop across that LED is 2.5 volts.  If we want to measure the voltage drop across both LEDs we can put one lead in parallel on ether side of both LEDs…

Here we can see that the total voltage drop is 4.97 which is darn close to the sum of the measurements from each LED (4.96).  That’s also the same as the total voltage in the circuit.   So that’s pretty straight forward.  If you want to measure voltage, you do so in this sort of out of band manner by placing the multimeter in parallel to the component you want to measure. The total voltage drop should equal the total available voltage in the circuit, in this case 5 volts.

Measuring current is much different.  To measure current you need to insert your multimeter inline in the circuit.  Doing so makes the multimeter a part of the circuit and it becomes required in order for the circuit to function.  Let’s take a quick look at doing that in the lab…

Here we have another simple series circuit.  This time we have a single blue LED with a single 100 ohm resistor in path between it and the 5 volts power supply.  The two red and black leads on the left hand side are the leads of the multimeter.  In this case, Im connecting the positive lead of the multimeter (red) to the negative side of the LED and the negative lead of the multimeter (black) to the negative side of the power rail.  In this configuration the multimeter is being used to complete the circuit.

You can see that we’re getting a reading of 11.45 milliamps on the multimeter.  In a series circuit the current will be the same at any point in the circuit.  But why are we getting 11.45 milliamps? If we go back to Ohm’s law, we can do some math to figure this out…

In this case, we want to solve the equation for I or intensity measured in amps.  We need to account for the voltage drop induced by the LED which in the case of the blue LED is 4 volts.  That leaves us with one volt considering we’re using a 5 volt power supply.  The resistance is also know to be 100 ohms.  If we solve the equation we can see that the circuit should have a current of 10 milliamps which is pretty darn close to what we’re measuring at 11.45 milliamps.

Let’s take another example this time with a red LED…

Here we can see that we’re measuring the current to be 24.5 milliamps.  Let’s do the math again but this time plug in the voltage values for the red LED which are different than that of the blue LED (these values were taken off the spec sheet for each LED)…

Here we can see since the red LED only takes 2.5 volts that the circuit has 2.5 volts left which need to be accounted for in the equation.  The resistance stays the same since we’re using the same resistor.  Solving the equation we get 25 milliamps which again is pretty darn close to what we got on the multimeter.  You’ll note that all of these examples were performed using series circuits.  There’s a significant difference between series and parallel circuits and I hope to cover that in a post soon.

 

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