Day Four – Parallel Resistors

Today’s circuit is building on the last circuit’s theme, showing us the properties of resistors in parallel instead of series. Let’s look at today’s diagram:

Pardon the dimness of today’s photo, I’m worn out from working on my car and didn’t realize it was so dim! Anyway, notice the way the 22K and 33K resistors are arranged; like parallel lines. When the select switch is set to B, current flows across one terminal of each resistor, then round to the other terminal of each resistor, instead of flowing through one resistor and then the other. If the select switch is set to A, current flows only through the 33K resistor. The diagram also shows the requisite battery, supplying 3VDC today, a power switch, and the meter. Here’s what it looks like all wired up:

So, what can we find out when we power the circuit up? Here’s the meter readout with the selector set to A:

The meter reads about halfway up the scale. So, let’s set the selector to B:

Woah, almost all the way at the top of the scale. What’s going on? What the book explains is that, in a parallel circuit, only part of the current goes through each resistor, and that in a parallel circuit, the total resistance is always -less- than the value of the lowest resistor connected in parallel. So the total resistance is something less than 22 kilohms, in this case. The Wikipedia article on parallel circuits does a good job of explaining this in more detail, including how to use Ohm’s law to calculate the resistance. At this stage in the game for me, I’m more interested in the hands-on results than the mathematical details of the physics (though I do read the articles as I write this, to attempt to learn some of it, as I encourage you to do, too!), so I whipped out one of my trusty multimeters to test the concept:

So, what you’re looking at above is the readout, in kilohms, of the 33K resistor. Unsurprisingly, the meter reads 33kohms. Next, I wired the two resistors in parallel, and attached the meter:

This fits pretty well with the explanation; a 22K and 33K resistor, wired in parallel, are reading 13.09 kilohms, a good bit less than the value of the smaller 22K resistor. If you want, check my practical work by using Ohm’s law, see what you find out. Either way, I’ll see you tomorrow with a new circuit.

Day Three, Introducing The Resistor

Today’s circuit, introducing the resistor, introduces us to a new component, the aptly named resistor. A resistor is a component that uses the electrical principle of resistance to reduce current flow. Among the things they can be used for is reducing voltages. Here’s today’s diagram:

This simple circuit is introducing us to the properties of resistors, and what they do in series, that is, wired in-line together, one after the other. There’s a select switch, what it’s doing is taking the resistor labeled 47K out of the circuit when the switch is set to ‘B’. The unit of measurement of resistance is the ohm, named after Georg Simon Ohm, a German physicist born in 1789. One of his most important contributions to science is a law known as Ohm’s law, which describes the relationship between voltage and current. We talked about this a bit on day one, and it will come up again. Using Ohm’s law, you can determine things about current from voltage, and things about voltage from current. For now, we’re getting off on a tangent though. The 47K stands for 47 kilohms, the 33K for 33 kilohms. A kilohm is, of course, 1000 ohms. So, other than the resistors and the select switch, we’ve got an on/off switch, a series of batteries supplying 9VDC, and that circular symbol there on the right. That’s the symbol for a meter. So what this circuit is going to do is show us, on a meter, the difference between having our current run through just a 33 kohm resistor, and both a 33 kohm resistor -and- a 47 kohm resistor. Here’s what it looks like all wired up:

So, let’s set the selector to A, turn the circuit on, and see what the meter does. Remember with the selector on ‘A’, both resistors are in the circuit:

Ok, with the switch on A, the meter swings not quite halfway to the right. So then we switch things to B:

The meter swings all the way to the right. With only one resistor in the circuit, much more current is flowing. To get a better idea of what’s happening, I used my multimeter, a tool that can act as a volt meter, ohm meter, or, as I’m about to show you, an ammeter, measuring the current in a circuit. Here’s the multimeter, set to measure milliamps. (An ampere, or amp, is the SI unit for measuring current. A milliamp is a thousandth of an amp).

Position A:

Position B:

This verifies the earlier results; With both resistors in the circuit, as in position A, the meter shows that 120mA of current is flowing. If only the 33K resistor is in the circuit, 293mA flow.

The manual explains when wiring resistors in series, you can add their ohm values together to determine the total resistance of the resistor set. So 33+47=80K. We can verify this by doing a little experiment. We’ll set the multimeter to another setting, one that allows it to act as an ohm meter, that is, a device for measuring the resistance of something. Take a look:

Since the details are a bit hard to see, what I’ve done is attached one probe of the meter to the first terminal on the 33K resistor, then a jumper wire from the second terminal of the 33K resistor to the first terminal of the 47K resistor, then the other meter probe to the second terminal of the 47K resistor. The meter is set to display in kilohms. As you can see, it shows just about 80K.

I’ll leave you with something fun to try. Read about Ohm’s law from the link earlier in this post. Look at the pictures that show the meter measuring milliamps. You know the resistor values are 33K, 47K, and that they add up to 80K. You also know that the batteries are supposed to be supplying 9 volts. Plug these numbers into Ohm’s law, and see what you learn.