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Fun With Snap Circuits 6: Resistance Is Futile. You Will Be Assimilated.

November 9, 2011

We cannot, by total reliance on law, escape the duty to judge right and wrong…. There are good laws and there are occasionally bad laws, and it conforms to the highest traditions of a free society to offer resistance to bad laws, and to disobey them.  ~Alexander Bickel

While you were building the LED project from my last post you noticed that you had to add a 100 ohm resistor to limit the current flowing through the LED to protect it. If you put too much current through an LED, it will burn out. With a POP! And sometimes pieces of plastic will fly off (“you could put an eye out with that”). And it will stink. Then you’ll have to buy a new LED.

Going back to the waterpipe analogy:

When water flows through a pipe, the pipe offers very little resistance to the flow of water.

If, however, a bunch of rocks are inserted into the pipe, this will increase the resistance against the flow of water through the pipe:

As electrons flow through a wire or conductor, there is very little resistance to the electron flow. If you were to connect an LED directly to the Snap Circuits battery block without a current limiting resistor too much current will flow throught the LED and POP! Another way to think of it is filling a water balloon with a garden hose. If you put a kink in the hose this will limit the flow of water making it easy to fill the water balloon, but if you let go of the kink in the hose, the water balloon will fill too quickly and burst.

There are five Snap Circuits resistors: the 100 ohm (the one you used in the LED circuit),  1K ohm (“K” stands for “Kilo” which means “one thouand” so, 1K ohm means “one thousand ohms”), 5.1K ohm, 10K ohm, and 100K ohm resistors.

100 Ohm Resistor

1K Ohm Resistor

5.1K Ohm Resistor

10K Ohm Resistor

100K Ohm Resistor

The greek letter Omega is used to represent the ohm unit

Ohm is a unit of electrical resistance. The higher the ohm value, the higher the resistance. The Snap Circuits 100K ohm resistor (100,000 ohm resistor) has a much greater resistance value than the 100 ohm resistor.

There are wirewound resistors, metal film resistors, and several other ways to manufacture resistors, but the most common is the carbon resistor and appears to be the kind of resistor used in Snap Circuits. The materials used in carbon compositon resistors are a combination of finely ground carbon dust similar to the graphite in your pencil (the conducting material) and clay powder (the non-conducting material). By mixing different amounts of carbon and clay together resistors are made to have different resistance values. If more carbon (the conducting material) is added, the resistor will have lower resistance. Which Snap Circuits resistor will have more conducting carbon powder and therefore lower resistance, the 100 ohm resistor or the 100K ohm resistor?

Metal wires, or connecting leads are attached at each end of the resitor and the resistor is protected by a plastic or paint insulating coating.

 

Connecting Resistors in Series 

Since there are only five kinds of Snap Circuits resistors, what would happen if you needed to limit the current flowing through an electronic component but didn’t have a single resistor that matched the value of the resistance that you needed? Let’s say that you needed 200 ohms of resistance. If you used the 100 ohm resistor, it won’t limit the current enough and you might burn out the component. If you used the 1K ohm resistor, it might limit the current too much and the circuit might not work. To solve this problem you can connect two 100 ohm resistors in series to limit the current by two hundred ohms.

In the diagram below, imagine that the battery is a pump and the paddle wheele is a resistor limiting the current:

 

In the next diagram, there is still a single battery, but a second paddle wheel has been added to represent a second resistor connected in series:

 

If the first paddlewheel, or resistor labeled “R1” is 100 ohms and the second resistor  labeled “R2” is 100 ohms, then the total resistance introduced into the circuit is 200 ohms.

Connecting Resistors in Parallel 

Now let’s say that you only needed 50 ohms resistance, but the lowest ohm value Snap Circuits resistor is 100 ohms. You can connect two 100 ohm resistors in parallel and reduce the resistance to 50 ohms. The diagram below may help you to understand what is happening with the two 100 ohm resistors in parallel:

 
The resistor is a useful electronic component. Resistors can be connected in series to increase resistance and they can be connected in parallel to reduce resistance. They can be used to protect other electronic components from overvoltage and they can be used in combination with other components to change the behavior of these components in a circuit.
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