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Series and Parallel Resistors

Experiment E2.

In experiment E1 we have seen how a resistor is used to control the flow of current. We now understand the basics of ohms law, which is for example that a 1000 ohm (1k) resistor allows 1 milliamp of current to flow for each volt that is dropped across the resistor. We know that if the resistor value is halved the current flow is doubled, and if the resistor value is doubled the current flow is halved.

Now we are going to experiment to see what happens when resistors are connected in series or in parallel.
Wiring the circuit.....

If they are not already fitted follow the wiring instructions in experiment E1 to fit R1, D1 and S1. Turn off the 5 volt supply by moving the slide switch to the left then follow these instructions:-

7. Bend the leads of two 470 ohm resistors as shown in experiment E1. Fit one 470 ohm resistor R2 between G8 & G11. Fit the other 470 ohm resistor (R3) between F11 & F15.

8. Use a pair of small pliers to squash the leads flat on the second push button.

9. The four leads of the bush button switch are arranged in pairs, each pair coming out of opposite faces of the square body. Fit the push button switch with two leads from one of these faces in I15 & I17 and the other two leads in G15 & G17.

10. Fit a bare wire link from J17 to Z17.

Testing the circuit.....

Switch on the 5 volt supply to the programmer module.

Press push button S1 on the plugboard and the LED will glow. Let go of that and press push button S2. Do that several times and see if you can detect any difference in the brightness of the LED. The brightness will be identical as far as the human eye can tell.

This shows that resistors in series add up directly. Two 470 ohm resistors in series have a total resistance of 940 ohms which is roughly the same as the 1000 ohms (1k) in the other part of the circuit, so the current into the LED is almost the same which ever button is pressed.

Now press both push buttons and the LED will glow much brighter. If two equal value resistors are connected in parallel the overall effect is the same as one resistor of half the value. When both buttons are pressed it is the same as having one resistor of just under 500 ohms.

Think about it.....

The first two experiments have shown us in practical terms nearly all we need to know about resistors. The importance of these experiments cannot be over emphasised.


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