Introduction: Tinee9: Resistors in Series

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Tutorial Level: Entry Level.

Disclaimer: Please have a parent/guardian watching if you are a child because you can cause a fire if you are not careful.

Electronic design goes way back to the telephone, light bulb, powered plants in AC or DC, etc. In all of electronics you run into 3 basic components: Resistor, Capacitor, Inductor.

Today with Tinee9 we are going to learn about resistors. We won't learn color codes for resistors because there are two package styles: Thruhole and SMD resistor which each have there own or no codes.

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Step 1: Materials



Resistor Assortment

Computer (that can connect to Nscope)

LTSpice (software

Below is a link to the Nscope and Resistor Assortment:


Step 2: Resistors

Resistors are like pipes that allow water to flow through. But different pipe sizes allow a different amount of water to flow through it. Example a big 10 inch pipe will allow more water to flow through it than a 1 inch pipe. Same thing with a resistor, but backwards. If you have a big value resistor, the less electrons will be able to flow through. If you have a small resistor value then you may have more electrons to flow through.

Ohms is the unit for a resistor. If you would like to learn the history of the of how the ohm became the unit named after German physicist Georg Simon Ohm go to this wiki

I will try and keep this simple.

Ohm's Law is a universal law that everything abides by: V = I*R

V = Voltage (Potential Energy. Unit is Volt)

I = Current (Simple terms number of electrons flowing. Unit is Amps)

R = Resistance (Pipe size but smaller is bigger and bigger is smaller. If you know division then pipe size = 1/x where x is the resistance value. Unit is Ohms)

Step 3: Math: Series Resistance Example

So in the above Picture is a screen shot of an LTspice model. LTSpice is software that help electrical engineers and Hobby people design a circuit before they build it.

In my model, I placed a Voltage source (ex. Battery) on the left side with the + and - in a circle. I then drew a line to a zig zag thing (this is a resistor) with R1 above it. Then I drew another line to another resistor with R2 above it. I drew the last line to the other side of the voltage source. Lastly, I placed a upside down triangle on the bottom line of the drawing which represents Gnd or reference point of the circuit.

V1 = 4.82 V (Nscope's +5V rail Voltage from USB)

R1 = 2.7Kohms

R2 = 2.7Kohms

I = ? Amps

This configuration is called a series circuit. So if we want to know the current or number of electrons flowing in the circuit we add R1 and R2 together which in our example = 5.4 Kohms

Example 1

So V = I*R -> I = V/R -> I = V1/ (R1+R2) -> I = 4.82/5400 = 0.000892 Amps or 892 uAmps (metric system)

Example 2

For kicks we are going to change R1 to 10 Kohms Now the answer will be 379 uAmps

Path to Answer : I = 4.82/(10000+2700) = 4.82/12700 = 379 uAmps

Example 3

Last practice example R1 = 0.1 Kohms Now the answer will 1.721 mAmps or 1721 uArmps

Path to Answer : I = 4.82/(100+2700) = 4.82/2800 = 1721 uAmps -> 1.721 mAmps

Hopefully, you see that since R1 in the last example was small the current or amps was bigger than the previous two examples. This increase in Current means there are more electrons flowing through the circuit.
Now we want to find out what the voltage will be at the Probe point in the picture above. The probe is set in between R1 and R2......How do we figure out the voltage there?????

Well, Ohms law says Voltage in a closed circuit must = 0 V. With that statement then what happens to the voltage to from the battery source? Each resistor takes away the voltage by some percentage. As we use example 1 values in example 4, we can calculate how much voltage is taken away in R1 and R2.

Example 4 V = I * R -> V1 = I * R1 -> V1 = 892 uAmps * 2700 Ohms = 2.4084 Volts V2 = I * R2-> V2 = 892 uA * 2.7 Kohms = 2.4084 V

We will round 2.4084 to 2.41 Volts

Now we know how much many volts are being taken away by each resistor. We use the GND sysmbol (Upside down triangle) to say 0 Volts. What happens now, the 4.82 Volts produced from the battery travels to R1 and R1 takes 2.41 Volts away. Probe point will now have 2.41 Volts which then travels to R2 and R2 takes away 2.41 Volts. Gnd then has 0 Volts that travels to the battery which then the battery produces 4.82 Volts and repeats the cycle.

Probe point = 2.41 Volts

Example 5 (we will use values from Example 2)

V1 = I * R1 = 379 uA * 10000 Ohms = 3.79 Volts

V2 = I * R2 = 379 uA * 2700 Ohms = 1.03 Volts

Probe Point = V - V1 = 4.82 - 3.79 = 1.03 Volts

Ohms Law = V - V1 -V2 = 4.82 - 3.79 - 1.03 = 0 V

Example 6 (we will use values from Example 3)

V1 = I * R1 = 1721 uA * 100 = 0.172 Volts

V2 = I * R2 = 1721 uA * 2700 = 4.65 Volts

Probe Point voltage = 3.1 Volts

Path to Answer Probe Point = V - V1 = 4.82 - 0.17 = 4.65 Volts

Probe Point alternate way of calculating voltage: Vp = V * (R2)/(R1+R2) -> Vp = 4.82 * 2700/2800 = 4.65 V

Step 4: Real Life Example

If you have not used the Nscope before please refer to

With the Nscope I placed one end of a 2.7Kohm resistor in a Channel 1 slot and the other end on the +5V rail slot. I then placed a second resistor on another Channel 1 slot and the other end on the GND rail slot. Be careful as to not have the resistor's ends on the +5V rail and GND rail touch or you may hurt your Nscope or catch something on fire.

What happens when you 'short' +5V to GND rails together, the resistance goes to 0 Ohms

I = V/R = 4.82/0 = infinity (very large number)

Traditionally we do not want current to approach infinity because devices can't handle infinite current and tend to catch on fire. Luckily Nscope has a high current protection to hopefully prevent fires or damage to nscope device.

Step 5: Real Life Test of Example 1

Once all set up, your Nscope should show you the value of 2.41 Volts like the first picture above. (each major line above channel 1 tab is 1 Volts and each minor line is 0.2 Volts) If you remove R2, the resistor that connect Channel 1 to GND rail, the red line will go up to 4.82 Volts like in the first picture above.

In the second picture above you can see LTSpice prediction meets our calculated prediction which meets our real life test results.

Congrats you have designed your first circuit. Series Resistor connections.

Try out other values of Resistance like in Example 2 and Example 3 to see if your calculations match real life results. Also practice other values too but make sure that your current does not exceed 0.1 Amps = 100 mAmps = 100,000 uAmps

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