RL filter on the scope
A signal generator into an inductor and a resistor with both ends on a two-channel scope: the RL corner frequency, drawn. There is no microcontroller in it: the simulator solves the 4 parts as a circuit, so it runs the moment you press Run, and nothing needs compiling.
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Beginner About 15 minutes to build and run. Runs in your browser. Free, and no account needed.
The parts retail for about $5.25. On Mokxi they're free.
What you'll learn
- How an RL low pass filter works
- The RL corner frequency formula
- Inductor reactance rises with frequency
- Comparing input and output on a two-channel scope
- Difficulty: Beginner
- Time: About 15 minutes
- Parts: 3
How it works
A signal generator feeds a 160 mH inductor in series with a 1k resistor. The scope watches both ends: channel 1 sees the input, and channel 2 sees the point between the inductor and the resistor. This makes an RL low pass filter.
The corner frequency is R divided by 2 pi L, which is about 995 Hz. The generator is set to 1 kHz, so the blue trace should be about 0.705 of the yellow one, and an eighth of a cycle behind. An inductor's reactance grows with frequency, which is the opposite of a capacitor's. Scale: 2 V per division, 400 microseconds per division.
Parts list
3 parts, plus the jumper wires and 2 bench instruments. The parts retail for about $5.25; on Mokxi they're free, and so are the instruments.
- 1 × Full-size breadboard
- 1 × Signal generator
- 1 × Inductor
- 1 × Resistor, 1k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × Oscilloscope
Wiring table
Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.
| Part and pin | Connects to |
|---|---|
| Signal generator pin GND | Ground |
| Resistor, 1k Ω pin 2 | Ground |
| Oscilloscope pin GND | Ground |
| Inductor pin 1 | Signal generator pin OUT |
| Oscilloscope pin CH1 | Signal generator pin OUT |
| Resistor, 1k Ω pin 1 | Inductor pin 2 |
| Oscilloscope pin CH2 | Inductor pin 2 |
The notes that come with it
There is no microcontroller in this one, so nothing is compiled: the simulator solves the circuit itself. The file in the editor is the notes that come with it.
// A generator into an RL low pass, both ends on the scope.
//
// L = 160 mH, R = 1 k. The corner is where the inductor's reactance equals
// the resistance:
//
// f0 = R / (2 pi L) = 1000 / (2 pi x 0.16) = 995 Hz
//
// The generator is set to 1 kHz, as good as on the corner, so the filtered
// trace (blue, CH2) comes out at
//
// |H| = 1 / sqrt(1 + (2 pi f L / R)^2) = 0.705
//
// of the yellow one, and an eighth of a cycle behind it. Count the divisions:
// 2 V/div vertically, 400 us/div across.
//
// Things to try. Take the frequency to 100 Hz and the two traces sit on top
// of each other; take it to 10 kHz and the blue one is down by ten. Swap the
// resistor and the inductor and it is a high pass. Halve L and the corner
// doubles: an inductor's reactance grows with frequency, which is the whole
// difference from the capacitor in the RC example, whose reactance shrinks.
Troubleshooting
The usual mistakes with this circuit, and the fix for each.
- The output trace is as big as the input This project
At low frequency the inductor passes almost everything. Set the generator near 1 kHz to see the corner. - Channel 2 shows the wrong signal This project
Channel 2 should be on the point between the inductor and the resistor, with the resistor's other end on ground. - The circuit under test shows clipped or distorted signals. Signal generator
A sine swinging below zero can exceed what an input allows. Add a DC offset or bias so the signal stays inside the supply range of the part. - Measurements are off or noisy. Signal generator
Join the generator's ground to the circuit ground. Keep amplitude and frequency within what the connected parts can handle. - A big voltage spike appears when current through the coil is cut, and it can damage a transistor or a chip. Inductor
Put a flyback diode across the coil, or a snubber, so the current has a path when the switch opens. - The coil gets hot or saturates because the current is higher than its rating, and the inductance falls. Inductor
Check the saturation and rated current of the part. Pick a larger core or a lower current. Resonance with a capacitor is at 1 over two pi times the square root of L times C. - The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure. - A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground.
Change it and keep it
Open it in the editor, change the circuit or the code, and keep your version in a free account.