Circuits

In this setup, I had a 16 ohm power resistor as my load and was using a micro limit switch to turn on 12V DC power to the load. The mechanical switch is inherently a bit noisy as the contacts inside close, so there is some interesting noise. I tried both with a small inductor inline, and without. The oscilloscope is hooked up to look at the voltage drop across the resistor. I am using a battery to eliminate the possibility of spikes in a wall power supply as it attempts to catch up with a sudden demand.

First I captured the switch closure with no inductor and saved it as a reference waveform in blue. The yellow one is with the 2.62 mH inductor added in between the switch and the load. It's interesting to see that the blue one is fairly noisy and you can see the contacts bouncing back and forth before the switch finally settles in the closed position. This all happens extremely quickly and the window is over 1.4 milliseconds. In the yellow one, the inductor manages to smooth out most of the noise at the beginning of the switch action, and ramps up to the on state fairly smoothly once the switch has committed to being on.

Just for fun, I also tried with a 220 uF capacitor across the load to see how it would smooth out the switch closing. As one would expect, it charges up gradually when power is applied and supplies some back when the switch bounces back. However, the key difference with the inductor is that the capacitor doesn't stop the initial spikes. The inductor is able to mostly filter out the blips at the beginning, while the capacitor isn't. Of course, they aren't being used in the same way but it's still interesting to see. The inductor is inline with the load and resists current changes, and the capacitor is across the load trying to keep a steady voltage.

Again, the blue waveform is with no inductor and the yellow one is with the inductor placed inline with the load. They are actually not on the same time scale (I had to blow up the blue one by 10x to see anything other than a straight dropoff). However, the point to be seen here is that the inductor releases its stored magnetic field to slow down the stopping of current when the switch is opened.

On the switching off test with the 220 uF cap, we can see an exponential decay that takes place over a much longer time period than either with or without the inductor. The time window is 70 milliseconds in this picture versus 140 and 14 microseconds for the yellow and blue waveforms respectively in the previous graph.

Here is a diagram of the boost circuit I put together (excluding the signal generator part). The transistor Q1 simply switches power through the inductor on and off, and the diode D1 only allows current to flow one direction. Capacitor C1 smooths out those pulses, and R1 is just a current limiting resistor on the base pin of the transistor for safety.