3. Initial Results

We first tested the system using the bench power supply and a small slide switch:

LC spike test setup with slide switch.

The power supply was set to 12 V. The test clips are 20 AWG and 36" long, which are typical of what might be used in most electronics prototyping benches.

We repeatedly flipped the switch until we captured a bounce-free transition (see the next section for more on switch bouncing):

Oscilloscope capture showing LC spike voltage (yellow) and current (green).

There are several noteworthy features to the above traces. Most striking is the magnitude of the first spike, which at almost 40 V is more than three times the 12 V we were attempting to apply to the circuit. We also see that there are four peaks above 15 V. The potentially destructive implications of the LC transients are clear: 12 V might seem like a safe voltage to apply to a device with a maximum voltage rating of 15 V, but the casual application of the 12 V repeatedly subjects the device to voltages outside the allowable range.

We also see that the frequency of the oscillations is just under 100 kHz, which with a capacitance of approximately 9 uF indicates that the wire inductance is a few hundred nH, which is consistent with what we would expect for the 36" leads.

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