12. Some oscilloscope views of signals

These images are taken with a Pico Technology ADC-212/3 oscilloscope for PC. The current was measured by measuring the voltage over a 0.1 Ω, 10 W thread winded resistor, which probably also has some inductance. The resistor was connected in series with the motor or lamp. The operating voltage was 18 V powered from a large power supply which can handle a maximum average current of 15 A. The small transient spikes that are seen at the positive flank of the voltage curves at 20 kHz probably occur because the power supply is not fast enough to keep the voltage stable for a short time. The DC motor (15 V, about 100 W) is running without load. The lamp is of normal car lamp type (12 V, 50 W).

1. PWM pulse over DC motor at 500 Hz and 30% duty cycle. The motor dynamics (voltage and current) have time to follow individual pulses, which indicates that the pulse frequency is too low.

2. PWM pulse over DC motor at 20 kHz and 30% duty cycle. Here the pulse frequency is so high that the motor behaves almost as if powered from a true DC voltage. However, some transient current spikes occur at the flanks of the pulses.

3. PWM pulse over car lamp at 500 Hz and 30% duty cycle. At this low frequency the lamp behaves almost exactly as a true resistor i.e. the voltage and current follow each other.