Although I’ve had PWM pump modulation working in my Gaggia Classic for a while now, and have been working on an updated design, I never actually put a ‘scope on the high voltage output side, mainly due to the difficulty of doing this safely without a differential probe. Today, I finally got around to trying it.
To limit the voltage to a sensible range, I used a potential divider made of a 180k and 1M resistor in series across the pump, then connected CHA and CHB inputs of the ‘scope across the 180k resistor. The ground clip of both CHA and CHB was connected to EARTH (the ‘scope is also earthed of course). This means that our voltage measurements will be relative to earth, rather than a differential measurement between two points. However, the math functions on the ‘scope (e.g. CHA-CHB) can be used to simulate differential measurements if needed (provided the voltage range of the ‘scope isn’t exceeded!)
The figure below shows the voltage measured at the pump for one cycle, using 1ms/div and 5V/div on the ‘scope. The PWM frequency was 1kHz, and the mains frequency was 50Hz. Therefore, there are 1000/50 = 20 PWM pulses visible in the graph below.
This graph is taken from CHB, measured at the point where the pump coil is connected to the IGBT collector. The CHA measurements look very similar. Frankly, I was surprised how clean the waveform looked! There is a little distortion, but it looks pretty good, considering the high frequency switching.
The PWM frequency isn’t synchronised with the mains frequency (being derived from the CPU crystal oscillator), so the PWM waveform slowly drifts relative to the underlying 50Hz mains sine wave. However, this shouldn’t make any appreciable difference to the output power of the pump, given the high PWM frequency.