Long, long ago, some friends and I in school used to try to repair old valve (tube) TVs. Most commonly the repair was just by cannibalizing another old set; we did not have any budget for spares but we brought some sets back to life.
Many of these sets had no power transformer and just directly rectified the mains. This was the UK and, at the time, the nominal supply voltage was 240V RMS so a peak of about 340V which the circuits were designed to run on. Hence it was not safe to touch the chassis.
I am wondering at what potential the chassis was with respect to ground. Let's make a few simplifying assumptions.
Supply is 240V RMS.
Neutral is 0V with respect to ground.
No transformer.
Full wave rectification with perfect diodes.
Smoothing is just a capacitor. I forget how sophisticated the smoothing might be.
I think that the smoothing is irrelevant. For half the AC cycle, the chassis with be connected to the neutral line and hence at 0V. For the other half cycle, it will be connected to the live. So, I want the RMS voltage of the resulting waveform. A little bit of maths gives me an RMS of 120V.
Does that seem correct?
Best Answer
The AC component will be 120 V, but with the positive peaks at Earth (Mains Neutral) potential so the peak voltage will be -338 V. This is worse than 120 V AC for electric shock current because the higher peak voltage punches through skin easier, but perhaps not as bad as 240 V AC because the shock frequency is 50 Hz rather than 100 Hz.