[OpenTRV-dev] OT: Voltage drops

[Kevin Wood]

The 2M2 resistor chains form a potential divider with the 39k resistors
taking the input voltage down to the measurement voltage which appears to
be about 2V at the peak of the UK mains waveform (340V). You can pretty
much work on the mains voltage being dropped equally across those 3
resistors, so the first sees 340V peak, the next about 226 and the next
113.

The X2 capacitor is a dropper to provide a power supply for the
"mains-side" active circuitry. As he says, it has a reactance of about
6.8k. Another way of looking at it is that it "dumps" its charge into the
bridge rectifier each half-cycle. The voltage at the bridge rectifier will
depend on the circuit's current consumption, since the capacitor can be
regarded almost as a current source. It will be limited to around 12v peak
by the zener diode across the DC side of the rectifier, however.

If you are connecting this to a continental mains plug which isn't
polarised, however, you should assume that the whole circuit upstream of
the opto isolators could be sitting at mains potential, since it's luck of
the draw which way round it gets plugged-in. It's probably best practice
to assume this anyway, to be honest, but you won't see large voltage
*differences* between any points downstream of the dropper resistors and
X2 capacitor. Unless there's a fault ;-).

Kevin

> I'm working a bit again on a circuit that allows safe and "almost" error
> free messurement on mains voltage (search givmon on openenergy)
>
> i have all the bits reasonble placed and observed the 3mm creepage between
> mounting holes etc...
>
> but i need to calc what voltages there are present where to make 100% sure
>
> of course i have googled but i cant find an easy to understand answer for
> these 2 q's:
>
>
> adding 3 2.2Mohm in series to both the neutral and line wires.. what would
> the drop be?
>
> and what would the drop be across a 0.47uF capacitor? the mains are
> connected via that to a bridge recitifier and need to know what voltages i
> can expect arround those
>
> /bo
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