[OpenTRV-dev] OT: Voltage drops

[Bo Herrmannsen]

Thanks...

both you and mike are right... i should not do this if i dont know.. had it
been back in 1995 i would have know ..


on the other side i want this done as it makes messurement and logging much
better and there would be almost no calibration to do :-)

however i will not release it before its right.... but from the above i can
almost understand that i need to keep the 3mm creepage on the whole board...

let see  what time will get me doing.....

/bo

2013/6/11 Kevin Wood <EMAIL ADDRESS HIDDEN>

> 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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