On Sunday 10 Mar 2013 19:50:40 James Courtier-Dutton wrote:
> As I will be doing ADC on Mains input voltage, I will be adding a
> digital isolation on the SPI bus.
> E.g. ti iso7241
Yep, that looks fine.
> So the ADC will be on the 240V AC side, but then the raspberry PI will
> be protected by the digital isolation by having the isolation on the
> link between the ADC and the PI. ADC -> Isolation -> SPI Bus of
> PI.
Don't forget you'll have to feed the ADC 3.3 Volts somehow.
>
> This also improves the accuracy and simplicity of the ADC side, no
> isolation transformers, and uses less power to measure it.
You could use a potential divider (two resistors) as an input stage, feeding
an optocoupler (low-voltage AC side), which then switches 3 volts to the ADC.
Add suitable protection diodes and filter capacitors as required. This will
give an on/off sense, but no effective voltage measurement.
If you want voltage measurement, you'll need a potential divider and a summing
amplifier to get a 0-3v signal which you can then sample.
For current measurement, look at current transformers, followed by a suitable
amplifer stage.
Hope this helps,
Tim B.
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