Battery saving idea for emonTX

I was toying with the emonTX schematic today and noted that the CT are routed to VCC and GND like this:


I may be wrong, but  I was thinking that this is a battery waster. multiply that by four for all the CT ports and the AC measuring entry point.

Wouldn't it be better to route that to a digital output and set that to HIGH only when measuring?

What do you guys think?



mharizanov's picture

Re: Battery saving idea for emonTX


I checked out and also did some calculations,

For all the four ports the consumption will be 0.0132 mA at 3.3V, so that can be ignored as insignificant.



Robert Wall's picture

Re: Battery saving idea for emonTX

You would be wasting power charging the capacitor each time so, depending on how often you take the measurement, your idea may actually consume more power. Charging capacitors that are then allowed to discharge with the stored energy being unused is a well-known problem in low-power designs.

Then there is the minor ( ? ) problem that you'd need to allow the input voltage many seconds to settle before you measured it (or implement a much more complicated filter in software to deal with the drifting offset).

It might be worthwhile if you are reading every minute or so. I do the sums like this: charge on the capacitor Q = C.V  = 16.5 uC  and equate that to charge passed down the resistors (in the quiescent condition) Q =i.t , put i = 3.5 uA and t comes out at 4.5 s, so that tells you that, neglecting settling time, if you read more than once every 4.5 s, it doesn't help you.

mharizanov's picture

Re: Battery saving idea for emonTX

Good comments, thank you

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