Options to supply power to the emonPi

Continuing the discussion from New install - Do I need to calibrate my CTs or Feeds?:

I ran in to difficulties installing my emonPi solar kit which exhibited a significant error in energy reporting and the cause boiled down to a power-up sequence between the micro USB supplying power to the Pi and the 9vac supplying power for power calculations.

Given that the emonPi kit was shipped to me with no installation instructions whatsoever (not even a link to the online installation guide) I think this indicates a lost opportunity to make the product accessible to a wider cross-section of users.

It seems to me that designing the kit with two power supplies that have a mandatory power-up sequence is just asking for trouble. IMO it would be far preferable if there were only one power supply that served both the 9vac measurement requirement and for there to be a rectifier inside the emonPi case to provide the 5vdc to power the computer.

A less attractive solution would be to remove the power-up sequence restriction.

Least attractive might be to publish the power-up sequence requirement far more prominently in the installation guide and include the guide in the hardware shipment to the customer.

Cheers, David.

Hi David,

Thanks a lot for your feedback. We have just launched the new User Guide for the emonPi in the last week or so and as such it’s not particularly well linked. We’re getting some stickers made up to include inside the shipping boxes for each emonPi that link to the guide.

As for the power supply , yes I total agree it would be awesome to have a single supply. The emonTx is powered directly from the AC adapter however due to the additional power requirement of the RasPi the emonPi requires the DC adapter to power the unit. Yes, it would be possible to have an internal transformer and rectifier to be able to power directly from a single mains cable, however this would introduce a high voltage component inside emonPi. We are keen to keep the units as safe as possible for users to work with, modify and tinker with therefore the safest options is the have the two isolated supplies. We have not ruled it out for future versions.

Thanks Glyn but I’m not quite with the logic here.

I get it that to operate my Solar Kit requires the sum of the power available from both wall-warts - the 9vac and the 5vdc and that today’s 9vac adapter doesn’t have enough welly additionally to supply the 5vdc requirement.

You mention an internal mains transformer having safety drawbacks - I’m not very competent at electronics, but why wouldn’t a larger capacity external 9vac transformer and an internal rectifier for the 5vdc do the trick?

I’m really enjoying the emonPi btw. Thanks for making it.

Cheers, David.

The issue is that we are using the AC adapter to provide a ‘clean’ AC waveform sample for Real Power calculations. If we try and draw power from the AC adapter the the AC waveform get’s distorted. On the emonTx we very carefully draw a small amount of current from the AC adapter at the correct time in the AC cycle to avoid overly distorting the waveform. The current the emonPi requires to Power the RasPi (full Linux computer) would be out of the question. See blog post explaining the single AC power supply design for the emonTx. On the emonPi the AC adapter is effectively a ‘sensor’ rather than a ‘power supply’, no current is drawn from the AC adapter.