The first AC-mains powered current node configuration used a resistive transformer-less supply. It took about an hour to charge with no load, and consumed about 0.26 Watt.
This is an improved version, using a capacitor:
(Whoops, I see I forgot to draw the 470 kΩ resistor across the 22 nF cap, to discharge it when disconnected!)
The 4.7 kΩ 1/2W resistor limits inrush current – the worst case being when the cap is empty and plugged in at the top of the AC mains cycle. It’s also a “fusible” resistor, meaning that it’ll act as a fuse when overloaded for any reason. This won’t be enough to protect the circuit, let alone a person touching this circuit, but it will prevent a fire in case of a catastrophic short (which could otherwise pull over 16 amps until the mains fuse blows).
As before, it’s charging the supercap – supplying nearly 1V in this case:
The 470 kΩ resistor right across that yellow 22 nF capacitor quickly drops the residual charge once unplugged.
Charging appears to go a bit faster, but there’s a problem because I’m running the ATtiny with a disabled brown-out detector (BOD). This means the ATtiny isn’t kept in reset as the voltage ramps up. As a result, it’s trying to run even at low voltages (and is bound to malfunction at voltage levels under 1.8V), but more importantly: it’s going to consume current while trying, which will prevent the supercap from charging! Which is is exactly what I see: the supercap voltage is barely rising above 1.34 V.
The BOD is an important hardware feature for circuits like these. It keeps the ATtiny in reset until the power reaches a certain level. That level is configurable for 1.8V, 2.7V, or 4.3V via the ATtiny’s fuses. In this case, 1.8V seems like the proper value to use – it will be too low for the RFM12B module which requires at least 2.2V, but this way the ATtiny can continue to run correctly even at lower levels, and then decide whether it wants to enable the RFM12B or not.
Unfortunately, I’m going to have to improve the sketch first. Right now, it just starts up and tries to do its thing, without consideration for the current voltage. Leaving the unit on for over two hours didn’t lead to a packet transmission, and only got the voltage up to about 1.8 V, whereas it keeps on rising with the ATtiny disconnected. Clearly, the ATtiny needs to become more aware of its current power state before it can act as a reliable AC current sensor. That’s the trouble with ultra-low power systems: it can be tricky to get them right!
Drat, it looks like I just messed up the fuses on the ATtiny, because I can’t reprogram it anymore. That’s the trouble with low pin-count controllers: it’s easy to mess them up!
Time to go back to separate power supply and JNµ test rigs. Let’s not muddle the issues any more than needed.
The good news is that this supply now consumes half of the resistive version, i.e. 0.13 Watt. Note that the power consumption of the resistive version could have been halved as well (see this comment). So in this case, the extra efficiency of the cap seems to be going into supplying a bit more current, i.e. charging the supercap faster.
Progress nevertheless (says the optimist): lower power consumption, faster start-up!