If you recall the 6800 µf supply test, I used the following setup:
The problem was that this uses a 10 mA supply current as test, which is 10x higher than I want for the final 220V capacitive transformer-less version.
Trouble is, increasing the resistor to 22 kΩ didn’t work, there wasn’t enough juice to keep the JNµ running.
In a comment, Koen remarked that it might be due to the zener soft “knee”. Zeners are not perfect, they tend to cut off the voltages at a specified point, but it really depends on how much current flows. With too little current, the zener-like properties are in fact far less pronounced. Here’s what I measured, roughly:
- 2.7 V @ 0.25 mA
- 2.9 V @ 0.5 mA
- 3.2 V @ 1 mA
- 3.8 V @ 5 mA
- 4.0 V @ 10 mA
That’s a lot of variation. In the case of the ultra-low power supply where very little current flows once the capacitor has been charged, it looks like a 1 mA “trickle” feed is not getting the energy to the right spot. Aha, found it:
Those zener diodes under 6V are pretty leaky! Unfortunately, I can’t seem to find any with better specs.
Time to try something else. How about the forward voltage drop of a regular diode? After all, that’s supposed to be somewhere in the range of 0.6 .. 1.0 V.
Here’s what I got with 5x the 1N4004 diode in series, and connected in conducting mode:
- 2.6 V @ 0.25 mA
- 2.75 V @ 0.5 mA
- 2.9 V @ 1 mA
- 3.1 V @ 2 mA
- 3.3 V @ 5 mA
- 3.45 V @ 10 mA
Hm, looks like that’s already a lot better! Next test is to replace the zener in the above circuit with 7x an 1N4004. With a bit of luck, that might provide a voltage in the proper range for the unregulated JNµ. Here’s what I get:
Not bad! The 470 µF 6.3V cap charges up to 3.8 V in about 2 seconds.
Even better is that with a 4.7 kΩ simulated 0.65 mA load, the voltage drops a bit but still stabilizes at ≈ 3.15 V.
There is a major drawback, though: the forward drop over a diode tends to be very temperature-dependent :(
Here’s an idea for a different approach: add a regulator and let the capacitor charge up to say 12V. That would give a lot more energy to draw from, even if a sizeable portion of those milliwatts get “wasted” as heat. Also, it turns out that the 12V zener I used (1N5242) has a considerably better behavior – less than 10 µA when I drop 0.1 V under the zener voltage (11.8V in the unit I tested), whereas the 4.3V zener eats up most of a 1 mA trickle feed.
And lastly, there’s the idea of tracking the supply voltage with the ATmega/ATtiny itself, to let it decide when to delay a power-hungry transmission. After all, the micro-controller is able to turn itself from a 10 µA to a 30 mA power consumer, just by changing its own mode and the RFM12B from mostly power-down to full-power.
Theoretically, an MPU could in fact regulate the power supply voltage by “modulating” its current consumption!
Ok, so the new target I’d like to aim for as ultra-low power source will be: 0.01 µF X2 cap (a mere 0.4 mA trickle), 12V zener, MCP1703 regulator, and an even smaller 100 µF 16V cap, since there’s a lot more voltage drop available if you start from a full charge @ 12V. Will it work? Well, there are some surprises ahead – stay tuned…