Much simpler and cheaper!
LEDs act a little bit like zeners, but they too have a somewhat round “knee” ar very low current levels. I’ve done some experiments, and have come up with a blue LED in series with a red one as suitable voltage reference:
As you can see, even with 1 mA of current, they are clearly visible (especially the red one, which is a low-current type). So this also makes an excellent power-on indicator – at no extra charge – if you pardon the pun. There’s also a 1N4148 diode to a 470 µF buffer capacitor. Here are the voltages this thing seems to stabilize on:
- 3.69 V @ 2 µA
- 3.80 V @ 5 µA
- 3.86 V @ 9 µA
- 3.97 V @ 25 µA
- 4.09 V @ 98 µA
- 4.13 V @ 150 µA
- 4.18 V @ 251 µA
- 4.22 V @ 399 µA
Taking the extra diode drop into account, this leads to a very acceptable 3.04 .. 3.57 V supply voltage for a JeeNode.
So for a capacitive AC mains supply, this could be doable with 7 components:
- a 10 nF X2 capacitor as reactive component
- a 4.7 kΩ fusible resistor to limit the inrush current
- a blue LED plus a red LED to create the necessary voltage drop
- a 1N4148 across the LED for the reverse current
- a 1N4148 from LEDs to feed the capacitor
- a 470 µF 6.3V electrolytic capacitor for energy storage
This circuit is dangerous when directly connected to AC mains, but if a direct reference to one of the input pins is not needed, then it can in fact be made a bit safer: replace the 10 nF cap by a 22 nF unit, and add a second 22 nF cap on the other power line input (plus a second 4.7 kΩ fusible resistor for extra security). Touching the “low-voltage” side limits current to 1 mA – this should cause at most a slightly tingling sensation when touched.
I don’t know about temperature sensitivity, but in a case like this where voltage stability is not so important, this circuit might in fact be the simplest way to build a 0.1 .. 0.4 mA ultra low power supply!
Update – as pointed out by Martyn in the comments below, this circuit is not safe in case of a fault. It’s still transformerless, so it has to rely on caps to do its work – both of which can fail by shorting out. Fusible resistors are not a good enough security in terms of safety, because they don’t blow at current levels below 1 mA – they only protect the circuit from large currents in case something goes wrong.
As always: be careful with 115V and 230V AC mains!