CMOS logic directly powered from mains

You could use the photodiode in an optocopuler in photovoltaic mode, put the mains through the LEDs and stack the diodes on the other side

AC line triggers inside a box are sometimes derived by wrapping an insulated wire around the hot AC line wire after the power entry connector. But 10 pF has an impedance of 260 megs at 60 Hz, so a fraction of a uA is available that way.

The usual transformerless way is to ride the circuit common on neutral and use a capacitor from the hot wire as a current limiter. That has some hazards.

PV optos are typically about 0.1% efficient. And running the LEDs off the AC line through a resistor might be a few percent efficient. The compound efficiency will be awful.

Wall wart?

Well, if you only need uA to run CMOS, I don't see the issue. And he's got a mains supply available, so it doesn't sound like efficiency is a huge constraint.

"I'm drawing 10mA from the wall to run three CMOS ICs! It's gonna break the bank!"

I can't imagine electrostatic charge scavenging would do much better...

If the CMOS is not interfaced with any external systems, i.e. is within an isolating box, why not just use a capacitive voltage divider followed by a rectifier ?

Using a simple series capacitor won't do, since it would pass through any peak voltages in a nasty way.

It's in Microchip's AN954 appnote. You should not touch any of the wires, of course.

This is the typical method- something like this:

The voltage rating shown on the cap is probably insufficient, but the general idea is okay. You can use a single-component bridge rectifier. The MOV improves survivability against transients. The 100R should be a flameproof type. R3 discharges the cap so users won't get a shock from the plug pins

It's not galvanically isolated, so Mr. Death (along with Ms. blown off scope probe ground) is always lurking nearby.

I would not recommend this if there is any reasonable alternative and without a thorough engineering review- a small wall plug adapter is cheap and pretty reliable and can supply many watts.

--sp

I have done such circuits as CHIPS for appliance controls where the danger was buried out-of-reach of the user. In one case I actually implemented the shunt regulation on-chip ;-) ...Jim Thompson

That's not cheap, though; a fusible resistor, capacitor, to single Zener diode will make -.7 to +10V waveform; resistor+filter-capacitor after that will get you a reasonably good supply for CMOS.

Or a primary lithium battery.

The absolute cheapest method I came up with was in conjuction with a zero crossing detector that used a couple CMOS gates directly connected through resistors to the mains.

The 'power supply' consisted only of a parallel capacitor and zener from Vdd to Vss, enough power came through the sense resistors to power the circuit and supply a robust thyristor trigger pulse every half-cycle.

Sadly, I didn't have the cojones to actually ship it like that, and a rather more conservative design won the day.

--sp

When the power needed is so low, you can just use resistive dropdown. That will have the benefit of being very robust against surges

Cheers

Klaus

If purely CMOS, electronic smog as a source? supercap pump storage if need to do something physical every now and then

You will need capacitive coupling. How little charge do you need? Can you put the hot lead to the center of a coaxial cable and use the shield to couple the power? Or, can you wrap the hot wire with copper tape? Voltage regulation can be sorted out later.

Or by two lemons as in the Motorola ad :-)

Yes, thank you all, I'll stick with wall warts. I wonder how much design review they do for a toaster oven with a CMOS timer though.

That's good, thanks. The examples I've seen in appliances obviously didn't have more than the basic circuit without safety consideratiuons.

I've powered LEDs from the AC line, with a cap and a surge resistor and a reverse diode. The LEDs tend not to last. It would probably be better to add a bridge rectifier and a filter cap before the LED.

LED night lights seem to be reliable. I should dissect one.