Computing stuff tied to the physical world


In Hardware on Nov 18, 2011 at 00:01

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:

Zener knee

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:

DSC 2777

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…

  1. JC, it is not surprising that you do not find zener diodes with ‘better specs’, as all zeners below 5V show the ‘real’ zener-effect, whereas zeners with higher voltages show more and more the avalanche effect. These are two completely different effects which leads to the different characteristics. I tried one 5V6 diode I had lying around (had BZX85-5,6 on the box) and it showed sharper characteristic, but still not good enough.

    I am curious about your next results as this would have been my preferred setup….

    BR, Jörg.

  2. note that LED’s have a larger forward drop (and give a visual “on” clue for free) Most LED’s I tested measured about 2.2 volt.

  3. color seems to dictate the forward drop value. red=low blue=high. I suspect that because the LED would conduct constantly, that temperature would be so high in the LED that temp fluctuations in the environment do not matter that much.

    • The power supply I’m after will push less than 1 mA through the LED. In normal use it’ll barely light up (except during packet transmission, makes a nice free indicator!).

    • A quick test with a blue 5mm LED here gives me a fairly sharp 2.3V knee, going up to 2.6V at 1 mA.

      Amazingly, that LED still light’s up clearly (but of course not brightly) with just 50 µA of current!

      Two of them in series, and then one or two 1N4148’s towards the buffer cap ought to get me into the 3.3 .. 3.8V range. I’ll try it out – thanks!

  4. look for low current SMD types, they seem to need 2mA (still twice your goal..)

  5. Be careful with LEDs, though – I think they have relatively small reverse breakdown voltage (is this the right term?) of only a couple of volts, so using them instead zeners in 230V AC gadget might not work.

  6. Kubik, correct. When this particular circuit uses a capacitor dropper, both the “reverse” mode (producing the ‘zener’ voltage) and the “forward” conduction mode are used (to discharge the capacitor ready for the next cycle).

    The LED is opposite, the “forward” sense is producing the ~ 1.8-4 volts. The reverse sense needs a diode in series if you want the blocking action to continue at higher voltages, a diode in parallel if you are substituting for the zener in this circuit.

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