Computing stuff tied to the physical world

Taking the JNµ to 3.0 V

In AVR, Hardware on Mar 1, 2013 at 00:01

As mentioned in yesterday’s post, lower supply voltages lead to lower power consumption.

There’s a new JeeNode Micro v3 revision coming which applies this logic to try and get the power consumption as low as possible. It also has changed slightly in its pinout:

Screen Shot 2013-02-26 at 09.30.47

The main physical changes are: it’s now 16 x 43 mm, and the ISP pins are no longer 2×3.

Let’s just focus on the power side of things for now, though. The JNµ v3 can be operated from several power sources:

  • Direct power: this runs without regulation, and requires a 2.2V .. 3.8V power source
  • Boost power: using the same circuit as the AA Power Board, runs from 0.9V .. 5.0V

The boost power circuit is very flexible as you can see, but it adds a certain amount of quiescent power consumption of its own, making the whole setup draw more idle current than when it is directly powered. What you do get with the boost power hookup, is that it’ll try to stay running as long as possible, down to a 0.5 V input under ideal conditions, so this thing will squeeze the last drop of energy out of its power source. Great for “empty” AA’s!

Boost power supplies are very clever and useful circuits, but they can’t break the laws of physics: a 100% efficient boost supply will draw 15 mA when asked to generate a 3.3 V @ 5 mA output from a 1.1 V input source. The current it draws will be triple that which it produces (more like four times, in fact, since the real circuit is far from 100% efficient).

Note that – in the light of yesterday’s story – it would really make no sense to pump up the voltage to say 5V, only to end up with a more wasteful circuit. That’s not just a quadratic increase, those losses would in fact be proportional to the third power of the voltage!

With a boost circuit, it really pays to work with as low an output voltage as possible. Which is why the JeeNode Micro v3 now comes with a 3.0V version of the boost chip, not 3.3V.

Let’s do the math, assuming a 100% efficient boost supply:

  • a 3.0 V booster draws 3.0 / 3.3 ≈ 91% of the current in comparison to a 3.3 V one
  • running at 3.0 V instead of 3.3 V consumes ≈ 83% of the power (with a resistive load)
  • combined, that means we’ll be using 3.0 ^ 3 / 3.3 ^ 3 ≈ 75% of the power at 3.0 V
  • so the total power consumed is 25% less at 3.0 V compared to what it’d take at 3.3V

That translates to 3 more months on a battery lifetime of 1 year – quite a bit!

This doesn’t just apply to the boost regulator: coin cells often have a 3.0V rating, and so do 2 alkaline AA batteries in series. Note also that 2 rechargable AA batteries will still work fine, as these will supply 2.4..2.6 V, well within the direct power range of the JNµ v3.

Note that for all the standard JeeNodes, nothing changes: these will continue to be populated with 3.3V regulators. One key reason for this is that they are already operating slightly beyond their specs (“overclocked”) with the clock running at 16 MHz. We shouldn’t push our luck with these settings, which are standard in the Arduino world. So the JN, JN SMD, and JN USB will all continue to be operated at 3.3V – no changes!

As for the many different JeePlugs: these all run at 3.3V, and the chips used on these plugs are normally qualified to run at either 3.0 .. 3.6V or 2.7 .. 3.3V, so we’re still fully in range.

Is it all win, then? Well, yes, but note that the noise margins are slightly reduced at 3.0V, which can slightly affect the accuracy of analog pin measurements and the cable lengths at which I2C chips still operate reliably. These effects are going to be minimal, I expect, but it’s something to watch out for.

Update – As pointed out in the comments, the above calculations are not correct. There is a small effect on booster circuit efficiency in all this, but the power savings are closer to 20% than 25% – not quite 3 months per year, as I claimed.

  1. Hmm, bad news. Sensor DHT22 work min. on 3.3V …

    • Ah, didn’t realise that, thanks. The lower-spec DHT11 goes down to 3.0V, if that’s any help. Otherwise, you’ll need to run it off an external supply – 3x Eneloop would give you 3.6..3.9V, or some other source with an external 3.3V regulator.

  2. DHT11 have poor temperature accuracy (±2℃) and range (only positive temperature 0-50℃)

    My favorite solution for long run is battery holder no.1025 at for CR2477 (3V, 950mAh) with booster to 3.3V, no 3V :)

    • What about the SHT11 sensor? Adding a booster for just 3.0 to 3.3 V is wasting relatively much on the extra quiescent current – with an SHT11, it would all run several times longer on just the coin cell as is.

  3. jcw, Are you sure you didn’t count the benefit one time too much ? Isn’t the fact that the regulator draws less current at 3V than at 3V3 already counted at the load side ?

    • It should be correct – to the power two for running at a lower voltage (power is V x I, both are lower), and one more time because the boost also needs to raise the voltage less.

    • On second thought: I think you’re right, I’ve been mixing up energy and power (again).

  4. So, what does that mean for real-life examples? For what types of uses is it better to use the direct-power over the boost-power version of the JNµ and vice versa?

    • It really depends on many factors: desired battery lifetime, what size and type battery can you fit, power draw from the sensors, etc. For the longest unattended use, it’s still best to use 3x AA without booster. For the smallest unit, use the coin cell, again without booster. If you use rechargeable AA batteries everywhere (as I tend to do), then 1x AA with booster is a good option.

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