How Low can the Prop go?
william chan
Posts: 1,326
If I don't use the brown out detector, how low a voltage can the Propeller run at 12Mhz internal RC?
Can it run at 2v?
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Can it run at 2v?
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Comments
The Propeller is rated for a minimum voltage of 2.7V. You may find some individual chips that will run at lower voltages at certain temperatures, but you'll have to do your own testing and selection. You may find that some parts of the chip will work at the lower voltages and other parts will not, but, without testing, it's not possible to tell exactly which parts and how that will affect things.
Even though the chip is rated for a minimum supply voltage of 2.7V, it may not work reliably if you set your supply at that. Remember that regulators vary in their output voltage with temperature, instantaneous load, etc. There may be small voltage drops between the regulator and the Propeller chip itself, not significant with a 3.3V supply, but crucial at the low end.
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·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."
- Bjarne Stroustrup
The datasheet at the top of the page Mike linked to makes for fascinating reading.
It looks like your prop will consume of the order of 10mA, but this depends on how many cogs are operating, and whether they are running spin or pasm (lower for pasm). Do you know this information yet, William?
Using 2 AA cells, figure 2 of the datasheet indicates you should get about 40 hours life. If you regard 0.8v as the end of the service life, you'd expect a service life of about 200 hours, so you're extracting only about 20% of the useful life. (Alternative calculation using 2850 mAh specified capacity gives a figure of '14%' of useful life)
There is a substantial effect of temperature, so make sure all the 30 milliwatts of heat generated by the prop is coupled back into the battery! And rub the battery vigorously for extra life. Its hard to read how much of a deal this is (figure 9 has a log scale)
Using 3 AA or AAA cells gives an open circuit voltage something like 4.7 volts. If you're consuming 10mA you can drop this using a red led (about 1.5 volts) and get a power indicator for free. You may need a couple of passives to make sure the power doesn't overshoot at really low startup currents. You may find 3 AAA's give you more life than 2 AA's using this technique.
If you're looking for a simple way to use a switcher, the Bodhilabs vPack board from sparkfun looks like a neat solution for us$10
I'll be interested in the results
tubular.
You might also consider lithium primary (non-rechargeable) batteries. The CR123, for example, is nominally a 3V battery with a starting voltage around 3.25V and a 1.5Ah capacity. Two nice things about lithiums: 1) Virtually zero self-discharge, leading to shelf lives measured in years, and 2) a very flat voltage discharge profile with a sharper "knee" at the end, compared to alkalines.
BTW, these are not lithium ion or lithium polymer batteries, which are rechargeable and operate closer to 3.6V.
-Phil
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'Just a few PropSTICK Kit bare PCBs left!
3 AA battery holders are difficult to find.
I think using and 3 AA batteries and a red LED to drop the voltage would result in the same lifetime as 2 AA batteries directly.
Lithium batteries are expensive ( compared to China AA batteries ). The battery holder is even more expensive.
Alkaline batteries have a bad sloping curve, but maybe if coupled directly to a large cap, the slope problem can be eliminated.
If the Propeller goes to sleep periodically, it can allow even weak AA batteries to still charge up the cap to 3v since the charging current is minimal near the end of each charge.
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Post Edited (william chan) : 12/16/2008 3:27:42 AM GMT
My maths was out for the 3 cell thing, sorry!
For the 3 cell thing to work you want a diode (or two) with a drop of 1.1volts at 10mA. This drops the voltage to 3.6 volts when the cells are new. Your prop stops working at 2.7 volts (theoretically) at which point the 3 cells will have a combined voltage of 3.8 volts, or 1.267 volts each.
1.35 vs 1.267 volts may not seem much but you're on a big bend in the curve. Figure 2 indicates you should go from 40 hours life to more like 100 hours life. But you need that magic 1.1 volts or thereabouts to achieve it.
tubular.
Only the loaded voltage reduce with cell depletion.
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Many mp3 players on the market use boost regulators so they can run on one AA or AAA cell to make it more compact. I would imagine that they also have low quiescent current.
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·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."
- Bjarne Stroustrup
--Rich