Battery Power
rokicki
Posts: 1,000
I'm considering using battery power for some of my propeller hacks, but I'm not very
familiar with doing so.
If I want to use battery power for the propeller demo board, would 6V be a high enough
voltage? (Does anyone know how much voltage "headroom" the 7805 needs?)
Has anyone used any of the DC->DC stepup circuits? Will these work for the propeller,
or are the filtering demands of a non-linear regulator excessive? It would be cool to
be able to power some propeller circuits with only a single 1.5V AAA.
Has anyone done any measurements on current consumption for the propeller?
How about suggestions on reducing the power consumption of polling cogs (like those
cogs handling serial communication and the like)? If I added a waitcnt 100 cycles,
say, in the polling loop, how much would this reduce current consumption?
I am probably going to try to test some of this stuff myself, but frankly I'm not
completely sure how to go about it. I could try using a small (1 ohm?) resistor
in series with the power line, and measuring the voltage drop across it. Or I could
just use the built-in ampere measurement of my multimeter (but I have no idea
what its resistance is or what effect the transient demands of the circuit might
have on the multimeter.)
If I decided to use battery power for one of the protoboards, and say I only needed
the 3.3V devices to operate, could I just apply, say, 4.5V directly to the 5V rail?
I am an electrical engineer by training but not by vocation, so while I do have a
basic understanding of "how things work", I'm afraid there are practical matters
that will mess me up, so any suggestions are appreciated. I'm not a big fan of using
9V batteries because their energy to weight rating is not so good, they are expensive,
and with the linear regulators most of the power consumed is just wasted. I'd
be much more comfortable using one (or three, or four) AA's (especially the
fancy, high-capacity NiMH rechargeables), but all the circuits I see for powering the
propellers seem to use standard linear regulators and wallwarts. What am I
missing?
familiar with doing so.
If I want to use battery power for the propeller demo board, would 6V be a high enough
voltage? (Does anyone know how much voltage "headroom" the 7805 needs?)
Has anyone used any of the DC->DC stepup circuits? Will these work for the propeller,
or are the filtering demands of a non-linear regulator excessive? It would be cool to
be able to power some propeller circuits with only a single 1.5V AAA.
Has anyone done any measurements on current consumption for the propeller?
How about suggestions on reducing the power consumption of polling cogs (like those
cogs handling serial communication and the like)? If I added a waitcnt 100 cycles,
say, in the polling loop, how much would this reduce current consumption?
I am probably going to try to test some of this stuff myself, but frankly I'm not
completely sure how to go about it. I could try using a small (1 ohm?) resistor
in series with the power line, and measuring the voltage drop across it. Or I could
just use the built-in ampere measurement of my multimeter (but I have no idea
what its resistance is or what effect the transient demands of the circuit might
have on the multimeter.)
If I decided to use battery power for one of the protoboards, and say I only needed
the 3.3V devices to operate, could I just apply, say, 4.5V directly to the 5V rail?
I am an electrical engineer by training but not by vocation, so while I do have a
basic understanding of "how things work", I'm afraid there are practical matters
that will mess me up, so any suggestions are appreciated. I'm not a big fan of using
9V batteries because their energy to weight rating is not so good, they are expensive,
and with the linear regulators most of the power consumed is just wasted. I'd
be much more comfortable using one (or three, or four) AA's (especially the
fancy, high-capacity NiMH rechargeables), but all the circuits I see for powering the
propellers seem to use standard linear regulators and wallwarts. What am I
missing?
Comments
I(ma) = .994 + .035f + .08c + 0.115cf
where f is the operating frequency in MHz and c are the number of cogs that are running. The first coefficient is leakage in the core, PLL and the brown out circuit. The second is the core's current consumption as a function of frequency. The third is the cog leakage current and the forth is the cog current consumption.
Using RCFAST at room temperature its:
I(ma) = 0.56 + 1.56c
It's static current consumption is less because the PLL isn't being used. Neither of these equations account for using counters or driving I/O which is application dependent.
Anytime you do a waitXXX, you are effectively reducing the number of cogs operating by 1 for the duration of the wait.
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Paul Baker
Propeller Applications Engineer
Parallax, Inc.
Post Edited (Paul Baker (Parallax)) : 1/4/2007 7:29:46 PM GMT
In my tests I have measured 50mA (max) to 80MHz generating only video TV.
That includes EEPROM (does not consume anything).
I have used several of the DC-DC step-up converter chips from linear, they work very well if you follow the datasheet instructions. Remember that power in to the converter will equal power out less a bit and that power is IV. That gives you an idea of the likely current draw from the battery for a given current into the load. One of the converters I mad e I used to sell, it was designed to step lipoly voltages up to 5v in order to drive pager motors for model aircraft, it weighed about 0.4g I think.
Rather than use a 7805 consider a LDO regulator, more headroom and less loss.
Graham
I`ve thought about your idea, and when I played a bit with my propeller I fond a battery pack with two AA batterys in, I plugged them into my demoboard just to see what happened. It worked! Without any regulators. I also got video output, but my keyboard didn't work, I suppose it's wired to 5v.
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