Power clarification, please

[Professor601]

I am a bit muddled on the actual power requirements for a Picaxe. The M2 manual says it can run safely on 3v, but most of the other docs say to use 4.5 to 5v. Can a stock Picaxe safely run on 3v (i.e. two AA cells) or is there some sort of special order part I need for that?

Also, has anyone actually tried using a Picaxe with a lithium cell, like a 3.6v 1/2 AA cell?

Courteously,
The Professor (601).

 

[neiltechspec]

I have a few projects running off single LiPo cells of varying sizes from 600maH to 2200maH.

They all go into low power mode when supply volts drop to 3.2v (checking with calibadc10).

Neil.

 

[techElder]

I run a 20X2 on a 3V CR123. I don't have any high powered stuff attached to it, but it runs "forever."

 

[westaust55]

The newer M2 and X2 parts as currently sold by Rev Ed are capable of operating at a eiders voltage range. 5 V is the max recommended but operation at 3 V is also equally possible and even a little lower - see the manuals and website.

While I have not built many projects operating at less than 4.5 volts, my model railway speedometer wagon due to space constrains runs on 2 AAA batteries.
See:http://www.picaxeforum.co.uk/showthread.php?27013-PICAXE-based-Model-Railway-Speedometer-Wagon
That runs for many hours on a set of 2 batteries with a 2x 7-segment display plus decimal point (=up to 15 LEDs on) plus a IR LED (5mA) for speed sensing.

 

[premelec]

@Professor601 I run stuff on a single Li cell ok - a large part of your practical answer is what you are trying to do with the PICAXE outputs - the PICAXE will run it's program fine with lower voltages but may not have high enough signal to properly actuate what it may be controlling... including LCD displays and such. As usual an integrated solution to your needs is what's best.

 

[AllyCat]

Hi,

The "base" PIC 08M2 chip is specified down to 2.3 volts and the 14 and 20M2s down to 1.8 volts. So yes, I regularly use 3.2 volt Lithium phosphate (LiFePO4) cells, or 2 x AA Alkalines, or sometimes a pair of NiMH cells (2.4 volts).

However, beware that the maximum output drive currents are considerably reduced at these supply voltages, so the chip might struggle to drive ordinary LEDs at a reasonable brightness and may be unsuitable to directly drive multiplexed (e.g. 7-segment) LED displays, etc.

Cheers, Alan.

 

[westaust55]

I have not tried operating at voltages less than 3 V myself, however threads by others on this forum has indicated that while the PICAXE chips will successfully operate at the lower voltages, there may be difficulties downloading a program. If this occurs then it may be necessary to program at a higher voltage (ie 3V or above).

The maximum Operating frequency may also be limited at the lower voltages.
By way of indication: from the 20M2 corresponding PIC datasheet, at 2.5V and below the max frequency is 16 MHz, and 2.5 to 3.6 Volts is 32 MHz.

 

[Professor601]

Thanks for the quick replies. The chip will be "driving" a few LEDs, and doing ADC reads and other inputs, so it's pretty low-power stuff. I am currently working with an 18M2 chip, but looking at a 20M2 for the additional PWM options it offers (using the PWM) to modulate a tri-colour RGB LED). Your notes sound encouraging, though, as I don't need high clock speed, and won't likely draw much more than 50mA at any one time.

I will report my progress when I have some.

Courteously,
The Professor (601)

 

[AllyCat]

Hi,

50 mA sounds like quite a lot of current to me (in the context of lower PICaxe supply voltages). But a blue LED will need 3 volts anyway and then there's the voltage drop across its essential series resistor and maybe the internal FET switches, particularly if driving with PWM and/or "active high" outputs. So I don't really see the minimum operational voltage of the PICaxe "core" being relevant (nor 2 x AA cells being a viable supply).

Note that the PIC(axe) output driver characteristics are shown in (I/V) graphs at 5v, 3.0v and 1.8v (Figure 31-41 onwards) around page 400 of the latest "base" PIC data sheet for the 14/20M2s ( PIC16(L)F1825/1829 ).

Cheers, Alan.

 

[Professor601]

Well, I admit I am mostly likely stupid here, and I fully admit I am a tinkerer rather than actually engineering my projects, but AAA cells have around 800-1000mA capacity, so for a 50mA draw that seems like it would work for quite some time. But allow me to clarify that the project I am tinkering on will likely only run for 30-60 seconds at a time, then be powered off with a hard switch, so my expectations are different from a continuous run project. Also, I expect the 50mA rate will only be when the tri-colour LED is running with all three lone at nearly full on, which will not happen very often.

That said, thanks again for the input and the data link. I had not reviewed the PIC manual before, and certainly I need to do so.

Courteously,
The Professor (601)

P.S. Hopefully one day I shall be able to answer questions on this forum instead of merely being the git who asks them.

 

[AllyCat]

Hi,

No, it wasn't the current that I was concerned about but the voltage. A blue LED (e.g. of a RGB module) requires a forward voltage drop of around 3 volts to emit a significant amount of light. An Alkaline cell at around "half life" delivers about 1.25 volts (the manufactuers' end of life is usually quoted for 0.9 volts or lower), so (at best) you may need to replace the cells rather frequently.

IMHO all AA{A} cell applications should assume a nominal cell voltage of 1.25 volts, whether using NiMH (rechargeables) or Alkalines. Lithium cells are an obvious answer because their voltage is more constant, but you still need to "do the sums".

Cheers, Alan.

 

[tmfkam]

Hi,

No, it wasn't the current that I was concerned about but the voltage. A blue LED (e.g. of a RGB module) requires a forward voltage drop of around 3 volts to emit a significant amount of light. An Alkaline cell at around "half life" delivers about 1.25 volts (the manufactuers' end of life is usually quoted for 0.9 volts or lower), so (at best) you may need to replace the cells rather frequently.

IMHO all AA{A} cell applications should assume a nominal cell voltage of 1.25 volts, whether using NiMH (rechargeables) or Alkalines. Lithium cells are an obvious answer because their voltage is more constant, but you still need to "do the sums".

Cheers, Alan.

I have to say I was rather shocked by your comment regarding cell voltages, naively I'd assumed that the terminal voltage of 1.5V was more consistent than you suggested. I realised that the internal resistance caused the terminal voltage to 'droop' under load (and that this resistance increased with age), but hadn't given a whole lot of thought to the chemical voltage falling quite as much over the life of the cell as it can. Thanks for your insight, I'll have to consider the battery as less of a 'fixed' voltage than I had previously.

This from Duracell, showing the discharge curve for an AA cell, under various loads.



I'd considered that the curve would be relatively flat from 1.5V - 1.4V for a much longer period.

Every day, a day at school...

 

[techElder]

But those charts are with "constant current". No application consumes current in that way.

Design the app to be useful first (not just blinking a light), and then consider long term power requirements.

That will usually point to a particular battery type or chemistry.

One size never fits all. Your mileage will vary.

 

[premelec]

There's a wonderfully amusing piece about the 'Batterizer' Joule thief device which discusses some about battery life - and the patent for this device which is based on 1.4 volt unrealistic battery cutoff voltage - https://www.youtube.com/watch?v=4iEshd6izgk enjoy Fake Advertising lives....

@Professor601 I've used 14M2s with their four PWMOUTs to control RGB LEDs in the past but currently am working with the APA102 RGB units with internal PWM and two line control easily programmed by a PICAXE - there's a thread on these units down a few days... They are worth a look -

 

[tmfkam]

Made a number of 'Joule Thieves' in my time. Mostly to power some white LED lights for my bicycle. While the claims for the Batterizer may be 'over optimistic' they can be very effective at getting the last energy from cells. I've got one here used in a torch with five parallel white LEDs run from a single 1.5V cell. Good illumination from otherwise 'flat' cells.

I could get two AAA NiCad cells to last an entire winter of cycling powering three white LEDs and three red LEDs (running both front and rear in parallel with balancing resistors) which would be equivalent to 15 hours or so at 15 minutes per day November - mid February. Decent brightness too, it needed to be, as the promenade where I cycled was totally unlit in winter and with no lights that solid concrete sea wall (and steps) could leap out of the darkness and catch you unawares.

 

[tmfkam]

But those charts are with "constant current". No application consumes current in that way.

There are some curves plotted by Eveready for their Energiser cells that plots the voltage discharge in various devices (radio, toothbrush, remote control and so on) with differing usages (5 minutes per day, 1 hour per day etc.) and the shape of the curve is very similar. Obviously the time the cell takes to discharge is different for the differing loads, but I am still surprised how quickly the cell voltage drops to what I might consider 'flat'.

 

[premelec]

@tmfkam - I have used and built many up converters - great! The piece about the Batterizer is an example of excessive claims for a product - almost all commercial products made these days operate well below the 1.4v the device marketer claims as cutoff. One of my annoyances is that up converter regulators aren't incorporated into commercial products like head lamps [worn on head...] which then dim constantly from initial brightness. Glad you are using adequate lighting for night riding! I've quit night cycling from aging vision and excessive bright auto lights - used to do it a lot with multi LED [and earlier tungsten halogen] lamps. Stay clear of leaping concrete abutments!