[Tinyos-help] Ask inner resistance of Micaz motes

[Urs Hunkeler]

Hi,

As MS already said, the "resistance" of the mote will vary based on the 
tasks being executed on the mote. If you need to estimate the current 
drawn by a mote and the remaining power in the batteries, then probably 
the best approach is to use a fuel gauge like the DS2770 as suggested in 
an earlier discussion (see below). I think, the DS2770 is not in 
production anymore, but you can find similar products:
http://para.maxim-ic.com/en/search.mvp?fam=batt_stat&295=Fuel%26nbsp%3BGauge&tree=powersupplies

You might also want to read this application note:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/121

The above recommendations are taken from an interesting discussion quite 
a while ago that I remembered. The two relevant posts are:
http://mail.millennium.berkeley.edu/pipermail/tinyos-help/2007-January/022092.html
http://mail.millennium.berkeley.edu/pipermail/tinyos-help/2007-January/022099.html

As for the help-list wisdom about the receive-mode drawing more current 
than the send mode: From the CC2420 datasheet, the CC2420 draws 18.8mA 
in receive mode and 17.4mA in send mode at the highest transmission 
power level (0dBm). I can confirm this with measurements that I made 
myself. The reason for this is probably (meaning, I have no citations) 
that sending is a pretty straight-forward process and the output power 
is limited by the laws for using the license-free radio bands (1mW in 
the 2.4GHz band). The receive mode, however, uses a much more 
complicated circuit to amplify and decode the received signal. Because 
of the low transmit power permitted by law, the radio module 
manufacturers try to extend the communication range by implementing 
sophisticated filters to eliminate background noise and the like.

When calculating the power consumption of communications consider that 
before sending a single message the radio enters a random back-off state 
during which the radio is in receive mode and typically draws more 
energy than during the actual transmission mode.

Cheers,
Urs

Michael Schippling wrote:
> Well, you see...in a word. No.
> At least I don't know of any.
> 
> But more to the point, it's a meaningless question
> because the current draw, and thus "inner resistance",
> varies with the functions being performed. E.g., if the
> radio is transmitting, more current is being drawn (and,
> according to help-list wisdom, receiving uses even more
> power....which makes no sense to me, but not much does
> anymore either).
> 
> If you have current measurements for the various states
> you can calculate the effective resistances and then
> guess how long the mote is in each state to arrive at
> a total power use.
> 
> MS
> 
> ma. pratama wrote:
>> yes..thats exactly what i do right now..using a small resistor and 
>> measure the battery each hour..but if i could know the inner resistance 
>> it should be much easier to measure the current..are there any 
>> repositories that revealed the micaz resistance??
>>
>> --- On *Sat, 6/6/09, Michael Schippling /<schip at santafe.edu>/* wrote:
>>
>>
>>     From: Michael Schippling <schip at santafe..edu>
>>     Subject: Re: [Tinyos-help] Ask inner resistance of Micaz motes
>>     To: "ma. pratama" <spidol_permanen at yahoo.com>
>>     Cc: tinyos-help at millennium.berkeley.edu
>>     Date: Saturday, June 6, 2009, 9:33 AM
>>
>>     That's a bit of a problem as the current draw changes with operation.
>>     Folks have used a small resistor in series with the battery to measure
>>     the current. Search this list for more info.
>>
>>     MS
>>
>>     ma. pratama wrote:
>>      > Hi all,,,
>>      >
>>      > I want to measure the current after usage in the micaz motes. I
>>     will using moteview to monitor the voltage in the micaz motes, so i
>>     only need the inner resistance inside the micaz motes to determine
>>     the current by simple formula : V =  I . R.
>>      >
>>      > Do anyone know how much the inner resistance of the micaz motes??
>>     thanks..