How to do power measurement.

[Michele]

hi guy,

i just realize 2 easy network( with beacon and without beacon ),

where i have 1 coordinator ( AC alimented)

                      1 device (battery alimented).

now, how can i do measure on  real power consumption of my device?

which instrument do i need?

is there documentation?

is there a mathematical model ?

i use  2 MC9s08GT60

thanks all.

[Michele]

thanks very much guy,

i think i have understand.

now i'll go to try to do my measurement

thanks very very much.

[bigmac]

Hello,

I assume you need to estimate the battery life for the device, and this will depend on the average current drawn.  Estimating the average current can be a little tricky since there is likely to be very wide variation of current, from a few microamps in stop mode, to many milliamps when a transmission occurs or the receiver is active.

The larger current values can probably be directly measured using a suitable DC meter, provided you can manually control the operating mode of the transceiver and MCU during the measurement.  However, for the measurement of microamp levels, the usually approach is to place a relatively low value resistor in series with the supply lead (preferably the negative lead) and monitor the voltage across the resistor.  If an oscilloscope is used, any short term variation can be observed, particularly with respect to pick up of hum and noise within the measurement setup.

It is best to include a Schottky diode in parallel with the measurement resistor so that, whenever the transmitter or receiver activates, the voltage drop will be limited to about 0.4 volts.  The resistor value should be chosen so that the maximum voltage drop is somewhat less than 0.4 volts for the other modes of operation.

The following thread may provide some additional information on low current measurement -

http://forums.freescale.com/freescale/board/message?board.id=8BITCOMM&thread.id=3287

The following thread gives an approach to estimating battery life -

http://forums.freescale.com/freescale/board/message?board.id=8BITCOMM&message.id=294

Regards,

Mac

Message Edited by bigmac on 2007-05-29 02:29 PM

[Curt]

Hi Michele

Not sure what you're asking here -- could be easy, could be hard since your system's power consumption will vary with time and activity.  Peak or average power?  How accurate do you need to be?

For rough work, your battery operated device power consumption can be determined by measuring battery voltage and current, so any multimeter that can measure DC volts and DC current will get you there (P = VI).  For the AC device, you'll probably want an AC power meter.

Hope that helps a little.