load resistance determination

[tR|GG3rX]

two questions...

(1) how can we know how much current that a load (e.g. microcontroller, other ICs) consumes, so we can determine when the battery power will entirely be used up.

(2) how can we know whether a load is resistive or non-resistive?

 

[flatulent]

The typical power draw for all of the parts on your board can be found from their data sheets and added up. Then when you have made one it can be measured.

Because of all of the bypass capacitors, most circuits have an equivalent circuit presented to the power supply of a resistor in parallel with a capacitor.

 

[tR|GG3rX]

oh i see. that's why they used bypassed capacitor to work with many ICs.
lets say i know the power consumed by an MCU, RAM, and other ICs. so can i combine the resistance or power consumed using thevenin's theorem? it'll be such complex calculation.

 

[batdin]

You don't combine resistances.Add currents (as flatulent said).Resistance is not the right representation of ICs as a load because their VI curves are by no means linear.
It can't be so hard to add the currents consumed by 10 ICs,right?
Furthermore,it is the current You will want to know in order to determine when the battery will be exhausted.

 

[Kevin Weddle]

You simply add the currents. The only problem is the reactive components which have apparent power to them. The current will be higher and lower, but it can easily be measured with an AC ammeter.

 

[yeechyan]

You can use osciloscope to check the waveform of the current and the voltage. If you see any phase differences between the current and voltage, it's an reactive load. If no phase differences, it's a resistive load.

 

[platonas]

Also as far as semiconductors are conserned (that is a PCB with MCUs or other ICs), you cannot use theorems to calculate consumption. It is not fixed. For a microcontroler for example this varies dramatically depending on the state and function performed by it. If yoou drive some external loads like LEDs the consumption is high, if it is in slip mode then only few microamps are drawn. Similarly for an Op-Amp, with high inputsignals, or output, the consumption is higher (always depends output load). Logic IC consume more with higher switching frequencies.
The only way to know the consumption is to measure it with a series ammeter. If you create a profile by taking various measurments you might have a better view of the average current needs of your board.