# POWER consumption

#### wheatstonin'

Joined Nov 27, 2007
3
Hi,
I'm building a microcontroller circuit as part of a project and we're trying to figure out the best way to quantize the power that is being consumed. Is there any formula/formulas that we could use to calculate this power? also, how do we calculate the power loss in the circuit? the power supply to the circuit is 3.3V.
thanks...

#### beenthere

Joined Apr 20, 2004
15,819
Power in watts is voltage (E) times current (I). All processors have variable usage of current, depending on the program instructions being executed. About the best you can do is monitor current over time and come up with an average.

#### eeboy

Joined Sep 27, 2007
90
Just to add a little to beenthere's post:

Consumption is monitored at the input to your circuit. Measure the current from the power supply and measure the voltage across the inputs. As he stated it's going to be very dynamic depending on the state (program execution) of your micro.

You have to be a bit more specific on the power loss. You could certainly argue that your power consumption is your power loss. It really depends on your circuit and what it is designed to do. To answer this you would really have to provide more details.

Just a trivial example... lighting an LED

The circuit would consume X watts. Y watts are dissipated as heat in the current limiting resistor and Z watts go towards the useful aspect of your circuit... that being light. The consumption X, is the sum of Y and Z.

#### Management

Joined Sep 18, 2007
306
Maybe you can put a 10 Ohm resistor in series with your power supply and your microcontroller, then measure the voltage across that resistor, get the current through that resistor, and then multiple that current by your power supply voltage and that should do the trick I think.

Anyone object. I was really just thinking out loud.

#### Salgat

Joined Dec 23, 2006
221
Maybe you can put a 10 Ohm resistor in series with your power supply and your microcontroller, then measure the voltage across that resistor, get the current through that resistor, and then multiple that current by your power supply voltage and that should do the trick I think.

Anyone object. I was really just thinking out loud.
Heck, a 1 Ohm precision resistor would probably be the best if you only wish to use a voltmeter. Great idea for easy measurements.

#### Management

Joined Sep 18, 2007
306
Heck, a 1 Ohm precision resistor would probably be the best if you only wish to use a voltmeter. Great idea for easy measurements.
HA yea 1 Ohm would be good. I don't know 10 was the first thing that came to mind.

#### eeboy

Joined Sep 27, 2007
90
True, that would work... but if you are using a DMM to take measurements then it doesn't make a lot of sense. Most DMM's have the capability to measure current as well. So, simply place the DMM in series. The problem with using the DMM for measuring voltage and current is they have a low bandwidth and probably will not catch surges of current during the operation of the device. Two DMM's would be necessary assuming the operation is somewhat dynamic. You would need to measure both quantities at the same time.

A DSO would be ideal here. This would capture both quantities over a period of time allowing for an accurate average/instantaneous power calculation.

There's not a lot of information here so we can only assume what type of equipment he has and how accurate he must be.

#### Management

Joined Sep 18, 2007
306
True, that would work... but if you are using a DMM to take measurements then it doesn't make a lot of sense. Most DMM's have the capability to measure current as well. So, simply place the DMM in series. The problem with using the DMM for measuring voltage and current is they have a low bandwidth and probably will not catch surges of current during the operation of the device. Two DMM's would be necessary assuming the operation is somewhat dynamic. You would need to measure both quantities at the same time.

A DSO would be ideal here. This would capture both quantities over a period of time allowing for an accurate average/instantaneous power calculation.

There's not a lot of information here so we can only assume what type of equipment he has and how accurate he must be.
Good Idea, a DSO would be best. If he only has a DMM then maybe he can set it to display the maximum voltage only. Maybe he would be able to see the spike in current draw during operation. Granted the DMM is fast enough to catch it right?

#### eeboy

Joined Sep 27, 2007
90
Were you referring to maximum current?

If he's looking for maximum instantaneous power consumption then you are correct. However, at maximum power you will be drawing more current from the power supply which will result in a voltage drop. This voltage drop could be (and probably is) negligible.

It all depends on what his circuitry looks like...

#### Distort10n

Joined Dec 25, 2006
429
DC power would be easy, and you would only need to look at the Vcc and Iq to determine Pq.
Dynamic power is another back of beans, and yes there are equations to calculate this. TI has an excellent application note on switching outputs; although it is meant for logic devices it should apply to all I/O's.
The caveat would be that not all outputs of a uC are digital I/O's. Some outputs may be analog in nature.
It certainly is a tough business when you want a dead on number for power consumption. It would be easier to deal with a worst case scenario to determine if you can handle Pmax in your design.
Most datasheets should specificy Iq under conditions. For a uC what devices are active when this was measured, etc.

thank you so much everyone for all your useful inputs!!! 