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Help for static power consumption measurement

Hi all,
I have to determine the power consumption on a device in different
states. I would like to have your inputs about the procedure I have
thought:
- Hardcoded the device in a specific mode
- Measure the voltage drop (Vdrop) across a series resistance (Rtest)
at batteries output (1.0 or 0.1 Ohm, I'm not sure what is best ???) (Do
I must considered cable resistance used to measure??? N.B. I use a
HP3456A 6 1/2 digits voltmeters)
- Knowing the batterie voltage (Vbatt), the power drain from batteries
is Pstate = Vbatt * Vdrop / Rtest
- Since I use a boost converter to raise the 1.5V from the 2 AA
batteries to 3V (voltage at which all the device components works), the
current drain by these components is approximatly 2 times less (if I
forget the switcher efficiency)?

Don't hesitate if you see major problem and/or have different methode
to propose!
Thanks in advance for all your input!
Fred
 
B

Ban

Jan 1, 1970
0
Hi all,
I have to determine the power consumption on a device in different
states. I would like to have your inputs about the procedure I have
thought:
- Hardcoded the device in a specific mode
- Measure the voltage drop (Vdrop) across a series resistance (Rtest)
at batteries output (1.0 or 0.1 Ohm, I'm not sure what is best ???)
(Do I must considered cable resistance used to measure??? N.B. I use a
HP3456A 6 1/2 digits voltmeters)
- Knowing the batterie voltage (Vbatt), the power drain from batteries
is Pstate = Vbatt * Vdrop / Rtest
- Since I use a boost converter to raise the 1.5V from the 2 AA
batteries to 3V (voltage at which all the device components works),
the current drain by these components is approximatly 2 times less
(if I forget the switcher efficiency)?

Don't hesitate if you see major problem and/or have different methode
to propose!
Thanks in advance for all your input!
Fred

But 2AAs in series have already 3V, (when 90% discharged 2V). That will
probably be enough to run your device. Is it a PIC?
What resistor you use is dependent on the current, for less than 100mA I
would take 1R, above 0.1R with fresh batteries or better a bench supply it
should be possible to determine the supply current.
 
P

PeteS

Jan 1, 1970
0
A small series resistor is fine for low power devices - but you need a
decent idea of the current drain profile under all circumstances.

A device that may draw 50mA statically may draw 2-3 (or more) amps on
startup depending on circuitry - and the power system must still
provide sufficient voltage. A series resistor of 1 ohm would be a
liability in that case.

I would suggest iterating the sense resistor (which is as good a
solution as any for a lab using 6 1/2 digit meters), starting with 0.1
ohm and watching the start transient. Note you have to watch out for
the power rating so your sense resistor doesn't burn (or worse, burn
*you*)


Obviously, the largest resistor that does not interfere with the power,
the better the measurement (within reason :).

There are other solutions, including magnetically coupled clamps (which
are non-inavasive), but they tend to be expensive and often have to be
pre-calibrated.

Cheers

PeteS
 
P

Pig Bladder

Jan 1, 1970
0
Hi all,
I have to determine the power consumption on a device in different
states.

In California, it's usually Volts times Amps.
I would like to have your inputs about the procedure I have
thought:
- Hardcoded the device in a specific mode
- Measure the voltage drop (Vdrop) across a series resistance (Rtest)
at batteries output (1.0 or 0.1 Ohm, I'm not sure what is best ???) (Do
I must considered cable resistance used to measure??? N.B. I use a
HP3456A 6 1/2 digits voltmeters)
- Knowing the batterie voltage (Vbatt), the power drain from batteries
is Pstate = Vbatt * Vdrop / Rtest
- Since I use a boost converter to raise the 1.5V from the 2 AA
batteries to 3V (voltage at which all the device components works), the
current drain by these components is approximatly 2 times less (if I
forget the switcher efficiency)?

Don't hesitate if you see major problem and/or have different methode
to propose!

(1) Ask the teacher what the **** he's asking for, or (2) Read that
chapter of the textbook again.
Thanks in advance for all your input!

You Betcha! ;-P
 
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