L
Lasse Langwadt Christense
Guest
Den tirsdag den 4. november 2014 20.24.20 UTC+1 skrev Tim Wescott:
get one of Dave Jones uCurrent
http://eevblog.myshopify.com/products/ucurrent
-Lasse
On Tue, 04 Nov 2014 17:38:01 +0100, blisca wrote:
Hi to all,
often i have to measure the current absorbed by small circuits,in
battery appplications Normally i deal with consumptions big as 20mA in
run mode and 2uA in sleep mode.
One method that sometimes i use is to desconnect the battery,leaving the
circuit feeded only by a 10,000 uF capacitor,seldom and quickly measured
by an high impedence multimeter,(of course not a 10 Mohm oscilloscope
probe).Measuring the difference in voltage at a specified times i can
calculate the consumption.The cap was previously measured,to avoid
tolerance error.
More often i need a more immediate measurement,so i feed the circuit by
a commutator switch,in parallel to 1,10 and 100 ohm shunt resistors.
The latter is constantly inserted so that the circuit is not completely
relying on itself during commutations.
When the uc goes to sleep and i need to have an hopefully realistic
rading of few uA i switch on the 100 ohm shunt,the voltage drop between
battery and circuit is below the mV,therefore irrelevant.
The voltage is read by a 6 and half digit DMM via a 60cm(2 ft)twisted
wire.
Is this a good method or others can be better?
Is there any advantage using an instrumentation op amp as buffer or
amplifier, wired very closely to the shunt?
Many thanks for your attention
Diego
I'm taking the liberty of cross-posting this to sci.electronics.design,
because this is really an electronics question, not an embedded question.
I think you'll get more answers from there, and many of them will even be
good ones.
I think your commutation idea will suffer in accuracy if the load is
fairly stiff in voltage -- like if you're driving a capacitor. In that
case the current with the low resistance switched in will be higher than
without.
If the circuit is otherwise isolated, the best way to do the measurement
may be to put your 100 ohm resistor between the circuit ground and the
negative terminal of the battery, then amplify its voltage by 10 or 100
using a chopper-stabilized op-amp. Then you can just read the voltage and
scale it appropriately to current.
get one of Dave Jones uCurrent
http://eevblog.myshopify.com/products/ucurrent
-Lasse