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'[PIC] measuring current in a buck dc-dc converter'
2006\05\04@200602 by rlistas

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Hi,


I need some suggestion on how to measure the output
current in a buck dc-dc converter used to charge
a battery.

I´m using a PIC 16F88 as PWM generator and charge
controller (MPPT).
The problem of  measuring the current is the ground referencial
that the microcontroller uses. I´m using a N channel
mosfet as the switch (so the negative is switched and the positive is
always conected) and therefore I´m getting trouble using the AD of the PIC.

I want to get the value of the current by means of a low value resistor
connected to the battery.

Any suggestion will be welcome.

thanks

Rubens

2006\05\04@203835 by PicDude

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face
Op-amp across the sense resistor?

-Neil.


On Thursday 04 May 2006 19:05, rlistas wrote:
{Quote hidden}

2006\05\05@063945 by istopher Holmes

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face
Hi Rubens,

 The INA138 from TI is a nice part for measuring current. Keep in mind
though that the minimum common mode input is 2.7V, so you couldn't use
it for charging a low voltage battery.

 You might also use an instrumentation amplifier. It's simpler than
building a differential amplifier with an op amp, plus you can easily
set the output's reference voltage to whatever you like. I did a circuit
like yours once using an AD623 to control the output of a switching
current source at very low voltage (i.e. low resistance load). Worked
pretty well.

Chris

rlistas wrote:
{Quote hidden}

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2006\05\05@071523 by Gerhard Fiedler

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rlistas wrote:

> I need some suggestion on how to measure the output current in a buck
> dc-dc converter used to charge a battery.
>
> I´m using a PIC 16F88 as PWM generator and charge controller (MPPT). The
> problem of  measuring the current is the ground referencial that the
> microcontroller uses. I´m using a N channel mosfet as the switch (so the
> negative is switched and the positive is always conected) and therefore
> I´m getting trouble using the AD of the PIC.
>
> I want to get the value of the current by means of a low value resistor
> connected to the battery.

If I understand you correctly, you can do that with an opamp or a high-side
current sensor amp (which is a specialized opamp). Look at the LT1637 or
similar. (For a one-off, Maxim also has such parts :)

Gerhard

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