nura100

Hello Forum,

i have a circuit which indicates when the charging current exceeds certain amperes, in my case 6 amps, i have used a 0.01 ohm current sense resistor in the negative of the battery and iam measuring the voltage developed across the sense resistor.

if the voltage across this resistor is 60 millivolts i know that 6 amps current is flowing thru the battery. an op-amp is used as a comparator to compare the voltage of sense resistor with a reference voltage of 60 mV.


but the circuit is not operating as i intended, even when the voltage across the sense resistor is beyond Vref (60mV) the output of the op-amp doesn't change state (ideally it should swing to 1 volt or so but stays at 3.81 V)

i don't know where the error is


Thank you

nura

Attached Images

Current Sense.jpg (92.5 KB, 42 views)

Nigel Goodwin

For a start you need a large resistor in the top end of the pot, as it covers far too wide a range, and most probably never goes low enough to reach the switching point.

Does your opamp work as close as that to the negative rail?, I would suggest using a split supply would be a better idea.

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ericgibbs

hi.
Using the 1:1 resistive divider R2/3, what do you think the voltage at the junction of R2/3 will be,,, not 60mV.!

As Nigel points out the variable control is too coarse and the end stop on the control probably will not go down to 30mV.

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Eric
"Good enough is Perfect"

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crutschow

As Nigel also noted, the LM1458N op amp will not work with the input equal to the negative supply voltage (in this case, ground). You need at least -3V on the op amp negative supply, or use an op amp with rail-to-rail capability.

(I know I'm beating a dead horse, but read the data sheet. If you don't understand data sheets then read a tutorial about op amps.)

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Carl

Nigel Goodwin

I don't think even 'rail to rail' opamps actually go that close to the rails?.

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crutschow

Yes. Many actually have an input common-mode range a few tenths of a volt above and below the supply rails (see Rail to Rail Input Operational Amplifiers at National.com for example.) The output also goes to the supply rails (but the available output current is reduced near the rail).

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Carl

Someone Electro

If the power supply for the op amp dose not have the same ground as the op amp it would be easy to use a resistor divider to move the 0V to 2,5V. In that case it should work.

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crutschow

It could be marginal without increasing the supply voltage. The LM1458 is rated at 6V minimum supply voltage.

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Carl

nura100

Thank you members for pointing out the errors, i am restricted to single supply only and cannot use dual supply. this circuit is actually to be interfaced to a micro which performs some other function based on the output logic state of the op-amp.

i am not good with op-amps, but i have tested it on the breadboard board by using two presets 0f 10k each on + and - pins of the op-amp with 5+ volts P.S fed to the LM1458.

here when the ref is set to 60mV on + pin and i adjust voltage on - pin to above or below 60mV the out put of the op-amp changes state accordingly,

but when i connect the circuit to battery charger circuit and try sensing the charging current across 0.01 Ohm resi the op-amp output doesn't swing .

can any member suggest a proper circuit which suits my requirement


Thank you

Regards

Arun

nura100

one more thing i forgot to mention, i am actually using a LM358 Dual op-amp and not LM1458, sorry for the mix up.

Attached Images

LM358.jpg (83.2 KB, 3 views)

ericgibbs

hi,
I will repeat my question.

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Eric
"Good enough is Perfect"

PIC tutorials:
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nura100

Hello eric

i know i have used two equal value resistors (potential divider) which results in half the voltage, i have removed the lower 10K resistor but the problem still continues.



Arun

ericgibbs

hi,
Look at these changes to your circuit.

The TC358 as a OPA is not suitable as a quality comparator.

A simple LM393 would have been a better choice.
Its a comparator.

Have you scoped the 'ripple voltage' across the Rsense resistor... have a look.

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Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

nura100

Thank you Eric, for the corrections, i am not lucky enough to own a Scope, since you have pointed about ripple voltage, should i have a 0.1uF polyester cap across the sense resistor and next to the 1000uF cap in the rectifier section.


one more thing, the +5V supply which powers the LM358 is derived from another trafo and only the GNDs of both the charger and 5volts supply are commoned.


regards


Arun