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)
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.
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)
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.)
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).
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.
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.
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
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.
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.)
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.