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solar charger problem

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Hlias

New Member
Hello everyone.
I have just built a small solar charger that charges a 6V battery. This in turn goes through a very simple op-amp circuit to check low voltage (in parallel) and then through a 5V voltage regulatorto a USB plug.
My problem, is that at the input, if I use my multimeter to check for short circuit/connectivity, if i use one polarity, it looks fine, but when i inverse the polarity of the multimeter leads, it appears short-circuited.
Could anyone suggest anything please?

thanks,
Hlias.
 

colin55

Well-Known Member
The is because the regulator contains transistors or diodes on the output and these have a low resistance in one direction. You will find the needle of the meter will deflect to a position equivalent to 0.6v on the scale.
 

Hlias

New Member
thanks Colin. So this shouldn't be a problem then. Also I dont know if it is the same reason but the LM741 that im using as a voltage comparator always seems to give a high output. May this be the same case?
 

audioguru

Well-Known Member
Most Helpful Member
Many lousy old 741 opamps do not work when the supply voltage for them is less than 10V. Their input voltage must be 3V or more higher than the negative power supply voltage (0V in your circuit) and must be 3V or more lower than the positive power supply voltage. So if your supply is only 6V then the input voltages for the 741 opamp must be no higher and no lower than 3V then it cannot have an input signal.
 

Hlias

New Member
Many lousy old 741 opamps do not work when the supply voltage for them is less than 10V. Their input voltage must be 3V or more higher than the negative power supply voltage (0V in your circuit) and must be 3V or more lower than the positive power supply voltage. So if your supply is only 6V then the input voltages for the 741 opamp must be no higher and no lower than 3V then it cannot have an input signal.
Hmmmm... that makes a lot of sense now. Thanks a lot. By the way, could you suggest any other opamp that would do the work @6V input and that will not require a lot of offset nulling?
 

audioguru

Well-Known Member
Most Helpful Member
An LM358 is a dual opamp with a minimum supply voltage of 3V. Its input voltage range is from the negative supply (0V in your circuit) to 1.5V less than the positive supply voltage. Its output voltage goes from almost 0V to 1.2V less than the positive supply voltage if the output current is low. It has the same input offset voltage error as ordinary opamps but since it is a dual opamp in an 8-pins case it does not have offset adjust pins.
An MC33171 and an MC34071 single opamp have offset adjust pins and have a 3V minimum supply voltage. Their inputs work at 0V and their outputs go almost to 0V.
 
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