Voltage follower generates voltage by itself

Question

I am currently working on a project that requires me to measure a capacitor voltage that can reach up to 250 V.

To measure such a high voltage I first created a voltage divider with very high resistor values so the capacitor doesn't discharge very fast (resistor1 = 10 MΩ, resistor2 = 200 kΩ). The problem is that with such high resistor values the Arduino doesn't measure the voltage exactly, since the max. resistor recommendation for the Arduino ADC is 10 kΩ.

I decided to wire an op-amp as a voltage follower to have lower impedance on the output, but the voltage follower doesn't measure capacitor voltages lower than 100 V (or 1.96 V after the divider) and what happens is that, when the voltage falls below that, the follower produces a small voltage by itself (around 1.9 V), thus tricking the Arduino into calculating the wrong voltage of the capacitor. The op-amp I'm using is the UA741 and I'm powering it with single rail supply.

Does anyone know why the voltage follower doesn't work?

The problem is that the output voltage of the single-supplied 741 cannot go below this value. If you still want the op-amp to be 741, you need to add a negative source with at least -2 V voltage (i.e. to make a split supply). The conventional solution is to use a rail-to-rail op-amp. — Circuit fantasist, Jan 14 '24 at 10:33
And why not use lower resistance resistors for the voltage divider, for example R1 = 100k and R2 = 2k? Take into account the power dissipated. The Arduino recommendations are about the equivalent input resistance (R1||R2). — Circuit fantasist, Jan 14 '24 at 10:44
Does this answer your question? Reasons not to use a 741 op-amp? — brhans, Jan 14 '24 at 14:14

ocrdu · Accepted Answer · 2024-01-14T12:44:51.653

The output of the 741 can't get closer to the rails than about 2 V, so with a single supply of, say, 0 and 12 V, the output would be limited to between about 2 and 10 V.

You could use a split supply, but it would be easier to use a rail-to-rail op-amp, and avoid the obsolete 741.

You could also try smaller voltage divider resistors.

BTW: The recommendation for the ADC is what it is, but the standard Arduino library for the ADC allows a very long time for the ADC to settle (which is why it is slow when you use the default settings), so you can get away with a higher impedance than the recommendation.

Voltage follower generates voltage by itself

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