# How do I not pass current with 0V input? is it possible to change the input voltage range?

#### tuto

Joined May 2, 2021
9
Hello everybody!

I have two questions.

I have this circuit where I want to vary the input voltage (V1) between 0V and 5V to supply power to R2. Is it possible to make the circuit not supply a power (0W) to resistor R2 when my voltage V1 is 0V?

When I do the simulation with 0V in V1 there is current going through R2 so my power in R2 is different from 0W. Is there any way of doing this? i always supply power because it is always current going through my R2. can i make this current go to 0mA only when V1 is 0V?

The other question is: if my voltage source V1 can only go between 0.55V and 3.3V, is there any possible connection to make between the voltage source V1 and the opamp such that my new input voltage goes from 0V to 5V?

thanks!

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#### MrChips

Joined Oct 2, 2009
23,541
Why do the have the negative supply of the op-amp connected to R1 and non-inverting input of the op-amp?

#### crutschow

Joined Mar 14, 2008
27,231
Besides the negative supply incorrectly connected to R1 as MC noticed, the op amp is open loop with no negative feedback, which will cause the output to go to either the positive or negative rail..
To use an op amp as an amplifier it must have negative feedback to the minus input.

You need to study how op amps work before you try to design a circuit using one.

And why are you using a high-speed, video op amp?

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#### Papabravo

Joined Feb 24, 2006
16,163
To sweep a voltage source use the .dc command in place of .tran(sient), or configure your voltage source to ramp the voltage up and down, or use a PWL source.

#### Ian0

Joined Aug 7, 2020
2,258
Two notes:
1. Do you have the negative supply pin connected to the right place? The circuit looks as though it was intended to be a constant current source, in which case pin 4 should be connected to ground, not to the negative input, and the inputs (pins 2 and 3) are reversed
2. If my assumption is correct then you will have a circuit with proper feedback, but it still won't work due to your choice of op-amp. You need an op-amp with a common mode input range that include ground, and the AD811 doesn't - it's not easy to spot but it's on page 4 of the datasheet.
The good old LM358 will work here, if you reduce its supply voltage below 32V, or you need to find an amplifier which will withstand 37V and has a common mode input range including ground - and they are not so common. OPA170 will manage 36V.