Voltage scaling

[reks]

hi i have a battery voltage sense circuit to be implemented.It should be such that battery voltage can go from 32V to 42V.
Hence i need to scale this 32-42V to 0-5V so as to give as input to ADC.Plz help me with this.
I can use an opamp for this purpose rt? but how to implement this, i dont hav any idea.

 

[Hero999]

You use a potential divider.
Voltage divider - Wikipedia, the free encyclopedia

 

[reks]

by using a pot divider ckt,i can scale the 42V to any level,but the 32 V how to reduce it to 0V, in the same circuit.

 

[Hero999]

Here's a guide which will enable to to do what you want.
**broken link removed**

First use a potential divider to scale the input voltage to a range safe for the op-amp. I used a potential divider consisting of 120k and 15k.

I'm not going to say any more for now, other than I went through the tutorial and calculated the resistor values and simulated the circuit.

Have a go yourself and I'll give you a hand.

EDIT:
You won't be able to get exactly between 0V and 5V but you should be able to get pretty close. According to the simulation, I managed to get a voltage range of 74.1mV to 4.99V, using E24 values for all the resistors.

 

[reks]

thanks for the notes.but i ve been askd to do the scaling using the differential opamp configuration by taking the voltage across the battery...

 

[Hero999]

I thought I could smell a college project.

Oh well as it's not what you want I'll post my solution, for the benefit of others.

Then you need a 32V voltage reference (the TL431 can do that) and a differential amplfier with a gain of 0.5.

https://www.electro-tech-online.com/custompdfs/2010/04/TL2FTL431.pdf
Operational amplifier applications - Wikipedia, the free encyclopedia

The op-amp will need to be run from a >24V supply and the voltage reference needs a continious supply voltage > 32V, say 36V.

It's not a very good way of doing it.

 

[reks]

by using two stages of the opamps, can this be implemented?1st stage senses the voltage across the battery or so, then it scales or somthing of that kind??i need to implement it using two or three stages opamp preferably.
can i give 32 or 42 V directly as input to a diff opamp?that is another doubt i ve.

 

[ericgibbs]

by using two stages of the opamps, can this be implemented?1st stage senses the voltage across the battery or so, then it scales or somthing of that kind??i need to implement it using two or three stages opamp preferably.
can i give 32 or 42 V directly as input to a diff opamp?that is another doubt i ve.

hi,
The 32V to 42V input directly to an OPA, is outside the range of most OPA's.

Use a simple resistive voltage divide to get the volts down to a more manageable level.

A 36K and 4.3K would bring it down to 3.4V to 4.8V, you can then use most OPA's to do the remainder of the level shifting, 0V to +5V.

Ok.?

 

[MikeMl]

If you are willing to negate the ADC reading in software, then you can do it with a single Rail-to-Rail Opamp operated off the same 5V supply that is used as the reference for the ADC.

 

[reks]

I am not well versed with differential opamp circuits.so i wanna clarify,if i apply 32V to input of opamp in differential mode, it wont work is it?

 

[MikeMl]

With most IC opamps, the common-mode input range is less positive than the voltage applied to the opamp's V+ terminal. If you power the opamp from 5V, the inputs cannot be more positive than 5V!

 

[reks]

But with suitable values of Rin and Rf, the applied voltage wont be scaled to reqd safe range for the opamp?

 

[MikeMl]

But with suitable values of Rin and Rf, the applied voltage wont be scaled to reqd safe range for the opamp?

But that is not what you asked!!!

 

[Hero999]

Eric,
A differential op-amp with a low gain can be run from a lower voltage supply because the inputs will be at a lower voltage.
**broken link removed**

Note how R2 and Rg form a potential divider?

If the gain is 1, the input voltage will be half the input voltage.

In this case the gain needs to be 0.5 so the input voltage will be lower.

Mike,
I already posted that circuit, you just used different resistor values and a rail to rail op-amp powered from 5V.

 

[reks]

Exactly sir, i was thinking that if i keep that Pot divider values such that 32V is reduced to 4V or small value, will that be given to the +ve terminal of opamp?

 

[ericgibbs]

Eric,
A differential op-amp with a low gain can be run from a lower voltage supply because the inputs will be at a lower voltage.

I agree, BUT this is NOT the question he asked.?

 

[ericgibbs]

Exactly sir, i was thinking that if i keep that Pot divider values such that 32V is reduced to 4V or small value, will that be given to the +ve terminal of opamp?

It would help everyone if you told us what you are trying to do and posted a circuit diagram.

At the moment we are all trying to figure out whats being asked of us.!

 

[reks]

sorry sir for being vague..
I need to sense battery voltage which will be 32V-42V.It has to be scaled to 0-10V.Voltage sense circuit should be using opamp.
the link posted fist **broken link removed** was very useful and it did the function very well.But then my guide doesnto want this circuit,.he suggested try sensing the voltage across the battery using a diff opamp, then may b scale it in next stage of opamp(which would b a non-inverting stage).
i hope i made my problem clear, please help me out.

 

[ericgibbs]

sorry sir for being vague..
I need to sense battery voltage which will be 32V-42V.It has to be scaled to 0-10V.Voltage sense circuit should be using opamp.
the link posted fist **broken link removed** was very useful and it did the function very well.But then my guide doesnto want this circuit,.he suggested try sensing the voltage across the battery using a diff opamp, then may b scale it in next stage of opamp(which would b a non-inverting stage).
i hope i made my problem clear, please help me out.

OK,
In that case use Mikes circuit post #9, if you need to invert the sense of the signal use a 2nd OPA as an inverter.

 

[MikeMl]

sorry sir for being vague..
I need to sense battery voltage which will be 32V-42V.It has to be scaled to 0-10V.Voltage sense circuit should be using opamp....

You changed your requirement. You said in your original post that you needed a 0-5V range. Which is it?