Voltage -/+3 in +/-10 out

[paul200]

I have a problem in how to convert an incoming voltage that can range from +/- 3 volts I need an output of +/- 10 volts.
I looked at op amps and the supply rail is common to the output so would need to change polarity? Help please

 

[gramo]

Use a non inverting op-amp setup,

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Gain is calculated by

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You want about 3.3 Gain, so that 3V coming in is amplified to 10V

With a 8.2K and 18K resistors that’s 18K / 8.2K + 1 = 3.2 (Gain)

You could use a potentiometer to make it more accurate

 

[gramo]

I should also add that you will need +10 volts and -10volts on the op-amp rails(+V and -V in the diagram) so that it can swing the full range on the output

 

[paul200]

That sounds ok, but how do you have 2 voltage supplies of different polarities on the same pins?

 

[gramo]

They arnt on the same Pin,

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+V is connected to +10V

and -V is connected to -10V

 

[paul200]

Ok got it!

Will a 741 op do the job?

 

[gramo]

Sure will

Here is its datasheet

 

[paul200]

Many thanks Gramo

Paul

 

[ericgibbs]

hi gramo,
From memory I think that the 741 can't swing its output rail to rail.

So if Paul wants +/-10Vout, he may have to use a higher rail voltage say +/-15V ??

Eric

 

[gramo]

Yeah, heres a sim comparing +-10V supplies with +-15V

https://www.box.net/shared/uvlciupd7o

 

[AllVol]

If you need 10 volts, and you have 10 volts, why go through the 741?

 

[gramo]

Anyone reccomend a good rail to rail op-amp?

 

[ericgibbs]

hi Allvol,

I think Paul has a variable +/-3 Voltage he wants to amplify to a +/-10V swing.

Looking at gramo's sim, he may be able to use +/-12v rails.

gramo, what software did you use for your sim?

Googled for 'rail to rail opa' lots available.

Eric

 

[Nigel Goodwin]

I think the OP ought to explain EXACTLY what he's trying to do, the original question is very vague - usually he's not asking what people think he is!.

 

[gramo]

Well I had a guess at what he wanted, and must have been on the right track, as you can see by the replys in the thread.

It seems our last problem is finding a dual supply, rail to rail opamp that can handle +-10V output.

Any suggestions?

 

[Nigel Goodwin]

Well I had a guess at what he wanted, and must have been on the right track, as you can see by the replys in the thread.

It seems our last problem is finding a dual supply, rail to rail opamp that can handle +-10V output.

Any suggestions?

Why rail to rail? - why not just use +/-12V supplies?. In my PIC analogue tutorial hardware I deliberately avoided the requirement for a rail to rail opamp, I don't think ANY opamp really does a good job of rail to rail.

 

[audioguru]

A rail-to-rail opamp has an output voltage that is rail-to-rail only when its output doesn't supply any current. It is Cmos.

He said he wants to convert +/-3V to +/-10v, not amplify it.
Isn't this a power supply project? A power supply without any output current is useless.

He didn't say how much current is needed at +/-10V so the DC to DC converter might be easy or it might be a nightmare.
He also didn't say how much current is available at +/-3v to power the thing.

 

[Hero999]

He also didn't say whether it was really dual polarity; it might be a nube thing saying +/- for a single power supply.

 

[paul200]

I take it there is no common connection ie. 0 volts

 

[paul200]

More information

I have a gyro sensor that outputs 0 volts when stationary,as the gyro is turned it can output between 0 to +/- 3 depending on which way the gyro is turned negative voltage turned one way, positive voltage when turned the other way.

The output which tracks the input voltage swing needs to to follow the input but amplify is propotionally ie.

0 volts in 0 volts out.
3 volts in 10 volts out
and anywhere inbetween.
The input has a small current and the output only needs a small current output.

Hope this gives a more clear view