Scalling Voltages upto 30V DC

[Suraj143]

Hi all
I can measure DC voltages from 0v to 5v using PICS analog input.

Now I need to measure the voltages above 5V which means 0V to 30V maximum.

I don’t know how to scale the OP AMP to my needs, if any method easy to understand it will be great.

I need the hardware part not the software.

Thanks

 

[KMoffett]

Google: resistive voltage divider

Ken

 

[Papabravo]

Easy to understand. OK you need an amplifier that attenuates a 0-30V signal into a 0 to 5V signal. This corresponds to a gain of 1/6 = 0.16666....
Are you with me so far? A non-inverting stage must have a gain greater than or equal to one so you will need to use an inverting stage. OOOPS you only have a single supply so there will be a problem.

RESET. Use a gain of 1 voltage follower and drive a voltage divider with a gain of 1/6 = 0.16666.... You must use an amplifier that can withstand 30V on the input. Start your search with an LM324. There may be other requirements, that you have not articulated, that will dictate an alternate choice.

The voltage follower will prevent the divider from loading the source and you should be able to conver the 0-5V output.

 

[Suraj143]

Easy to understand. OK you need an amplifier that attenuates a 0-30V signal into a 0 to 5V signal. This corresponds to a gain of 1/6 = 0.16666....

Totally understood.

Are you with me so far? A non-inverting stage must have a gain greater than or equal to one so you will need to use an inverting stage. OOOPS you only have a single supply so there will be a problem.

RESET. Use a gain of 1 voltage follower and drive a voltage divider with a gain of 1/6 = 0.16666.... You must use an amplifier that can withstand 30V on the input. Start your search with an LM324. There may be other requirements, that you have not articulated, that will dictate an alternate choice.

The voltage follower will prevent the divider from loading the source and you should be able to conver the 0-5V output.

Thanks for the info.But I have to study this.

Those items very new to me.I don't know what they are.

*A non-inverting stage
*voltage follower
*drive a voltage divider

For the time being I'll use resister voltage divider.Thanks for the support.

 

[ericgibbs]

For the time being I'll use resister voltage divider.Thanks for the support

Hi Suraji,
When you calculate your resistive divider, dont forget to keep the divider impedance seen by the PIC's ADC <= 10K0.

 

[Suraj143]

Hi Suraji,
When you calculate your resistive divider, dont forget to keep the divider impedance seen by the PIC's ADC <= 10K0.

That is very usefull.Thanks for that.

 

[Hero999]

Even better, include the PIC's input impedance in the voltage divider calculation. If the PIC has an exact inpedance then you won't need another resistor, just one series resistor will do.

 

[Nigel Goodwin]

Even better, include the PIC's input impedance in the voltage divider calculation. If the PIC has an exact inpedance then you won't need another resistor, just one series resistor will do.

That's no good, it's not the impedance that's the problem, it's the charging current for the sample and hold capacitor - source impedance needs to be no more than 2K or so, input 'impedance' is massively greater than that.

Essentially you really need a buffer, except for very rare occasions - which is why my tutorials use an opamp, with the attenuator at it's input (not feeding the PIC).