Hi
I need to derive a 0 to 5 volt signal proportional to battery voltage to control an auxiliary load on a 12 volt battery system. The auxiliary load device has a 0 to 5 volt speed controller input and I want to run the device at the highest speed that the battery will support. The battery voltage will vary from 12 to 14.5 volts DC depending on engine speed that powers the charging circuit.

I want to limit the speed so that this auxiliary load does not lower the battery voltage below 12 volts when engine speed is at idle. But when engine speed is high and excess electrical power is available then the auxiliary device can be run at maximum speed.

With a resistor voltage divider across the battery with say 10k and 5 k ohm resistor this would give 4.9 volts across the 5 K resistor when battery is at 14.5 volts and 4 volts when battery is 12 volts. So I need to remove the 4 volt offset of this voltage to get it to 0 to .9 volts and then amplify by a factor of 5 to 6 to get my desired 0 to 5 volt signal for speed control of the auxiliary load.

It has been 30 years since I have worked with op amps and could use some help setting this up.
thanks for any help
Rudy

 

Some calculators -


https://masteringelectronicsdesign.com/differential-amplifier-calculator-2/

http://www.ti.com/download/kbase/adc/DifGain.htm

https://www.daycounter.com/Calculators/Op-Amp/Op-Amp-Voltage-Calculator.phtml


Regards, Dana.