TL072CN OpAmp Question.

Discussion in 'General Electronics Chat' started by R!f@@, Dec 1, 2015.

  1. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
    Hey Guys.
    I have a little confusion about the TL072 opamp (DIP).
    You see I am trying to make a charger for the SLA for my access controller.
    I am not happy with the ones I see as they really does not do what it says. I have tried and it does not to switch to float. The Kit one I attached in the other SLA Battery thread.
    So I am trying to make my own that does what I want.
    The schema is attached. It is not complete but I ran the simulation and it works fine.
    700mA Charging at 14.7V ( can be adjusted ). The charging current is sensed by the PIC ADC and switches on the float 13.85V ( can be adjusted ) when the Ibatt drops to 70mA.
    Simulation works.
    My concern is the TL072. It is wired across the Current sense resistor. OPAMP is used to sense the current draw to switch to float by PIC. The data says it works at single 5V rail. I needed a max 5V output as it is sensed by the PIC. The diode at opamp out put is a 5.1V zener for protection. I tried with main input (19VDC) but opamp ouput goes above 5V when sensing the current. At 5V Supply it won't. I can get a good OPAMP output variation (Voltage) from 0mA to 720mA (charger Current sensing).

    The questions are.
    1. Will the TL072 work at 5V Supply or do I need another Opamp.
    2. The sensing voltage is differential but above IC's Vcc ( if at 5V Vcc ) but below 15VDC with in what the data says. So is this safe. I checked the data but since I have never used this OPAMP like this I like to confirm with some one more knowledge on differential amps and about this OPAMP running at minimum voltage.
    Last edited: Dec 1, 2015
  2. dl324

    Distinguished Member

    Mar 30, 2015
    Why aren't you using the charging supply to power the circuit?

    I built a 12V SLA charger and powered everything from the charging supply. Mine had an opamp, comparator, timer, flip flop, and voltage regulator but no PIC.
  3. wayneh


    Sep 9, 2010
    From the data sheet:
    "(4) The magnitude of the input voltage must never exceed the magnitude of the supply voltage or 15 V, whichever is less. "

    The op-amp works fine at 5V, but not if you need to accommodate inputs up to 15V.
    R!f@@ likes this.
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
    Think about sensing the current on the ground side with something like a LM358 or 324,. It is usually easier and more accurate.
  5. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
    Most of the circuits uses low level sensing and for my application I cannot use a ground side sensing as the charger PSU is also used to power the access controller when charging.. Ground is common and I am using high side to switch on and off. A common ground will affect the charger during charging. I thought about that and tht is why I need high side sensing.. Besides the circuit is simple and accurate and works for me.
    I guess I need to figure out a way to limit the the ADC voltage.

    Why 15V ? Doesn't the TL072 works up to 36V single sided?
  6. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
    OK..I switched to LM358. IC is powered by 19VDC and output stays below 5V. So far so good.