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Using LM2941CT with backup battery

Chris Inskip
Prodigy 20 points
Other Parts Discussed in Thread: LM2941, UC2909-EP, BQ24450, UC2906, LM5050-1

Greetings from retired Mech engineer!  I will be grateful for help:
Re PROPOSED CIRCUIT DIAGRAM 2016/03/17 (voltageRegulator02.jpg) below:
........................

This is a simple burglar alarm system.  The Alarm control unit should maintain 13.8V at the battery, but is defective, and allows voltage up to more than 15V to be sustained on the connection to the battery.

Therefore I contemplate the scheme in the attached diagram.

As you can see, it has addition as shown in red, ie a voltage regulator using an LM2941CT is inserted between the unit and the battery, set at 13.8V.

QUESTION:  Will 13.8V be maintained at the battery?

.........................

The purpose of the battery is, of course, to power the burglar alarm system when the 220 V domestic supply is interrupted.  This definitely happens at times here in South Africa.  At such times current must flow not from the control unit to the battery, but in the opposite direction.  Therefore I have included in the scheme a further addition, namely the diode (in red) which bypasses the LM2941CT.

QUESTION:
  Will this work satisfactorily?  If yes, what type of diode will be suitable?  Ideally, I imagine it should:

- have forward current capacity of 1A, with small voltage drop.  (the demand is <500mA when the 220 V supply is down, and the unit is triggered and operating the siren.);

- pass a very low reverse current when the voltage at the battery is 13.8V, and the voltage on the input side of the LM2941CT is above 15V or a bit more, so as not to counteract the voltage regulation by the LM2941CT.

.........................

QUESTION:
Do I need to provide also something to protect the LM2941 from damage when the 220 V supply is interrupted but there is still 12 V or more from the battery on the output side of the LM 2941?

  • 0
    TI__Mastermind 22610 points


    QUESTION: Will 13.8V be maintained at the battery?
    A: Perhaps, depends on the state of the battery. Any time the battery voltage is is more than a few millivolts below the voltage set by R1 and R2, the current out of the LM2941 charging the battery will be equal to the LM2941 short circuit current limit (typically around 1.9A) If the battery voltage is 12V, and the ACU voltage is 15V, and 1.9A is through the LM2941, then the dissipation will be ((15V-12V x 1.9A)= 5.7W. So that's going to call for a large heat-sink to keep the LM2941 from going into thermal shutdown.

    QUESTION: Will this (diode) work satisfactorily? If yes, what type of diode will be suitable?
    A: Probably. It depends on what's lurking behind the +/- terminals of the "... defective ..." ACU. For a diode, I'd suggest a Schottky diode, If>= 2A, Vr>= 40V.

    QUESTION: Do I need to provide also something to protect the LM2941 from damage when the 220 V supply is interrupted but there is still 12 V or more from the battery on the output side of the LM2941?
    A: Probably not. Ideally the voltage at the LM2941 IN pin will never lower than one diode drop (of the external return diode) below the voltage at the LM2941 OUT pin. If there is any possibility that the external return diode may fail, then yes the LM2941 should be protected from reverse current.

    A comment:
    While this LM2941 configuration will probably charge your battery, it does not have the 'smart' charge, diagnostic and protection features that ensure to everything is in proper operation. I would strongly suggest the use of a device designed to maintain lead-acid battery:

          UC2909-EP - Enhanced Product Switch-mode Lead-Acid Battery Charger with Differential Current Sense

          www.ti.com/.../UC2909-EP

          BQ24450 - Integrated Charge Controller for Lead-Acid Batteries

          www.ti.com/.../BQ24450

          UC2906 - Linear Lead-Acid Battery Charger

          www.ti.com/.../UC2906

    Additionally, I would probably replace the external return diode with a MOSFET driven by an OR-ing controller, or 'smart-diode':

          LM5050-1 - High Side OR-ing FET Controller

          www.ti.com/.../LM5050-1

  • 0 in reply to Donald Jones
    Prodigy 20 points

    Thanks, Donald Jones, for helpful response 2016/03/29.