UPS Power supply for alarm or emergency


    I rescued this unit from the trash, mainly because of the high quality toroidal transformer.  It looked quite new and clean with a nice metal cabinet painted white.  It is a simple, regulated power supply intended to keep a 12 V lead-acid battery floating so that it can provide power when the mains power supply goes out. 
    I reverse engineered the schematic and tested the unit.  It had a couple of bad contacts which I easily repaired and the lead-acid battery was missing but other than that it was fine.  I am not sure what the original purpose might have been because it appears that the cabinet held a battery but not any alarm or lighting system.  It seems it just supplied 12 V to some other outside system.  It could be used to power an alarm system or emergency lighting. 
   
power supply
power supply
power supply
power supply schematic

    Here is a basic explanation of how it works.  Pin 6 of the LM723 provides a 7 V reference voltage.  This is divided by R5 & R4 and the lower reference voltage is input to the comparator on pin 5 of the LM723.  The output voltage of the system is sampled and divided by P1 & R6 and the result is input to the inverting input of the comparator on pin 4.  This works so that the circuit tends to maintain the set output voltage independently of load.  The output voltage is set by adjusting P1. 
    R7 & R8 in parallel form a 0.25 ohm resistor which measures the outgoing current.  When the current is sufficiently high then the current limiter transistor of the LM723 will conduct and this will limit the current going out. 
    After studying the circuit I would suggest some improvements and simplifications.  The first thing I would do is use the 7 V reference voltage from pin 6 directly into pin 5 and adjust the resistors feeding pin 4 accordingly.  I am not sure why a lower reference was used in this case but I imagine it could be a generic circuit which was adapted to this particular use. 
    In the original circuit P2 is 450 Ohm adjusted to 18 Ohm which is really not good.  To account for component variation we need a range between about 11 and 28 Ohm so the ideal would be a 10 Ohm fixed resistor in series with a 20 Ohm pot but if we can only find a 50 Ohm pot then I would put it in parallel with a 68 Ohm fixed resistor.  That would be for production in series because for a single home-built unit I would probably just connect pin 2 directly to the emitter of Q1 and adjust the value of R7/R8 by trimming it.  To build my own low value resistors I use nichrome wire and this way it is easy to trim the value so that the current limit is exactly what we want.  Nichrome wire can be found and recovered for free from old heaters.
    My alternative for the adjusting pots for voltage and current limit gives much more control and ability to fine tune the circuit.
    In the original circuit the battery is charged through R10 (10 Ohm) and is discharged through D4.  I do not know the original purpose of this circuit but in general this design seems to me like not very good because the battery would take forever to get to a full charge.  Of course, this may have been the original intention but if this is an emergency power supply then I would want it to recover the charge as fast as possible after the mains power returns.  For this I would make R10= 0 (short) or a very low value.  D4 also causes loss of voltage and power when the battery is discharging so shorting R10 has the added benefit of eliminating this loss. 
    One thing I do not like is mixing on the PCB the 230 Vac from the mains with the rest of the low voltage circuit.  I would try to keep the mains voltage totally off the PCB or at least keep it clearly separate and marked. 
   
power supply schematic