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Alarm power supply with battery back-up This power supply is designed to go with the Multi-Sector Burglar Alarm described in the June 1990 issue of SILICON CHIP. It's built on a single ·pc board and features a regulated 12V output and battery back-up. By GREG SWAIN Although the burglar alarm described in the June issue of SILICON CHIP can be powered from any regulated 12V supply, an effective alarm system must have battery back-up. Any alarm that can be disabled simply by pulling a fuse at the fusebox is virtually useless. This easy-to-build power supply is the answer to that problem. Nor- mally, it derives power from the mains and delivers a regulated 12V rail to the alarm circuit. However, if the mains supply fails for any reason, it trips a relay and this switches in a back-up battery to power the alarm. Virtually any rechargeable 12V battery can be used to provide this back-up, provided it has sufficient capacity to power the siren if the alarm is tripped. This battery is trickle charged when the mains supply is on to ensure that it is maintained at full capacity. The circuit has also been designed so that the switch-over does not trigger or reset the alarm. This is necessary to avoid false alarms during legitimate power failures and to ensure compliance with noise pollution regulations. Note that although the circuit has been specifically designed for the alarm in the June 1990 issue, it can also be used with most other 12V alarm systems. By combining this power supply with the Multi-Sector Alarm and the Alarm Keypad (SILICON CHIP, July 1990), you can build a fully-featured system for far D7 1N4004 18VAC ►-+--~--..----~--...---t--t-""'11N L~~1o P"ou..,_r___._...,__ _ _ _ _.,__.,.__--41_--, ADJ + 1.Sk 4700 35VW - 4700 _ 3SVW - 10 2SVW I' 10k '\.. 10 2SVW + 4700 - 2SVW 12V TO ALARM CIRCUITRY + 1.Sk _ 0.SW - - - - - - - - - - - - - - - - - -""\ 06 ' 1N4004 B . ,~. ' EOC VIEWED FROM BELOW + 0.1 OUT 12V ALARM POWER SUPPLY Fig.1: 3-terminal regulator IC1 provides a + 12V output to power the alarm, while regulator IC2 provides a + 13.8V output to charge the battery. If the mairis supply fails, Qt & Q2 switch off and the relay contacts close to connect the battery to the alarm circuit. 48 SILICON CHIP 1.Sk o.sw T I 112V 1 BATTERY ...I.. PARTS LIST 1 PC board, code ZA-1454 (copyright Dick Smith Electronics) 3 2-way PC-mounting terminal blocks 1 finned heatsink 3 9mm x 48A bolts 3 48A nuts and washers 1 1 2V miniature SPOT relay (DSE Cat. P-8008) 2 2000 miniature trimpots (horizontal mount) Semiconductors 1 LM350T 3-terminal regulator (IC1) 1 LM31 7 3-terminal regulator (IC2) 1 1 4 4 1 All the parts are mounted on a single PC board, with external connections made via three terminal blocks. Note the heatsinking arrangement used for ICl (LM350). IC2 is bolted directly to the board. less than the cost of equivalent commercial units. Circuit details Ref er now to Fig.1 which shows the circuit schematic. As can be seen, the design is based on two 3-terminal regulators (ICl & ICZ) plus a couple of transistors and a relay. ICl pro~ides the regulated + 1 ZV rail to power the alarm circuit while ICZ functions as the battery charger. The two transistors (Ql & QZ) provide the changeover function when the mains power fails. In greater detail, power for the circuit is derived from a mains transformer with an 18V secondary and this drives bridge rectifier Dl-D4. This circuit, combined with the two 4700µF filter capacitors at its output, gives an unregulated DC rail of about 25V and this is fed to the inputs of the two regulators. We'll briefly explain how these regulators work before going on with the rest of the circuit description. Fig.2 shows an adjustable positive regulator circuit based on the LM350. Capacitor Cl filters the DC input to the regulator while VRl, Rl and RZ set the output voltage. In operation, the regulator produces a fixed 1.25V between its output (OUT) and adjust (ADJ) terminals. This means that VRl sets the current through Rl and RZ and this in turn sets the voltage on the ADJ terminal. By suitably adjusting VRl, we can thus set the output voltage to the required value (it will always be 1.25V higher than the voltage on the ADJ terminal). Since the current flowing from the ADJ terminal is negligible, we can derive a simple formula for the output voltage as follows: VouT = 1.25 (1 + RZ/(VRl + Rl)) In Fig.1, Rl is 4700, RZ is 4.7k0 and VRi' is a 2000 trimpot. Thus, Fig.2: basic scheme for an adjustable 3-terminal regulator circuit. The regulator maintains a constant 1.25V between its OUT and ADJ terminals. BC548 NPN transistor (01) BC558 PNP transistor (02) 1 N5404 3A diodes (D1 -D4) 1 N4004 diodes (D5-D8) 1 2V 400mW zener diode (ZD1) Capacitors 3 4700µF 35VW axial electrolytic 2 1 OµF 25VW tantalum 1 0. 1µF ceramic Resistors (0.25W, 5%) 1 2 1 2 1 OkO 4.7k0 1.5k0 1.5k0, 0.5W 2 1 kO 1 4700 2 3900 VRl can be varied to give a theoretical output voltage range of 10.0ZV to 13.75V. In practice, VRl is adjusted to give an output voltage of exactly 12V. This output voltage is filtered by a 4700µF capacitor and then fed to the output terminals. The parallel lOµF capacitor is included to protect ICl against high frequency oscillation while the lOµF capacitor at its ADJ terminal greatly improves the ripple rejection. The 1.5k0 resistor across the output terminals sets the minimum load current to ensure correct operation of the regulator. ICZ (LM317) works in exactly the same way as ICl but has a lower current rating (1.5A vs. 3A). Its output is set to 13.8V by VRZ and this is used to trickle charge the 1 ZV backup battery. As before, a 1.5k0 resistor is used to set the minimum SEPTEMBER 1990 49 capacitor at the output of IC1 prevents glitches on the supply output during changeover, to prevent false triggering. Construction Fig.3: here's how to install the parts on the PC board. Take care with component orientation and be sure to use the correct transistor types for Qt & Q2. The two 1.5k!l resistors near the relay must be rated at 0.5W. load current while the · 0.1µ,F capacitor ensures regulator stability. Note that no bypass capacitor has been used on the ADJ terminal here since output ripple from this regulator is not an important consideration. Diodes D6, D7 and DB protect the regulators against discharge currents from the external electrolytic . capacitors and the battery if the mains power fails. In addition, the regulators also include built-in short circuit and thermal overload protection. If their ratings are exceeded, the devices shut themselves down by current limiting. Changeover circuit Now take a look at transistors Ql and QZ. As mentioned previously, these control the changeover function. Basically, Ql functions as a simple comparator. It compares the voltage at the output of the bridge rectifier with a reference voltage on its emitter as set by zener diode ZD1 (12V 400mW). Normally, when mains power is present, Ql is turned on and so Q2 and the relay are also on. The relay contacts are therefore held open and the battery is disconnected from the output. However, if the input to the regulators falls below about 15V due to mains failure, Ql, Q2 and RLY1 all switch off and the relay contacts close. This connects the battery to the output and so it now supplies power to the alarm circuit. The two 4700µ,F filter capacitors at the output of the bridge rectifier ensure that the regulators remain in regulation during the changeover time. In addition, the 4700µ,F All the parts (except the power transformer) are mounted on a PC board coded ZA-1454. Fig.3 shows the assembly details. Before installing any parts, check the copper pattern of the board for possible defects. In particular, check for open circuit tracks and shorts between tracks due to incorrect etching. Once this has been done, install the two wire links and all the resistors. Refer to Table 1 for the resistor colour codes when you are installing the resistors. Alternatively, you can use your digital multimeter to check the resistor values. The diodes, trimpots, transistors and the smaller capacitors can now be installed on the board. Before soldering their leads, check that all polarised components (diodes, transistors and tantalum capacitors) are correctly oriented. Also, be sure to use the correct transistor type at each location and note that diodes D1-D4 must be 1N5404 types which are rated at 3 amps. External connections to the board are made via three 2-way PCmounting blocks and these can be installed at this stage. You can also now install the relay but leave the big 4700µ,F filter capacitors off the board for the time being. The next step is to mount the heatsink. This is supplied predrilled with the kit (see panel) and is secured to the board using two 3mm bolts and nuts, one of which also secures the LM350T regulator (IC1). Bend the regulator's leads at right angles and smear its metal tab with heatsink compound before TABLE 1: RESISTOR COLOUR CODES □ □ □ □ □ □ □ 50 No. 1 2 3 2 1 2 SILICON CHIP Value 10k0 4 .7k0 1.5k0 1k0 4700 3900 4-Band Code (5%) brown black orange gold yellow violet red gold brown green red gold brown black red gold yellow violet brown gold orange white brown gold 5-Band Code (1%) brown black black red brown yellow violet black brown brown brown green black brown gold brown black black brown brown yellow violet black black brown orange white black black brown Smear heatsink compound on the metal tab of the LM350T regulator before bolting it directly to the heatsink. Check that the edge of the heatsink doesn't short against any nearby component leads. bolting it down. Don't forget to solder the regulator 's leads on the copper side of the PCB. The leads of the LM317 regulator must also be bent at right angles. No heatsinking is required for this device - it is mounted directly on the PCB (metal tab down) and secured with a 3mm bolt and nut. Assembly of the PCB can now be completed by installing the three 4700µF capacitors and the relay. Watch the polarity of the capacitors - you'll strike problems if you install them the wrong way around. Testing & adjustment To test the unit, you will need a mains transformer with an 18V AC secondary. The recommended transformer is the type M-1990 which is rated at 2.2A and is available from Dick Smith Electronics. Alternatively, you can use a DC supply capable of putting out about 20V DC (anything from 18-25V will be OK). If you are using a DC supply to test the unit, it should be connected to the inputs of the bridge rectifier (ie, directly to the terminal block on the PCB). You don't have to worry about the polarity of the DC supply - the bridge rectifier will take care of that. Don't connect the alarm circuit or the back-up battery at this stage. Now apply power and check that the relay operates. If it does, use your multimeter to check the voltage at the 12V OUT terminals. Adjust trimpot VRl for a reading of exactly 12V. Similarly, check the voltage at the BATTERY terminals and adjust VR2 for a reading of 13.BV. Once the outputs have been adjusted, switch off the power and check that the relay contacts close. If the relay fails to operate when power is applied, check Ql, Q2 , ZD1 and their associated resistors. Check also that D5 is correctly oriented. If you are unable to adjust either output to the correct value, check the resistor values and diodes around the relevant regulator. The completed project can be installed in a lockable steel case, along with the alarm PC board, the power transformer and (if it fits) the back-up battery. Make sure that you install the power transformer and its associated mains wiring in a professional manner. The 12V back-up battery should have a minimum rating of 1.2AH. 1§;1 Where to buy the kit This project was developed by Dick Smith Electronics and is available from all DSE stores or by mail order from PO Box 321, North Ryde, NSW 2113. You can also order by phone on (02) 888 2105 or, from outside the Sydney area, on (008) 22 6610 . The kit consists of a PC board plus all the on-board components (including the heatsink) but does not include the transformer or a backup battery . The price is as follows: The power supply is ideal for use with the Multi-Sector Burglar Alarm described in June 1990 SILICON CHIP. This unit features variable exit & entry delays, timed & latched outputs, and two separate sector inputs. The number of sector inputs can be easily increased by means of an add-on board. 12V Alarm Power Supply (Cat. K-8402) .............. .. ..... .... .. ... $39.95 Power Transformer (Cat. M-1 990) .. ................... .... .. .. .... .. ... $22. 95 Postal orders should include another $4 .50 for postage or $6 .50 if the power transformer is included in the order. Please quote the catalog numbers when ordering . Note: copyright of the PCB artwork associated with this project is retained by Dick Smith Electronics. SEPTEMBER 1990 51