Monday, April 23, 2012

12V 10A High current Power Supply with battery backup

The full circuit diagram of the 12V 10A Power Supply is shown in Fig.1.Power of about 18V to 25V is applied to screw terminals pins 1 (+V) and 2 (–V).Although the power supply was originally designed to run packet radio transceivers, the unit is not just confined to this type of radio. In fact, any radio can be used with this power supply. It can also be used as a main source of power; ie the battery, or used as a standby source of power in the event of a power failure. There would be a float charge for the battery when mains voltage is applied, and the battery can be relied on to supply current to equipment when the mains supply fails.

The input current to the circuit is limited by a 5A fuse (FS1) for protection purposes. Relay RLA is a small ‘homemade’ reed type that is set to close the contacts when the current drawn through this relay coil is in the order of about 1.5A. When this occurs, RLA’s contacts close, current is drawn through resistor R1, LED1, R2 and onto R3. The voltage across resistor R3 is sufficient to turn on FET TR2, which supplies power to a 12V cooling fan.
12V 10A High current Power Supply with battery backup schematic

Capacitor C1 in the circuit forms a timing function, where a delay of about one second is experienced at switch on, and about five seconds at switch off.

Zener diode D2 limits the gate/source voltage of TR2 to 2.7V. Field effect transistor TR2 is used to switch a cooling fan on in order to cool the circuit down during high and prolonged current demand. A 1.3W 6.2V Zener diode (D1) is selected for an 18V input, or 12V for a 25V input. The Zener diode can be removed and a shorting link replace it if a 24V fan is used. An old redundant computer fan was used in the prototype to assist in the units cooling. When the reed relay contacts of RLA1 are closed, LED1 is illuminated to indicate that a current higher than 1.5A is being drawn from the mains derived power supply, and cooling is being effected.

Voltage regulation
Transistors TR1, TR3, TR5 and IC1 form a voltage regulator that is commonly seen in power supplies. TR1 is a Darlington NPN transistor with a high current gain that acts with TR3. These two transistors act with resistor R4 and the parallel resistors R5 and R6 to form a current limit device.

The preset potentiometer VR1 is used to set the current limit value, when there is 0.7V across the base/emitter junction of TR3. In the prototype the current limit was set to 3A.
During a current limit condition (usually a fault condition in a conventional series-regulated power supply), the circuit can become somewhat unstable. To increase the stability of the circuit, a 100µF electrolytic capacitor (C3) is included to smooth the supply to the base (B) of transistor TR1. In addition a 100nF capacitor (C4) is added in parallel with C3.

Transistor TR5 and IC1 act together as a high current voltage regulator. Including R7 and TR5 increases the current handling of IC1, which, on its own  is limited to 1.0A. Choosing a value of 1ohm for R6 ensures that TR3 bypasses current around IC1 which is set at approximately 450mA, less than half the maximum current handling of this device, IC1.

Charging circuit
Diode D6 provides isolation between the battery and the charging circuit. Should the mains supply fail for any reason, this diode inhibits the battery from discharging through the charging circuit. Transistor TR4 supplies current to the relay coil RLB, which is then energised and hence the relay contacts are closed. The battery can now be charged and utilised in the power supply.

In the base circuit of TR4 is a 10V Zener diode (D5), this simplearrangement ensures the transistor is turned on when mains power is applied and the units output voltage is above 10.7V. Hence, relay RLB is energised when the mains supply is connected or the battery is showing a healthy state of charge. This occurs when the output from the regulator
(IC1) is above 10.7V. This also ensures the voltage at the battery is sufficient to power; in the event of a mains failure. Once the battery voltage drops below about 10.7V, relay RLB1 contacts will open; disconnecting the battery.

Charging of gel-cell batteries
The technique for charging gel-cell batteries uses a constant-voltage, current-limited system. The constant voltage is generated and set to a specific value, and for a 12V battery it’s 13.2V (2.275V per cell) – (Yuasa gel-cell, NP12-12 series – 12V, 12AH).

The maximum charging current should be set to one quarter of the batteries AH (amp/hour) capacity. For a 12AH battery, this would equate to 3A. The maximum charging current could be set lower, however the battery will take a little longer to reach its full capacity after a deep discharge.

Should a deeply discharged battery be attached to this power supply, the battery could take a very large current at switch on, and the output of the power supply could drop severely. The charging current will limit at the value determined by preset VR1, and the battery should slowly rise to a value around 12V within minutes.

Reed relay
Reed relay RLA’s coil is fabricated with approx. 18½ turns of 1.5mm enamelled copper wire wound on a 3.5mm drill; the drill is used as a mandrel, or coil former. The coil leads are formed to fit the PCB – see Fig.2. The coil is an air wound type, 18½ turns closely wound with an internal diameter of 3.5mm. The glass-encapsulated ‘reed’ insert (contacts) is set centrally within the coil, and should have an actuating current of approximately 1.5A. In the prototype, a rubber sleeve was positioned around the reed to help space it and keep it centrally mounted within the assembly.

reed coil construction

Against convention
Details of the interwiring between the circuit board and off-board components is shown in Fig.4. Note resistor R13 and LED are located off the PCB and provide a power on indication.
Going against convention, where power supplied to a circuit is generally supplied via a socket. In this project 3-pin XLR connectors have been used. The output to the battery has a chassismounted plug, and the battery a cable socket.

complete wire scheme
This is deliberate, as it was considered that should anything accidently short across a socket, it would be preferable if it shorted across the lowest current source, ie, the power supply. The higher source of power, a gel-cell battery could deliver well over 100A if the terminals are shorted across, and explode as a consequence. The power supply current,
even under a short circuit condition, is set by VR1 to 3A.

The connecting wire and connectors on the input supply, and from the output of the circuit, must be adequately rated for the current drawn. The prototype used 5A wire on the input and 25A connecting wire on the output side.

The choice of the output relay RLB really limits the current-handling capacity of the power supply, since its contacts will be passing the full load current. The footprint of the relay on
the PCB provides a choice of contacts available from DPDT to SPDT, with contact ratings up to 15A in SPDT arrangement, which would be suitable.

In this project, a DPDT relay was used and was found to be adequate in passing the 10A which the unit was designed to supply. It must be stressed that the PCB copper tracks carrying this level of current must be built up with solder to reduce the track resistance, and hence the track current carrying capacity.

Setting up
For the initial setting up, include the 47ohm ‘test’ resistor. If not, a current limited power supply supplying 18V to 25V could always be used, with a current limit initially set to 100mA.

Ensure the preset pots VR1 and VR2 are both set fully anti-clockwise. This will ensure that the current limit is set to the minimum value, and the output voltage is set to maximum. Setting the output voltage to maximum will ensure that contacts RLB1 are closed.

No equipment or battery should at this point be connected to the circuit. Wait until it has been fully tested and the output voltage set to the nominal output voltage of 13.2V to 13.6V.
As a temporary measure, connect a pushbutton switch between pins 3 and 4 on the circuit board, and hold the pushbutton down as you adjust the output voltage until the relay contacts of RLB close. The output voltage of the circuit is adjusted with the aid of VR2 and is set to 13.2V. The output voltage is measured across terminals 3 and 6 on the circuit board.

Connect up the circuit supplying current to the power supply unit, slowly increasing the input voltage set to 18V (using a bench power supply and a 47ohm series test resistor), and constantly monitor the current at this point. The current drawn should be in the order of 123mA with the relay RLB energised. At this point LED2 should be illuminated and LED1 extinguished. The output voltage is monitored with a DVM and set to 13.2V for a 6-cell gelcell battery (float charging the battery at 2.275V per cell) as VR2 is adjusted).

The charging current is set by the manufacturer, and can, as a general rule of thumb, be set to a maximum current of one quarter of the maximum amp/hour (AH) capacity of the battery. For example, a 12AH battery can be charged at a maximum current of 3A. This is the current limit value set by the unit, using trimpot VR1. Setting preset VR1 to the 11 o’clock position will set the current limit (maximum charging current) to 3A. Y o’clock – 2.5 amps, and w o’clock – 2.0 amps.

Limiting current
To set the limiting current accurately, a discharged battery could be connected in circuit, in which case the 47ohm test resistor (if used) would have to be removed from the input side of the power supply. Monitor the current drawn by the circuit using an ammeter and set the current by adjusting VR1 to match the battery used. Important: Do not set the charging current too high, as this will cause premature failure of the battery. If a discharged battery is attached to the power supply, the voltage at the battery may be low and hence the charging circuit voltage. The battery voltage will depend upon the state of charge, low enough to want to draw the power supplies limiting current. In this case, an ammeter on the input side can be used to read this current.

Adjusting VR1 will adjust the current limit of the circuit and hence the maximum charging current for the battery. As mentioned earlier, the maximum value for this current will actually be dependant on the capacity of the battery. A larger capacity battery can, of course, be used, but from a deep discharge will take longer to reach a full charge.

In use
The power supply has been very successful in use with the homebase station radios, and no appreciable problems with 12AH and 24AH batteries. It has to be remembered that the power supply has to be left on after use to top up the battery; leaving it on overnight should not be a problem. 

Source: EPE


  1. Hello,

    Nice updating about the 12V 10A High current power supply. It gives long time of battery backup. Thanks for sharing...

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