5-Volt/3 Amp Power (277-0119) Operation Faxback Doc. # 15758 Supply AC to DC Converter The AC to DC converter consists of the transformer, bridge rectifier, filter capacitors and bleeder resistors. The primary of transformer T1 is energized when the input line is connected to a 120 VAC source through switch S1 and Fuse F1. The secondary of T1 is used in conjunction with the Bridge Rectifier BR1 to produce two power supply voltages V+ and 2V+. In the circuit, the bridge effectively acts as two full-wave rectifier circuits tied together, producing a plus and minus voltage of equal potentials. We reference the voltages to the - terminal of the equivalent circuit, giving V+ and 2V+ voltage outputs. The V+ voltage supplies most of the current and connects directly to the series pass transistor. The 2V+ voltage is needed to supply a higher voltage to operate the IC regulator chip. Resistors R17, R20 and R21 are bleeder resistors to discharge the filter capacitors after the input power is removed. Series Pass Regulator The Series Pass Regulator is a Darlington Power Transistor. A Darlington Transistor consists of two transistors connected together so that the emitter of one transistor drives the base of the other transistor. Such a combination gives a very high current gain equal to the gain of the first transistor multiplied by the gain of the second transistor. The series regulator acts as an adjustable power resistor which changes value in response to an error signal from the Precision Voltage Regulator IC. The series regulator will change its resistance so that the output voltage will remain constant under varying input and load conditions as well as eliminating the ripple that is present at the input. Precision IC Regulator The heart of the 5V Power Supply is the RS723 IC Precision Voltage Regulator. This regulator provides a temperature compensated voltage reference, an error amplifier (operational amplifier), an output (driver) transistor and another transistor which can be used for current limiting - all in one package. The regulator works by comparing the output voltage sampled by the sense lines to a 5-volt reference which is obtained by dividing down the IC reference voltage with a voltage divider network. Resistors R5 and R6 are used so that the source impedances as seen by the error amplifier are equal, which provides minimum temperature variation. Any change in the output voltage causes a change at the input of the error amplifier which is amplified and applied to the regulator's output driver transistor. The output drive transistor then drives the series pass regulator changing its resistance in such a way that the output stays at 5 volts which means that the inputs to the error amplifier again become equal. This whole process happens so quickly that the output remains constant. Foldback Current Limiting The short circuit current limit resistors (R sc) R11 and R12, are in the output current line and transistor Q1 continuously monitors the voltage drop across R sc. If a short circuit or increased loading occurs, the current through R sc increases which in turn increases the voltage drop across R sc. Whenever the voltage drop across R sc reaches 0.65 volts, Q1 conducts, developing a voltage across R7 which turns on the current limiting transistor inside the Precision Regulator IC. The current limiting transistor then in turn removes the drive from the series pass regulator. The current output will remain constant (constant current limiting) and the output voltage will drop until the output drops two diode voltage drops (equivalent to about 1.3 volts below the 5-volt reference). As the output voltage drops below this point, a voltage divider composed of R9 and R10 comes into action as CR1 starts to conduct and the foldback current limiting starts coming into play. As the output voltage drops below the turn on point of CR1, a voltage develops across R10, which adds to the voltage drop across R sc to make Q1 conduct heavier, which will in turn make the series regulator conduct less and the voltage drop across R sc will decrease. The net effect is that the voltage drop due to R10 and R sc will still equal the 0.65 volts necessary to turn on Q1 and as the drop across R10 increases, the drop across R sc will decrease. R10 is adjusted so that with a short circuit applied to the output, a small amount of current is drawn through the load. If R10 is adjusted for zero current the circuit will latch up and it cannot be reset unless the load is removed and the power is turned off until the circuitry discharges and then is turned on again. If R10 is adjusted correctly and the short or load is removed or decreased, the output voltage will rise since less voltage will be developed across R sc and this will tend to turn Q1 off and allow more base drive for the series pass regulator. If the load is further reduced, the power supply will go back into the normal constant voltage output. The point at which the power supply starts turning into a constant current source is called l max and the current drawn during a short circuit is 1 sc. Figure 18 illustrates the foldback current limiting. The maximum output current and the short circuit current both decrease as the power supply heats up, since the voltage necessary to turn on Q1 is temperature dependent and decreases with increasing temperature. Diodes CR4 and CR5 protect the power supply if the sense lines are inadvertently opened. If the sense lines are opened, the output will go to approximately 1.2 volts higher than the adjusted 5-volt value. Without the diodes, if the sense lines would open, the power supply output voltage would be uncontrolled. Operation of Remote Sense You may use remote sensing to eliminate any error caused by the voltage drop through the power leads to the load. The remote sense lines will compensate for 0.5 volt drop in the power leads for a load up to 2.5 Amps and 0.2 Volt drop in the power leads for a load up to 3.5 Amps across each output power line. What happens is that the remote sense lines keep the voltage at the load constant regardless of the load current by compensating for any drop in the pickup line. CAUTION: For proper operation, the sense leads must be connected to the appropriate power leads at the load or jumpers must be placed between the appropriate sense and output terminals. The sensing wires should be 22-gauge or larger to prevent any error due to the sense leads. NOTE: If you do not plan to use remote sensing, connect the sense terminals directly to their respective output terminals. The size of wire needed for the power leads can be easily calculated if the length desired, resistance drop of the wire and the maximum current needed are known. The following data and examples on the next page will determine the correct wire size. MAX. CURRENT MAX CURRENT 2.5 AMPS 3.5 AMPS WIRE GAUGE OHMS/FT. MAX. LENGTH MAX. LENGTH 22 0.0162 12.5 ft. 3.5 ft. 20 0.0101 20.0 ft. 5.6 ft. 18 0.00639 31.5 ft. 8.9 ft. 16 0.00402 50.0 ft. 14.2 ft. EXAMPLE If you want to use the Power Supply at 3.5 Amps with 18 gauge wire, how long can your power leads be? 18 gauge wire = 0.00639 ohm/ft. Maximum drop per lead = 0.2 Volt for 3.5 Amps 0.2 Volts _____________________=8.94 ft. 3.5 Amps X 0.00639 ohm/ft. So, each power lead could be up to 8.9 ft. long CHECKOUT AND OPERATION You'll need the following items to checkout your Power Supply: Test Equipment Range Accuracy Multimeter 0-10 VDC 3% or better 0-5A DC Scale 5% or better Four 10-ohm, 10-watt resistors or a known load of 2 to 4 ohm resistance, 20-watts or greater. CHECKOUT Perform the following steps to check out your 5-VOLT DC REGULATED POWER SUPPLY. 1. Adjust the FOLDBACK adjust pot (R 10) full clockwise. 2. Jumper the +SENSE to +OUT and -SENSE to -OUT. Be sure to add these jumpers. Do not operate with sense lines unconnected. 3. Connect the multimeter, set up to read 5 VDC, to the +OUT and -OUT terminals. 4. Set POWER switch to OFF and connect the Power Supply to 120 VAC. 5. Set POWER switch to ON and adjust the VOLTS pot (R3) for a 5 (+/- 0.1) VDC output. 6. Set POWER switch to OFF and connect either the four 10-ohm, 10-watt resistors in a parallel to form 2.5-ohm, 40-watt load or a suitable high power load to the output. Use the remote sense leads to monitor the loads. 7. Set the POWER switch to ON and you should read 5.0 volts across the load. If you read zero or very low, refer to the troubleshooting section. Adjust METER pot (R18) for 2 AMPS (5 volts/by 2.5 ohms = 2 Amps). NOTE: If you have a suitable DC AMPS scale on your multimeter (2 to 10 Amps), you can connect the meter in series with the load and set the METER pot R18 such that the Power Supply ammeter reads the same as the DC AMPS reading on the multimeter. 8. Adjust the FOLDBACK adjust pot (R10) fully counterclockwise, then short the +OUT and -OUT terminals. The voltage and current should both go to zero. If you remove the short, the output should stay at zero. This condition is called foldback latchup and is normal since we have not done the final adjustment of the foldback yet. Foldback Adjust 1. With the +OUT and -OUT terminals shorted, adjust FOLDBACK pot (R 10) until the ammeter indicates 0.5 Amps. Now remove the short and the output should go to 5 volts and 2 Amps. CAUTION: Do not adjust the foldback current to more than 1.5 Amps during a short. 2. If, during high current operation, applying a short circuit across the OUT terminals causes a latchup, readjust FOLDBACK pot (R10) to give at least 1 small deflection from zero (100 mA) while the Power Supply is still warm. Operation The 5-VOLT 3-AMP POWER SUPPLY will power any equipment that utilizes TTL logic. Connect the +OUT and -OUT leads to the V cc and GND inputs, respectively, of the equipment being powered. If you use remote sense, also connect the +SENSE and -SENSE leads to the Vcc and GND inputs of the equipment being powered. See "Remote Sense Terminals" for connection of sense terminals. Connect an external voltmeter to the input terminals of the equipment being powered or to the + and -SENSE terminals of the Power Supply. Connect the Power Supply line cord to a 120-VAC source and set the POWER switch to POWER. Observe the voltmeter for a measurement of 5.0 volts. If necessary, you may adjust potentiometer R3 to provide the desired voltage level. CAUTION: Do not try to drive the load from the sense leads only. (br/all-06/28/95)