10 Amp 13.8 Volt Power Supply Circuits
This circuit uses the LM723 IC which gives us excellent voltage regulation. The circuit uses 3 pass transistors which must be heat sinked. Resistor R9 allows the fine tuning of the voltage to exactly 13.8 volts and the resistor network formed by resistors R4 through R7 controls the current limiting. The LM723 limits the current when the voltage drop across R5 approaches .7 volts. To reduce costs, most commercial units rely on the HFE of the pass transistors to determine the current limiting.
The circuit in Figure 2 senses when the voltage exceeds 15 volts and causes the zener diode to conduct. When the zener diode conducts, the gate of the SCR is turned on and causes the SCR to short which blows the 15 amp fuse and shuts off the output voltage. A 2N6399 was used for the SCR in the prototype but any suitable SCR can be used. While over voltage protection is a good idea, it should not be considered a substitute for large heat sinks. I personally feel the best protection from over voltage is the use of large heat sinks and a reliable current limiting circuit. Be sure to use large heat sinks along with heat sink grease for the 2N3055 transistors.
Component/Parts List
R1 : 1.5K ¼ Watt Resistor (optional, tie pins 6 & 5 of IC1 together if not used.)
R2,R3 : 0.1 Ohm 10 Watt Resistor (Tech America 900-1002)
R4 : 270 Ohm ¼ Watt Resistor
R5 : 680 Ohm ¼ Watt Resistor
R6,R7 : 0.15 Ohm 10 Watt Resistor (Tech America 900-1006)
R8 : 2.7K ¼ Watt Resistor
R9 : 1K Trimmer Potentiometer (RS271-280)
R10 : 3.3K ¼ Watt Resistor
C1,C2,C3,C4 : 4700 Microfarad Electrolytic Capacitor 35 Volt (observe polarity)
C5 : 100 Picofarad Ceramic Disk Capacitor
C6 : 1000 Microfarad Electrolytic Capacitor 25 Volt (observe polarity)
IC1 : LM723 (RS276-1740) Voltage Regulator IC. Socket is recommended.
Q1 : TIP3055T (RS276-2020) NPN Transistor (TO-220 Heat Sink Required)
Q2,Q3 : 2N3055 (RS276-2041) NPN Transistor (Large TO-3 Heat Sink Required)
S1 : Any SPST Toggle Switch
F1 : 3 Amp Fast Blow Fuse
D1-D4 : Full Wave Bridge Rectifier (RS276-1185)
T1 : 18 Volt, 10 Amp Transformer Hammond #165S18 (Digi-Key HM538-ND)
The circuit in Figure 2 senses when the voltage exceeds 15 volts and causes the zener diode to conduct. When the zener diode conducts, the gate of the SCR is turned on and causes the SCR to short which blows the 15 amp fuse and shuts off the output voltage. A 2N6399 was used for the SCR in the prototype but any suitable SCR can be used. While over voltage protection is a good idea, it should not be considered a substitute for large heat sinks. I personally feel the best protection from over voltage is the use of large heat sinks and a reliable current limiting circuit. Be sure to use large heat sinks along with heat sink grease for the 2N3055 transistors.
Component/Parts List
R1 : 1.5K ¼ Watt Resistor (optional, tie pins 6 & 5 of IC1 together if not used.)
R2,R3 : 0.1 Ohm 10 Watt Resistor (Tech America 900-1002)
R4 : 270 Ohm ¼ Watt Resistor
R5 : 680 Ohm ¼ Watt Resistor
R6,R7 : 0.15 Ohm 10 Watt Resistor (Tech America 900-1006)
R8 : 2.7K ¼ Watt Resistor
R9 : 1K Trimmer Potentiometer (RS271-280)
R10 : 3.3K ¼ Watt Resistor
C1,C2,C3,C4 : 4700 Microfarad Electrolytic Capacitor 35 Volt (observe polarity)
C5 : 100 Picofarad Ceramic Disk Capacitor
C6 : 1000 Microfarad Electrolytic Capacitor 25 Volt (observe polarity)
IC1 : LM723 (RS276-1740) Voltage Regulator IC. Socket is recommended.
Q1 : TIP3055T (RS276-2020) NPN Transistor (TO-220 Heat Sink Required)
Q2,Q3 : 2N3055 (RS276-2041) NPN Transistor (Large TO-3 Heat Sink Required)
S1 : Any SPST Toggle Switch
F1 : 3 Amp Fast Blow Fuse
D1-D4 : Full Wave Bridge Rectifier (RS276-1185)
T1 : 18 Volt, 10 Amp Transformer Hammond #165S18 (Digi-Key HM538-ND)
