Lab Variable Symmetric Power Supply circuit 2x30V 3.5A

Posted on Jun 1, 2017

The power supply circuitry consists of a 220/2 * 18V / 3.5A transformer, a rectifier, a smoothing filter, a power amplifier, LM301, and two regulators LM317 and LM337. The theoretical of the circuit is shown in the figure. The voltage from the transformer is rectified by the rectifying bridge. Parallel to the bridge diodes, C1, C2, C3, C4, have been installed to suppress noise. Then, through the main filter capacitors C5 and C12 and other small capacitances, which further filter the current, it is led to the voltage regulators, consisting of the LM317 & LM337. These integrated are responsible for regulating the positive and negative tendencies, respectively. In the positive and negative feed lines, LEDs are inserted to show us at any moment that the circuit is being fed correctly.

Lab Variable Symmetric Power Supply circuit 2x30V 3.5A
Click here to download the full size of the above Circuit.

The diodes around the LM317 & LM337 are used to protect them from accidental short circuits at the output of the power supply or by discharging their electrolytic media, which could damage them.

The positive voltage setting is done with the potentiometer R3, while the negative voltage, which is an LM301. This is connected to the non-inverting input, via a voltage divider, consisting of R4, R5 & R6, to the positive and negative supply voltage.

The inverting input is connected to the earth via a resistor of R9. If for any reason the positive voltage changes, then the LM301 causes exactly equal variation in the negative supply voltage.

The capacitor connected to the legs 1 and 8 prevents the power amplifier from being immobilized. The basic resistors present at the output serve as a load for the operation of the circuit and for discharging the capacitors after the power supply is switched off. The capacitors in the positive and negative output lines, as well as the coils, act as frequency and noise cut-off filters from the devices we feed.

The RF Ripple Factor is calculated by:

RF = VAC / VDC * 100%

Where VAC or AC voltage component (Vpp / 2) at the specified DC voltage VDC.

Where Vpp = 2.24 mV thus Vp = 1.12mV or VAC = 1.12mV

And VDC = 10V

RF = (Ripple Factor) = 1.12 * 10-3 V / 10V * 100%

= 1.12 * 10-4 * 100% = 0.000112%


C1 = 100nF ceramic
C2 = 100nF ceramic
C3 = 100nF ceramic
C4 = 100nF ceramic
C5 = 4700 / 35V Electrolytic
C6 = 100nF ceramic
C7 = 4.7μF / 400V Electrolytic
C8 = 100μF / 35V Electrolytic
C9 = 100μF ceramic
C10 = 100nF / 100V Electrolytic
C11 = 100nF ceramic
C12 = 4700 / 35V Electrolytic
C13 = 100nF ceramic
C14 = 100nF ceramic
C15 = 100nF ceramic
C16 = 100nF ceramic
R1 = 1,2K
R2 = 220Ω
R3 = 4.7K Potentiometer
R4 = 12K
R5 = 1K Potentiometer Trimer
R6 = 12K
R7 = 1,5K 2W
R8 = 1,5K 2W
R9 = 4.7K
R10 = 1K
D1=  LED Red
D2 = 100V 2A Bridge no.
D3 = LED Red
D4 = 1N4001
D5 = 1N4001
D6 = 1N4001
D7 = 1N4001
U1 = LM317
U2 = LM337
U3 = LM301
T1 = 2x18V L1 = VK200
L2 = VK200



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