Introduction: (written 5-20-2007)

Fall 2001 - the beginning of my Junior Year in High School - the class assignment in Electronics II was to assemble a 0-9VDC power supply kit, but my teacher, Mr. Doug Ripka, aware that I had already learned most of the Electronics II curriculum from the books I'd read during my summer vacation, allowed me to design & construct my own more advanced power supply. I had spent all of summer 2001 studying books like Basic Electronics Theory, Power Supplies, Switching Regulators, Inverters, and Converters, & Building Power Supplies, and having already built a few gadgets over the summer, I knew very well the thrill of invention and was eager to revive the feeling with my design, assembly, & operation of the following project.

±30V DC 1.5A Adjustable Power Supply

Devin R. Ott
Design - November 2001
Final Assembly - December 2001

Description:

Two independently adjustable DC power supplies; one positive (V+), one negative (V-). High/Low output ranges.

• 1.5A maximum output current. • 0.01%/V typical line regulation. • Integrated short circuit protection & thermal shutdown.

Schematic:

Circuit Operation:

120VAC enters back panel via 16AWG power cord. Earth GROUND is hardwired to aluminum chassis (10" x 3.5" x 5.8"). Fused AC power is switched by SW1, mounted on back panel near fuse holder.

When powered-ON, 120VAC feeds the primary windings of two center-tapped 2A power transformers (RadioShack #273-1512B), each yielding an isolated 36V-PEAK (25.2V-RMS) on the secondary winding. The 36V-PEAK is rectified by a 1.5A bridge rectifier (Fairchild #KBP005M).

Each rectifier outputs a 60Hz pulsing 35V-PEAK DC potential: one is positioned above GROUND and filtered by C1; the other is positioned below GROUND (V-) and filtered by C2.

Bleeder resistors {R7, R8} are placed across filter capacitors {C1, C2} allowing them to discharge when device is powered-OFF. Capacitive time constant approximation T=RC ≈ 1 minute.

Green LED indicator is biased by the voltage across C1, forcing 9mA forward current when the device is powered-ON.

The 35VDC on each capacitor is further filtered and regulated by a LM317/LM337 adjustable (positive/negative) voltage regulator, rated for 1.5A and mounted to a heat sink. Each regulator outputs a low-ripple DC voltage determined by two external resistances:
    • resistance between V-OUT terminal and ADJUST terminal, {R1, R2}.
    • resistance between ADJUST terminal and GROUND reference, {R_ADJ}.

The regulated voltages are determined by the following relationship:

LM317:           V_OUT = (1.25V)[1+(R_ADJ÷R1)] + (I_ADJ)(R_ADJ)

LM337:           -V_OUT = (-1.25V)[1+(R_ADJ÷R2)] + (-I_ADJ)(R_ADJ)

*If the maximum value of R_ADJ is low (a few kΩ), the voltage contributed by the (I_ADJR_ADJ) term is negligible so it can be omitted from the VOUT expression.
*Always breadboard & test a new circuit design before you begin its final layout & assembly. A circuit may work theoretically on paper or in PSpice, but that doesn't guarantee it'll work in the real world or in your particular environment of operation. In my own designs, I usually draw out circuits on paper and then breadboard the design in stages, one after another, testing each stage as I go, until the entire circuit is on the board. And for troubleshooting purposes, I always try to leave the breadboard intact while I layout & assemble the final prototype.

{R1, R2} are fixed at low values.
*see my Voltage Regulator Experiment for more information.

R_ADJis adjusted via potentiometer {R5 or R6} to vary the regulator's output voltage across two Δ15V ranges.
Switches {SW2, SW3} select one of two V_OUT ranges by either inserting or bypassing a fixed resistor {R3, R4} in series with each potentiometer.

Positive Supply Voltage: LM317
    • SW2 closed ( low range: +1.2 to +15V ) --->    R_ADJ = R5
    • SW2 open ( high range: +15 to +30V ) --->      R_ADJ = R5 + 5kΩ
Negative Supply Voltage: LM337
    • SW3 closed ( low range: -1.2 to -15V ) --->    R_ADJ = R6
    • SW3 open ( high range: -15 to -30V ) --->      R_ADJ = R6 + 4.7kΩ

Output capacitors {C3, C4} prevent the load from exhibiting certain low level capacitance (a few nF) that can cause oscillations on the regulator's output. And like all bypass caps, they reduce power line impedance and effective pick-up loop area.

Final Assembly:

Improvements:

Improve regulator performance & stability: My high school electronics teacher, Doug Ripka, always used to say that "a voltage regulator is like a high-gain amplifier."
• 0.1uF bypass capacitor on regulator's V-IN terminal - decouples high frequency input noise.
• 10uF bypass capacitor on regulators ADJUST terminal - improves low frequency ripple rejection,

*consider diode protection to prevent capacitor from discharging through low-current paths inside the regulator, causing damage.

*See my 9VDC Supply for SONY Stereo.

• Electrolytic or tantalum output capacitor (at least 1uF) - prevents low-values of load capacitance (a few nF) that can cause oscillations on the regulator's output.

*diode should be used to prevent output capacitance from discharging through the regulator.

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