2N3055 and LM317 power supply regulatable 0 28V 20 Amp

Part list for 20 Ampere regulatable power supply:

• 2 x 15 volt 20+- amps
• D1…D4= Bridge rectifier MB2504 (25 amps cooled)

  or eight BYW29 8 amp diodes (TO220 pinning cooled)
  or 8 x MR750 (MR7510) diodes (MR750 = 6 Ampere diode) or 16 x 1N5401 (1N5408) diodes.

• F1 = 2 Amp
• F2 = 25 amp
• R1 2k2 2,5 Watt
• R2 240 ohm
• R3,R4,R5,R6 0.1 ohm 10 watt
• R7 6k8
• R8 10k
• R9 47 Ohm 1 watt
• S2 mini switch
• R10 8k2
• C1,C7,C9 47nF
• C2 four times 4700uF/50v or one 22000uF/50v
• C3,C5 10uF/50v
• C4,C6,C9 100nF
• C8 330uF/50v
• C10 1uF/16v
• D5 1N4151
• D6 1N5401
• D10 MR750
• D11 LED
• D7, D8, D9 1N4001
• IC1 LM317
• Two 2N3055 transistors
• P1 5k
• P3 10k trimmer
• relay = 30 volts AC, 2×10 amp switching

S2 switches between +- 3 Amps and full output current

The relay is used to switch off the power supply voltage when the mains (S1) are/is switched off. So no delay do to the discharge of C2, and so preventing output voltages from not return to zero immediately.

A MB2504 is used as it is a 25 ampere rectifier bridge which also should be cooled. Or you could use eight BYW29 8 amp diodes (TO220 pinning) mounted on a heat sink.

Mount a little heatsink on the LM317 IC

Remember to isolate the 2N3055 transistors from the chassis/radiator! Use a radiator (heat sink) of appropriate size and surface area; insulating and heat-conducting spacer or at least a thin mica; hot adhesive and thermal paste.

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12 to 28V Boost Regulator LM2585

This boost regulator is for those times when you have a 28v relay, but want to use it with a 12v source. The schema is built around the National Semiconductor LM2585, and uses the energy stored in an inductor to boost the 12 to 28v. Output voltage can be varied by adjusting the ratio of resistor values on the feedback pin.

The voltage regulator schema does it’s switching around 100 Khz, but generates no noise if SMT components are used. Output is good for about half an amp continuous, enough to power two or three large microwave relays. The board measures 1.5″x2″.

It is important to note at least these three cautions before powering up the board:

• A short-schema on the output will kill U1 and D1. Always use a 1 ohm 5w resistor, or a 2.5A fast fuse on the 12v input lead.
• Do not omit the LED (D2); It provides a visual indicator of a properly operating boost condition, but more importantly, it also provides a minimum load for the output, preventing an output “spike” which will otherwise appear when the load is disconnected abruptly.
• Keep the ratio of r2 and r3 to 22 or less to keep the output voltage within the ratings of C4 (C4 on my board is rated at 35wvdc). This ratio plus 1, multiplied times 1.25v, determines the output voltage.

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