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LM317-based Wide-Range Adjustable Voltage Power Supply Kit

These ECROS Technology adjustable-voltage power supply kits use the Fairchild Semiconductor LM317MT or LM317T voltage regulators.  These regulators have the following features:

  • wide output voltage range, adjustable from 1.3V to 37V (with high enough input voltage and reduced current above 16V)
  • up to 500mA output current for LM317MT
  • up to 1.5A output current for LM317T (with Flexi-PSU's large heatsink)
  • internal protection against overcurrent
  • internal thermal shutdown to protect against overheating
  • input-to-output voltage up to 15V at full current and up to 40V at reduced current
  • dropout voltage of 3V maximum at full load
  • minimum load current of 3.5 mA (typical, 12 mA maximum)
  • 0.1F input bypass capacitor for stability

Price and Ordering:

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Mini-PSU Parts Kit contents:

U1 LM317MT or LM317T †
J1 2.1mm Power Jack
C1 0.1F Ceramic Capacitor
D2 1N4006 (or equivalent) Diode
R1 1 kohm and 2 kohm Trimmer Potentiometers
R2 150, 180, 220 and 270 ohm Resistors
D3 1N4148 (or equivalent) Diode
C2 10F 35V Electrolytic Capacitor
C4 330F 35V Electrolytic Capacitor
- Heatsink, Screw and Nut

Additional Parts in Flexi-PSU Kit:

R3 2 kohm Resistor
LED1 Red LED, T1 (3mm)
SW1 Switch (0.5A @ 50V) ‡
- Terminal Block, 2-way
- Larger Heatsink

† The regulator must be inserted as U1, not U2.

‡ Because of the modest current rating of the slide switch supplied for SW1, it is recommended that an external switch, with a higher rating, be used if it will be frequently operated while a high load current is flowing.

(Use the menu on the left hand side to go back to the Mini-PSU introduction.)

Technical Specifications

Output Voltage Range - For lower output voltage ranges, install as R1 the 1 kohm, 100mW trimmer potentiometer supplied in the kit.  To obtain the range you want, select R2 from those supplied as follows:

To obtain higher output voltages, do not reduce R2 further or the power rating of R1 will be exceeded.  Use the supplied 2 kohm trimmer instead.

If you are ordering assembly and test please be sure to specify your selection of R1 and R2 from the above choices.  Unused parts will be sent along with your assembled PSU.

Input Voltage - For the regulator to operate properly, the input voltage to the PSU must always be the output voltage plus the regulator dropout voltage plus the voltage drop across D1, if used.  For example, with the output set at 5V and D1 bridged with a wire link, the input voltage must be at least 8V (5 + 3).  This includes supply ripple.  Continuing the example, if, at full load, ripple causes the input voltage to drop 0.5V below its average value during each half-cycle, then the average input voltage should be 8.5V.

Power Dissipation - Heat dissipated in the regulator, in watts, will be the product of the load current in amps and the voltage dropped across the regulator.  Again, using the above example, suppose you apply an input voltage of 12V.  The regulator will drop 7V (12 - 5).  At 0.5 amps, the heat dissipated will be 3.5W (7 x 0.5).  With the compact heatsinks supplied in the 0.5A kits, ECROS Technology recommends that you limit power dissipation in the regulator to 4W.  This is based on personal safety concerns.  At higher power, the circuit will continue to function but the heatsink will become very hot.  If you touch it, you may not pull away fast enough to avoid some lasting discomfort.  The 1.5A kit with the Flexi-PSU PCB has a large heatsink and power dissipation is not a concern.

(Use the menu on the left hand side to go back to the Mini-PSU introduction.)