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LD1117-based 3.3V / 5V / Adjustable Power Supply Kit

This ECROS Technology power supply kit uses the ST Microelectronics LD1117V33 voltage regulator.  This device has the following features:

  • fixed output voltage of 3.3V (but read on for some interesting options!)
  • 2% accuracy (3.235 to 3.365V) over the full ranges of temperature and load current
  • over 0.8A output current with internal protection against overcurrent
  • low dropout voltage of 1.1V typical (1.2V maximum) at 0.8A load current
  • internal thermal shutdown to protect against overheating
  • input voltage of up to 15V
  • only 10F output bypass capacitor required for stability
  • no minimum load (regulation down to zero load current)
  • can also be easily configured for 5V or adjustable output!

Price and Ordering:

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Mini-PSU Parts Kit Contents:

U1 LD1117V33 Regulator †
J1 2.1mm Power Jack
D1 1N4006 (or equivalent) Diode
C1 0.1F Ceramic Capacitor
R1 1 kohm Trimmer Resistor
C2 10F 35V Electrolytic Cap.
C4 470F 25V Electrolytic Cap.
- Heatsink, Screw and Nut

Additional Parts in Flexi-PSU Kit:

R3 470 ohm 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.

‡ When switching more than 0.5A, an external switch is recommended.

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

Technical Information

Regulator - Install the regulator as U1, not U2.  This is unusual for a fixed regulator, but the LD1117 family includes both fixed and adjustable devices (read on for how to take advantage of this).

3.3V Fixed Output - For a fixed output voltage of 3.3 volts, connect a wire link across the footprint of C2 to short pin 1 of the regulator to ground.  The LED in the Flexi-PSU kit can be used as a power indicator in the normal way with a resistor of about 220 ohms installed as R3, stretched to connect to the output side of the PSU (see Technical Details).

5V Fixed Output - For a fixed output voltage of 3.3 volts, connect a red or orange LED at the footprint of C2, anode to the positive side.  If it is more convenient, you could connect the LED anode to the wiper of R1 (in the middle, connected to C2) and the cathode to pin 1 of R1 (square pad, grounded).  The LED will boost the output voltage by about 1.7 V and serve as a power indicator into the bargain!  If the LED is not bright enough, you can add 3.3 mA of LED current with a 1 kohm resistor installed as R2.

Adjustable Output - For an adjustable output from 3.3 to about 8 volts, connect a 1 kohm trimmer as R1 and a 10F capacitor as C2.  As recommended in Technical Details, if you want a power indicator with an adjustable output supply, it makes sense to connect it to the input side.

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 dropped across D1, if used.  For example, with this regulator configured for 3.3V output and with D1 installed, the input voltage must be at least 5.3V (3.3 + 1.2 + 0.8).  This must allow for supply voltage "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 5.8V.  This can be reduced to 5.0V if D1 is omitted, in which case care must be taken not to connect the input the wrong way round.

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 average input voltage of 9V.  The regulator will drop 4.9V (9 - 3.3 - 0.8).  At 0.5 amps, the heat dissipated will be 2.45W (4.9 x 0.5).  ECROS Technology recommends that you limit power dissipation in the regulator to 3W with the compact heatsink supplied in the Mini-PSU kit.  For higher power applications (higher input voltages and/or higher load current), please purchase the Flexi-PSU kit.  The much larger heatsink removes all concerns about power dissipation.

General Notes - The maximum input voltage of the LD1117 family of regulators is only 15 V.  This will typically be exceeded by connecting a 12 V nominal unregulated input from something like a wall-wart power pack.  For applications such as an adjustable supply going up to 12 volts, it is better to use the LM317-based kit.

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