Overviewσ25 ("sigma 25") is a simple positive regulated power supply based on the common 78xx series 3-terminal fixed voltage linear regulator IC (TO-220 package). The LM340 and LM2940 are also supported.
Its compact size and low cost makes the σ25 suitable for a variety of applications. Being a linear regulator, it has lower noise than most switching power supplies. For even lower noise, use the σ78 voltage regulator module instead of the standard IC regulators.
σ25 can be configured to output from 5V to 24V DC using the appropriate 79xx regulator, or 1.8V to 24V using the σ78.
A negative output version of the σ25 is the σ26.
σ25's small circuit board has the same dimensions and mounting hole locations as the σ24 EI-30 tranformer mounting board, and could be used together with that board in a "stacked" configuration for minimum footprint, or mounted side-by-side to minimize height. σ25 could also be used with a chassis-mount power transformer for maximum flexibility.
σ25 could also accept a DC voltage input. The input voltage needs to be at least 5V higher than the desired output voltage under load, to ensure that the voltage regulator stays in regulation under all operating conditions. The input voltage should not exceed 10V higher than the output voltage to avoid overheating the regulator, particularly if the load current would be high. Note that if you use a DC voltage as input, it does not matter which way you wire it to the "AC IN" connector, the input will handle either polarity. However, the input DC source should be fully isolated (i.e., neither of the input wires should be externally connected to σ25's + or - outputs). You should not use DC as input in a dual-rail system using both a σ25 and σ26.
The specified heatsink should be sandwich-mounted between the voltage regulator IC and the board surface with an impertial #4-40 or metric M3 machine screw and hex nut. No isolation pad is necessary.
WARNING: Building the σ25 will require that you work with AC line voltage, which is a lethal shock hazard. If you don't know what you're doing, please ask someone who is experienced to help you. (See the disclaimer).
Schematic diagramσ25's schematic diagram is shown below. It is a straightforward implementation of a 78xx-based voltage regulator with only few parts.
Circuit descriptionThe power transformer's secondary winding is connected to σ25's J1 connector. C1 helps to shunt away high frequency noise and transients. D1 provides full-wave rectification, and in conjunction with bulk capacitor C2, converts the AC sine wave from the transformer into DC. C3 is a bypass capacitor for C2 and lowers high frequency impedance. This raw DC voltage is then sent to U1 for voltage regulation. The regulated DC output is additionally decoupled by C4 to improve transient response. D2 is a reverse voltage protection diode for the voltage regulator. R1 is the current-limit resistor for the LED, which provides a small load for the voltage regulator even when it's not connected to anything. The LED can be mounted on the board or wired to a front panel for power on indication. The regulated output is at J2 pin 1. In most cases J2 pin 2 is "ground".
Parts listThe following is the parts list to populate one σ25 board. The DC output voltage is determined by the choice of U1 and the power transformer.
Note that the value of R1 (the LED current-limit resistor) should also be varied with the output voltage. The listed R1 values are calculated based on an operating current of approximately 3mA on an LED with Vf of 2V. The default value should work well with most LEDs. Decrease the resistor value if you find that the LED is too dim, or increase the value if too bright.
** The listed U1 vendor part numbers are examples. Many other usable variants are available.
Power transformer selectionThe power transformer secondary voltage (under load) should be selected based on the following table. If the listed secondary voltage is not available for the chosen transformer type, select the closest available higher voltage. If you use the σ78 regulator, the transformer secondary voltage can be lowered for less heat dissipation, because σ78 is a low-dropout design and does not require as much input voltage to maintain good regulation.
The VA rating of the transformer should be based on the intended load, while also taking into account the power dissipated by the voltage regulator. For example, if the desired output voltage is 5V, and the intended load will draw 100mA of current, then the power consumed by the load is:
(5V * 0.1A) = 0.5WThe power loss on the voltage regulator is the voltage dropped across it, multiplied by the current passing through it. For a transformer with 9V secondary voltage, the rectified DC voltage is approximately:
(9V * 1.4142) = 13VSubtract 2V from that figure for the bridge rectifier drop, and subtract the 5V output, we find that the voltage drop across the regulator to be 6V. Multiply that by the current of 0.1A and the power dissipation would be 0.6W.
Add the two together we have:
(0.5W + 0.6W) = 1.1WFrom this figure, it would be appropriate to choose a 1.5VA transformer to allow losses in the transformer itself. You should go through this calculation for the specfic voltage and current for your application.
Even though the voltage regulator is rated for 1A to 1.5A, it is recommended that the power dissipation on the regulator to be kept below 1.5W (continuous duty) with the default heatsink. For higher current applications please use the σ11 or σ22 power supply.
Please also note that EI-30 transformers are available only in limited VA ratings (typically 1VA-4VA). For applications requiring a higher powered transformer, you should forego the σ24 mounting board and use an appropriate chassis-mount transformer.
Please mount the power transformer far away from sensitive low-signal audio circuitry to avoid magnetically-induced hum and noise.
Dual-rail power supplyA σ25 and a σ26 board (and optionally two σ24 transformer boards) may be used to build a dual-rail (positive and negative) power supply. The σ25 and σ26 should each have its own transformer secondary winding. You can use a transformer with two secondaries, or two separate transformers. Note that most parts have reversed polarity on the σ26 compared to those on the σ25.
The circuit boardThe σ25 board is made of high quality FR-4 glass epoxy, double copper layer with top-side silkscreen and soldermask on both sides. The dimensions of the board is 2.0" x 1.2" (50.8mm x 30.5mm). The thickness of the board is 0.062" (1.58mm). The dimensions and mounting screw locations of the σ25 and ε24 boards are identical, therefore they can be "stacked" together with standoffs. The following image shows the board layout:
The following is a 3D rendering of a populated circuit board (heatsink not shown).
Build instructionsClean both sides of the blank σ25 board with paper towel and isopropyl alcohol or electronics flux remover before soldering any parts on it.
This is a simple project, just solder all parts to their respective locations on the board, noting the polarity and orientation of the diodes, electrolytic and tantalum capacitors, voltage regulator, LED and Molex connectors. Install the lowest profile parts first and work up.
The 78xx voltage regulator pins should be pre-bent 90° to match the mounting hole and pads. Do not solder the voltage regulator pins to the board until after it is mounted to the board along with the heatsink. Apply a small amount of heatsink thermal compound to the back of the regulator and the mating heatsink surface, and apply thermal compound on the back of the heatsink where it will mate against the board. The board's ground plane will augment the total heat dissipation. Use a #4-40 (or Metric M3) machine screw and nut to secure the voltage regulator and heatsink on the board, then solder the three voltage regulator pins to the board.
Clean up the solder flux residue from the board with isopropyl alcohol (or electronics flux remover) and a brush. Do not connect the AC input and DC output until you've passed the initial check phase, outlined in the section below.
The AC and transformer wiring is beyond the scope of this document, but you should understand what you're doing if you are doing this project.
Initial checkAfter you're done building the board and wiring everything up, use your multimeter in Ohms mode to check for short circuit across the two pins of the AC IN and DC OUT connectors. You should read very high (or "infinite") resistance. Do not connect the board and apply power if this check does not pass.
Connect the AC IN, then turn on the power and test the voltage at DC OUT with your multimeter. It should be within a close tolerance to the voltage regulator's specifications.
If this passes, turn off the power and connect the DC output to your intended load. Pay attention to polarity! Power on again and re-check the output voltage.
Pay attention to the voltage regulator temperature. If it gets too hot then one of the following problems exist:
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