From Curuxa

Very simple AC to DC voltage regulator with short circuit protection, 5V output, max 1A.

This module can be used for powering the rest of the circuit at 5V DC when your power supply provides an alternating current.


Electronic circuit


Bill of materials:

  • 1x 7805
  • 1x Electrolytic capacitor (read section below)
  • 1x 100nF cercamic capacitor (optional)
  • 4x 1N4007 diode
  • 4x Straight male headers (usually sold as 40-pin strips)
  • 6x Straight female headers (usually sold as 40-pin strips)
  • Some thin wire
  • Some heat shrink tubing
Layout: top view
Layout: bottom view


How it works

PWR-DD5 can take an input alternating voltage from around 7Vrms to 25Vrms and provide a constant 5 volt supply. Pin 1 is always the positive one, pin 2 is ground.

This module is based on the simple, cheap and common 7805 voltage regulator. It is very inefficient, so it will dissipate a lot of heat when more than a few milliamperes flow though it. That heat would be wasted energy, and it could require a big heatsink to continue working, so this module is recommended for supplying digital, low-power circuits. If you need to supply high power devices (such as DC motors) you might want to avoid voltage regulators (at least for that part of the circuit) by using MC2A and powering those motors directly at high voltage.

This is the most common and simple AC-DC circuit. The four 1N4007 act as a diode bridge, taking the alternating voltage and obtaining a fully-rectified sine wave. That wave is then smoothed by the capacitor, avoiding it to fall under 7V, which is the minimum voltage required for the 7805 to work properly. The 7805 takes that smoothed, irregular wave, and outputs 5V DC.

In case you have an input voltage of 8-35V DC instead of a alternating voltage, you might be interested in building a PWR-DD5 instead of PWR-AD5.

If you want, you can use PWR-AD5 in all cases. This circuit can support inputs of 7Vrms-25Vrms AC or 8V-35V DC.

You can power a circuit up to 1.5A continuously, but you'll have to replace the 1N4007 by other diodes.

If you wish to use as few components as possible you can avoid the smaller 100nF ceramic capacitor.

More information


A proper capacitor must be chosen for this module. It is required to smooth the rectified voltage so it doesn't fall under 7V, the minimum (safe) voltage at which the 7805 works properly.

This capacitor must be always electrolytic.

If you don't want to spend time choosing it, you can buy a 4700µF electrolytic capacitor, rated at least at 35V. It would work in almost all cases.

Capacitor selection

The problem is that bigger-than-needed capacitors would waste money and electricity, so if you are going to supply PWR-AD5 always at the same input voltage, you should buy the smallest possible capacitor.

The capacitor value depends on output current, input voltage, and input frequency. This chart shows the minimum required value for the given parameters. You must always buy a capacitor bigger than the specified in the chart, but the smallest possible.

Note that you also have to buy a capacitor rated at at least a few volts above your input AC current. Buy a capacitor rated at least at double your input voltage when possible.


No heatsink

Maximum current without heatsink

The chart on the left shows the maximum current than can be supplied by PWR-AD5 at a given input AC voltage and ambient temperature with no heatsink.

Higher currents would make the 7805 reach its maximum operating temperature so it will shutdown by itself.

If you want to get higher currents you'll have to use a heatsink, but that's not recommended. If your circuit requires so much regulated power from PWR-AD5 you should use a more efficient (and complex) circuit, so you won't be wasting so much energy as heat.

Choosing a heatsink

Heatsink selection, output current=100mA

If you want to use a heatsink with this module, take a look at these charts. They indicate the maximum thermal resistance your heatsink should have.

The lower thermal resistence, the better, but the more expensive and bigger it will be.

Heatsinks with bigger thermal resistances than the specified in these graphs would make the 7805 reach its maximum temperature.

Also, make sure your heatsink is suitable for TO-220 packages.

Heatsink selection, output current=1A
Big heatsink selection, output current=1A
Heatsink selection, output current=500mA
Big heatsink selection, output current=500mA


Low-voltage AC input, 100mA

If you want to power a circuit that requires 5V and no more than 100mA, but you only have a 13Vrms, 60Hz AC power supply, an electrolytic capacitor of at least 400µF would work, rated at least at 16V (25V would be better). You can connect your power supply to PWR-AD5 input, and PWR-AD5 output should be connected to the rest of your circuit. You wouldn't need a heatsink for this circuit.

Low-voltage AC input, 1A

If you want to power a circuit that requires 5V, 1A, but you only have a 10Vrms, 50Hz AC power supply, you need an electrolytic capacitor of at least 1900µF (1.9mF), rated at least at 16V. You can connect your power supply to PWR-AD5 input, and PWR-AD5 output should be connected to the rest of your circuit. You will also need a heatsink for TO-220 packages with a thermal resistance lower than 6.8ºC/W.

Mains/powerline (100-240Vrms, 50-60Hz)

PWR-AD5 supports input AC voltages below 25Vrms. If you want to power Curuxa Main Boards or Modules from the power line you need to buy a transformer whose primary is rated at your mains voltage and frequency (120Vrms, 60Hz in the US. 230V, 50Hz in most Europe, Africa, Asia and Oceania) and secondary rated between 7 and 25Vrms.

Transformers with secondaries rated at low voltages (7-13V) would be more efficient and require smaller heatsinks (or none at all), but would be usually bigger, more expensive, and require bigger capacitors.




After you finish builing this module you should supply its input with an AC voltage between 7 and 25Vrms or DC voltage between 8 and 35V. Then use a voltmeter to check the output voltage, which should be somewhere between 4.8 and 5.2 volts.