PWR-DD5

From Curuxa

Very simple DC to DC voltage regulator with short circuit protection, 5V output.

This module can be used for powering the rest of the circuit at 5V when your power supply or batteries provide at least 7.5V.

Warning: all calculations and graphs are referred to this model. Other companies manufacture 7805 integrated circuits with slightly different characteristics.

Contents

Electronic circuit

Schematic

Bill of materials:

  • 1x 7805
  • 1x 1N4007 diode
  • 2x Straight male headers (usually sold as 40-pin strips)
  • 2x Straight male headers (usually sold as 40-pin strips)
  • Some thin wire
  • Some heat shrink tubing


Wiring

Printable version: Single, Quad

How it works

PWR-DD5 can take an input DC voltage between 7.5 and 35V from the male headers, and provide a constant 5 volt supply at the female headers. Pin 1 is always the positive one, and 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.

Input-output voltage relation

As seen in the graph, an input voltage of at least ~7.5V is required for getting the desired 5V output. If the input voltage is lower than 7.5V the output voltage will also be lower than 5V.

The output stabilizes at around 5V when the input is at least 7.5V. There is a small voltage drop for inputs bigger than 7.8V but you can ignore it.

The strange behavior at Vin=~5.75V (Vo jumps from 2.40 to 3.80V) probably happens because that's the voltage at which two semiconductors become forward-biased after their forward voltage drop has been reached.

The diode at the 7805 input pin avoids inverse input voltages. If we remove that diode and supply the 7805 with an inverse power supply, the circuit would become almost a short-circuit, the 7805 would not burn, but your power supply, connectors, other electronic devices and even wires would burn. Don't forget to include that diode.

More information can be found in the 7805 datasheet.

Heatsink

Maximum current without heatsink

The graph on the left shows the maximum current than can be supplied by PWR-DD5 at a given input 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 the PWR-DD5 you should use a more efficient (and complex) circuit, so you won't be wasting so much energy as heat.

Heatsink selection
Small heatsink selection
Big heatsink selection

If you want to use a heatsink with this module, take a look at these three graphs. 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.

Heatsink 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.

For example, if you want to power a circuit that requires 5V, 1A, but you only have 9.3V batteries, you can connect your batteries to PWR-DD5 input, and PWR-DD5 output should be connected to the rest of your circuit. You will also need a small heatsink for TO-220 packages with a thermal resistance lower than 23ºC/W, but if you live in a very hot city you would need a heatsink with less than 17ºC/W.

Construction

PWR-DD5 PWR-DD5

Test

After you finish builing this module you should supply its input with a voltage between 7.5 and 35V. Then use a voltmeter to check the output voltage, which should be somewhere between 4.8 and 5.2 volts.