There's a range of other options with voltage regulators which are worth exploring.
In last month’s FUNdamentals, we presented several ways of deriving a fixed, clean, regulated voltage for your project when the power supply voltage is greater than your circuit requires; and spoke of some reasons why this may be the case. This month, we look at some different methods to regulate the voltage, as well as covering some devices that step the voltage up.
MORE ON VOLTAGE REGULATORS
Last month we covered the traditional, low-cost regulators that require the supply voltage to be at least 2.5 V greater than the output voltage. However, this is not always practical. Take, for example, producing a 5V USB supply from four AA batteries, totalling 6V when new.
This would be impossible with a 7805; however, there is another three-pin, fixed regulator that can help us. The LM2940CT-5.0 is a member of a family of regulators referred to as “low-dropout” regulators, meaning that the difference between the input and the output voltage can get quite low before the devices “drop out”. The LM2940CT-5.0 only requires an input 0.5V higher than the output, making them perfect for our scenario above [1].
The LM2940CT-5.0 even has the same layout as the 78XX series, with input on the left, ground in the middle, and output on the right (when viewed from the front). The manufacturer’s datasheet for the Texas Instruments product does recommend using both an input and an output capacitor, however. The family of regulators are also of interest to the maker because there is a 3.3V variant, the LM3940. The low dropout regulators are anywhere between three and ten times the price of the 78XX series though – depending on the item chosen, and the supplier – so they are not always ideal if their particular attributes are not essential.
MORE POWER!
One of the challenges of both the 78XX series, and the low-dropout regulators presented above, is that they only have a current rating of 1A (over which, they will self-limit the current.) This is not a big drama for a typical logic circuit, but is a challenge for anything driving light arrays, motors, actuators, and often much more. In such cases, there is a way to boost the current capacity of these devices. The manufacturer’s datasheets from several suppliers list a similar circuit that has been commonly used for a long time. This involves the regulator being attached to the base of a power transistor, which handles the high current as the series pass element, while staying regulated by the regulator IC.
The circuit diagram [2] shows this (note: variations on this circuit exist and work fairly well). The choice of power transistor determines the current that can be passed and, in turn, this would rely on appropriate heat sinking.
One limitation of a circuit such as this comes from the fact that breadboards are not well suited to large component legs (even the TO220 package can stretch some breadboard springs and result in inadequate contact when used with fine resistor legs). Also, larger packages such as the TO3P don’t fit at all. In addition, breadboards are not rated for very high currents, and 4A would be too much for many.
Fortunately, there is a solution. Both Altronics and Jaycar Electronics sell kits based on a low-voltage adjustable regulator, such as the LM317, which includes a circuit board, all required components, and instructions. These kits will, with added heat sinking, supply a voltage between 3V and 15V at a current of up to 4A. The kit has provision for fitting with screw terminals instead of the supplied PC stakes, so you could use fine wire to supply the logic on your breadboard and thicker wire to supply the high-current-driven load.
GOING UP!
All of this is well and good if you need to reduce a supply voltage, but what if you want to run your 5V device from a 3.7V lithium polymer battery? Sure you could put two in series for 7.4V and use a linear regulator, but this is inefficient on battery and far too close to the nominal value of 2.5V above output voltage for standard regulators (the 7.4V is nominal and is usually slightly higher when charged), leaving only the more expensive low-dropout types. In this case, the answer comes from a more recent switch mode power supply (SMPS), or switching regulator. The SMPS device uses a tiny coil to store energy when turned on and release that energy to increase the overall voltage out. SMPS devices switch on and off very fast as high speed allows them to use a smaller coil. SMPS can run between 20,000 and 50,000 cycles per second depending on the design. At high frequency, smaller copper windings are required, enabling tiny transformers to be used. SMPS can be designed to step-down the voltage (Buck mode) or step-up the voltage (Boost mode). Switchmode circuits can also be designed to increase or decrease voltage, Buck/Boost mode.
One such device from Jaycar is the XC4512 5V DC-to-DC converter. This takes a voltage from 2.5V to 5V, and steps it up to a regulated 5V DC. The current rating is stated in the datasheet as 500mA. This device is a small board, the size of many USB flash drives, and has a USB socket for the output, being aimed directly at microcontroller boards.
Another Jaycar product is the XC4609 DC-DC Boost module. This is a larger module with voltage display that can take an input from 3.5V to 35V and give an output between 5V and 56V, at a current of 2A. Altronics are also at the party, with the Z6334 DC-DC Converter Module being an adjustable switchmode supply, with input ranging from 3.5V to 35V and output from 5V to 25V. Also in the line-up is the Z6339 DFRobot Boost module, taking two AA batteries and yielding 5V, and the Z6337 DC-DC Boost/Buck module, which can be configured to increase or reduce voltage at currents up to 1A.
Of course, there are many other products available from online retailers; however, examples of these were not obtained. If in doubt, ask for datasheets, as these always contain the required information and more.
Hopefully this article has helped you understand that the supply voltage does not have to be limited to 5V from a USB port, and that supplying high power projects with a regulated supply rail does not need to be difficult or expensive. Likewise, lightweight, portable projects can be powered from single-cell battery supplies while still powering logic circuits.