Fads to Obsessions and Beyond...

Free domain for life, exceptional technical support, website transfer

Power Supply - DC-DC Step-down SMPS

A variable 1.23-37V 3A DC-DC step-down (buck) switch mode power supply is constructed based upon the LM2596 adjustable version. The datasheet recommended circuit is utilised to explore line and load regulation using commonly ("junk box") available capacitors and inductors.

The vast majority of electrical/electronic projects will require an external source of electrical power (except those solely powered by battery, solar energy or similar). Further, generally in the home setting (and or office or similar location) the wide availability of AC electricity (120 or 240 VAC depending upon location) predisposes electrical/electronic projects to be powered by DC produced from such domestic/commerical AC mains supply.

For the typical DIY'er or hobbyist, avoiding directly dealing with mains AC should be a primary objective to safely practice hobby electronics. This is the major advantage of using "wall warts" as a power supply option for hobby electronics projects. Also conveniently, "wall warts" are generally readily available for "free" due to surplus/disused and or damaged electrical applicances which are often powered by "wall warts" of one variety or other (and or can be cheaply purchased online).

However, matching the output voltage and current of whatever wall warts may be on hand at the time to the requirements of a particular DIY circuit often requires the use of an intermediatry voltage regulator (e.g. 12V DC wall wart output converted to 5V for PIC microcontroller circuit). Such DC-DC conversion can be handled by a linear regulator such as the LM317 but this is wasteful of electrical power, and if relatively large current is involved, produces a lot of waste heat that needs to be dealt with via heat sinks, fans etc.

An alternative to DC-DC conversion via linear regulator IC's is to use a switching regulator to form a DC-DC step-down (called a "buck converter") switch-mode power supply (SMPS). A SMPS uses a switching transistor to alternately power the load via the source DC and a combination of an inductor and capacitor (which are also "charged" by the source DC) (1). This enables a more efficient use of the source DC and less wastage of electrical power as compared to the "usual" linear voltage regulator method.

Therefore, a SMPS buck-convertor for the DC-DC conversion potentially allows a wider range of wall-warts to be more effectively used to power the typical 5V (and increasingly 3V) DIY microcontroller circuits. This is particularly beneficial in the typical case of wall warts that can supply relatively large currents (e.g. > 3A) but also output relatively large voltage (> 12V) such as surplus laptop chargers, which means considerable waste power when used in conjunction with a 5V or 3V linear regulator (and hence need for heat sinks etc).

LM2596 based buck-mode SMPS

There are a wide variety of potentially suitable IC's upon which you can base a step-down (buck) SMPS for DC-DC conversion. The appeal of the LM2596 for DIY projects is the very low price via ebay (even though the LM2596 is an old design running at relatively low frequency, which partly explains the low price!).

A LM2596 3A 2-17V output based DC-DC converter board can be purchased from ebay for less than $1 (including shipping). Whereas, the cost of the actual LM2596 IC's (TO-220 package, quantities of 10) is ~$0.25/ea. This shows that the ebay sale price of the LM2596 converter boards (which includes necessary capacitors, inductor, potentiometer etc) must be close to the cost of components giving low profit margin.

Alternatively, lower quality and hence cost, with large bulk quantity discounts, components are being used. The old adage "you get what you paid for" is no doubt generally true, however, these low cost LM2596 converter boards have been tested with apparently good results (at least for the DIY setting), e.g. (2), (3).

The LM2596, by the standard datasheet circuit, requires a couple of capacitors, an inductor and a potentiometer (if desired to have an adjustable version) which is only a couple of components more than the usual linear voltage regulator circuit, but potentially giving the advantage of less power wastage when stepping down a relatively large input voltage to a relatively low output voltage. Hence, enabling a more efficient use of a wider range of "surplus" wall-wart's to power DIY circuits.

Instead of just using the ebay available LM2596 converter boards, I was interested in testing the utility of using the LM2596 IC in conjunction with "commonly" available capacitors and inductors (i.e., whatever is available in the "junk box") - in particular, capacitors and inductors salvaged from disused and or disgarded consumer electronics (see Tear-Downs).

This then would indicate if "special" (i.e. tight specifications, "high" quality - hence increased cost) capacitors and inductors are necessary (as alluded in the datasheet) or would "ordinary" components suffice for reasonable results (hence also giving the advantage that a LM2596 buck converter could be incorporated into a DIY design PCB for a custom circuit).

The following sections detail the results of testing the line and load regulation of the LM2596 adjustable version using the datasheet recommended circuit with a range of capacitors and inductors salvaged from disused and or disgarded consumer electronics.

Text (5). Text

The usual schematic for the LM317 as per the datasheet is as follows.

basic circuit

"Basically" the output voltage (Vout) is determined by the ratio of the feedback resistors R1 and R2, using the following formula:

Vout = 1.25 x (
R2 / R1

The actual internal operation of the LM317 (and linear voltage regulators in general) is given in detail by an Analog Devices publication (7). Such detail/knowledge is likely of importance to assess the exact interaction of the LM317 with external circuitry. However, for the typical DIY scenario, the LM317 can be largely considered a "black box" that converts an input voltage to a regulated output voltage.

Refer to LM317 Component Testing for further details about utilising this IC in power supply circuits and some test results with a DIY electronic dummy load to check LM317 output voltage regulation etc.

Note: Image loading can be slow depending on server load.

  • Wall Wart SchematicWall Wart Schematic

    Silver Membership registration gives access to full resolution schematic diagrams.

    Wall Wart Schematic

This project did not require a PCB.

The construction was done using prototyping board. See the photographs and schematic diagram sections.

Qty Schematic Part-Reference Value Notes
1R2, R33301/4W, 10% 
1R142701/4W, 10% 
1C40.33 uFelectrolytic
1C50.1 uFelectrolytic
2C11,C1210 uFelectrolytic
1D3,D4Red and Green LED 
Integrated Circuits
1U2LM317TLinear voltage regulator datasheet
1U3LM10863.3v Linear voltage regulator datasheet
2SW2,SW3Switchsingle pole, single throw, 1A






No comments yet.

Add Comment/Question

Only Logged-In Members can add comments

"If we could sell our experiences for what they cost us, we'd all be millionaires".

Please donate any amount to help with hosting this web site.

If you subscribe (only $2/annum) you can view the site without advertisements and get emails abouts updates etc.



Only logged-in users with Silver Membership and above can download.

Silver Membership is only $2, so join now!