Introduction to the BBPS-1
A bench power supply with multiple outputs adjustable for voltage and current is just right for some applications. However, there are times when you need only a single voltage supply and it would be an added benefit if it were compatible with a solderless breadboard.
This project will supply all the information required to build a rock solid linear voltage regulator in any of 9 voltages from 2.5V to 15V that will provide enough current for many small circuits. The same low cost printed circuit board can be used for the voltage of your choice, and Gerber files are provided later in this article. Coupled with a readily available "wall wart" AC to DC converter, the regulator (called the Breadboard Power Supply 1, or BBPS-1) will quickly become a favorite on your workbench. A 3.3V BBPS-1 is shown in use below.
The circuit for the BBPS-1 is far from revolutionary, but offers some features not available in other breadboard compatible regulators. The schematic is reproduced below; click on the image for a larger version.
J1 is a power jack where the external AC to DC converter connects; D1 is a Schottky diode which prevents reversed polarity voltage from reaching the other components. SW1 is used to switch the DC input on and off, and R2 provides a quick discharge path for C1, thus avoiding a slow bleed down. C1 through C4 provide filtering, noise suppression, and oscillation suppression for U1, which is the actual regulator IC. Resistor R1 and LED1 provide a visual confirmation that U1 is working.
PH3 and PH4 are pin headers which plug into the power and ground rails on the solderless breadboard. PH1, PH2, PH5, and PH6 are also pin headers which allow both +V and ground to be selectively disconnected from the power rails on the breadboard.
Note that the schematic above is for a 5V regulator. Other voltages require a different U1 and R1; more information about that is provided in the next section.
The photo below shows a bare printed circuit board on the right and a finished assembly on the left. As you see, no surface mount parts are used. The reference numbers on the PCB agree with those in the schematic diagram above and the parts list in Table 1.
Table 1 lists the parts required to construct a BBPS-1 regulator.
|Reference No.||Description||Qty.||Source||Part Number|
|J1||Jack, Power, Coaxial, 2.1mm Center Pin||1||Jameco||2144690|
|D1||Diode, Schottky, 40V, 1A||1||Digi-Key||1N5819-TP|
|SW1||Switch, SPDT, Toggle, Right Angle, PCB Mount||1||Jameco||75871|
|C1||Capacitor, Electrolytic, 1000uF, 50V, Radial Leads||1||Digi-Key||UVY1H102MHD-ND|
|C2||Capacitor, Ceramic, .33uF, 50V, Radial Leads||1||Digi-Key||490-8842-ND|
|C3||Capacitor, Ceramic, .1uF, 50V, Radial Leads||1||Digi-Key||399-9877-1-ND|
|C4||Capacitor, Electrolytic, 10uF, 50V, Radial Leads||1||Digi-Key||493-5915-1-ND|
|R1||Resistor, .25W, Carbon Film, Axial Leads||1||Digi-Key||See Table 2.|
|R2||Resistor, 100 Ohm, .25W, Carbon Film, Axial Leads||1||Digi-Key||100QBK-ND|
|U1||Voltage Regulator, Linear, TO-200 Package||1||Digi-Key||See Table 2.|
|LED1||Diode, Light Emitting, T1 3/4, Green||1||Jameco||693901|
|PH1-PH6||Header, Pin, Straight, 40 Position (Make from strip.)||1||Jameco||2168211|
|N/A||Shunt, 2-Position, .1" Centers, Closed, Black||2||Jameco||19141|
|N/A||Shunt, 2-Position, .1" Centers, Closed, Red||2||Jameco||112432|
|N/A||Heat Sink, TO-220||1||Jameco||158051|
|N/A||Grease, Thermal, Silicone, White||1||Jameco||615312|
|N/A||Spacer, Nylon, Tapped 4-40, .5"||2||Digi-Key||36-1902C-ND|
|N/A||Screw, Machine, 4-40, Pan Head, Phillips, .25"||3||Jameco||2094389|
|N/A||Nut, Machine, 4-40||1||Jameco||40943|
|N/A||Printed Circuit Board, Custom Made, FR4, 1.6mm Thick, 1 oz. CU||1||Itead||See text.|
Table 2 is used to select two parts (U1 and R1) that are listed in Table 1 but not defined there. The choices for U1 and R1 are based on the desired output voltage from the BBPS-1. For example, if you want a BBPS-1 regulator with a 5V output, you should locate "5" in the VOUT column, and see that U1 should be an LM7805CT and R1 should be 560 ohms. Consequently, you would order one LM7805CT-ND and one 560QBK-ND.
|U1||Digi-Key PN||VOUT||IOUT (Max.)||VIN||R1 (Ohms)||Digi-Key PN|
Important Technical Information
Note that Table 2 also contains a column that shows the maximum current available from each voltage regulator IC. This is a theoretical maximum under ideal conditions, and is probably not achievable with the small heat sink used in the BBPS-1. Fortunately, all of the voltage regulator ICs listed have a thermal shutdown feature that automatically turns them off if they overheat; once cool, they will resume functioning. Also be aware that the AC to DC converter selected must be capable of supplying at least as much current as is listed in the IOUT column for the corresponding regulator IC; more available current from the AC to DC converter is better.
Finally, note that Table 2 contains two columns that show the minimum and maximum voltage input requirements for each of the voltage regulator ICs. The AC to DC converter that is used with the BBS-1 must provide voltage that is not less than the minimum listed and not more than the maximum listed. You should also know that the higher the input voltage is, the more heat that will have to be dissipated by the voltage regulator IC. Thus, it is best to use an AC to DC converter that provides a voltage that is as close to the minimum as is practical.
The Printed Circuit Board (PCB)
The printed circuit board for the BBPS-1 was created based on a schematic drawing and a PCB layout designed with a software program called DipTrace. DipTrace was then used to create a set of Gerber files for the design that were sent for prototyping to the manufacturer in China, a company named Itead. Itead manufactured 10 of the PCBs and shipped them to the US for a total cost of about US$14; it took approximately three weeks from the time they were ordered until they arrived.
The original DipTrace schematic drawing and PCB layout files are available for downloading for those of you who want them for use in making your own PCB. In addition, the Gerber files are also available for those who want to order PCBs from Itead or another supplier. These files are for hobby use only and are not to be used for any commercial purpose. The author has successfully used these files and believes them to be correct and suitable for the purpose of producing functional PCBs as shown and described in this article, but there is no guarantee; use them at your own risk.
Assembly and Operation of the BBPS-1
The BBPS-1 uses no surface mount devices, and all the component locations are identified by the silk screen on the PCB. The components are well spaced and the pads are amply sized for soldering. Thus, by following the silk screen indications and referencing the photographs in this article, assembly of the BBPS-1 should be easy.
- In general, it is better to install the shorter components on the PCB first and the taller components later.
- Pay special attention to the polarity of the electrolytic capacitors, the diode, the LED, and the IC; the silk screen and the photographs show proper orientation of all parts.
- Use a very thin layer of thermal grease between the back of U1 and the heat sink; fasten the heat sink to U1 with a machine screw and nut.
Operation of the BBPS-1 is equally straightforward.
- Orient the solderless breadboard such that a ground rail is at the topmost edge.
- Insert the pin headers into the ground and power rails on the breadboard, making sure that the "G" pins insert into a ground rail and the "+" pins insert into a power rail.
- Note that connection of the BBPS-1 to the ground and power rails may be individually disconnected by removing the appropriate shunt(s) from the pin headers.
- Connect the AC to DC converter to the BBPS-1 by inserting a 2.1mm power plug into jack J1; +V should be on the center pin and ground on the shield.
- Turn the BBPS-1 on by moving the switch lever toward the power jack; the green LED should light.