Power Supply Introduction


In this first (and following) stages, the builder should remember that one of the most common causes of errors is soldering. It pays to review materials on soldering, get help from Elmers, or whatever you can do to make your solder joints as clean and properly conductive as possible!

The second most common cause of errors is installation of the WRONG component and/or installing the component in the wrong ORIENTATION. The old rule of "measure twice, cut once" clearly applies to this project. Be especially careful and beware that it is very easy to install the wrong resistor depending entirely in color codes. While color codes are helpful in initially sorting resistors out, it is imperative that you validate that you have the correct resistor by double checking with your ohmmeter. While the ohmmeter reading will never be the exact value, for most of the resistors in this kit, the ohmmeter will get you to within 1% (a very few some are within 5%) of the stated value

The remaining one-tenth of one percent of the causes of errors is the defective component - most suspect the component immediately; the intelligent rarely look first at possible component failure.

Theory of Operation

This stage provides the +5 volt power rail for the radio. The incoming voltage (from 9 - 12 Vdc) is regulated by U1 to a nominal 5 Vdc (4.5 - 5.1 Vdc range). D1 serves to protect the circuit from accidentally reversed polarity.

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Power Supply Schematic

(Resistor testpoints (hairpin, top, or left-hand lead), as physically installed on the board, are marked in the schematic with red dots)

(Click for Full Schematic)
Power Supplyschematic

(above schematic has clickable areas that can be used for navigation)

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Power Supply Bill of Materials

Stage Bill of Materials

(resistor images and color codes courtesy of WIlfried, DL5SWB's R-Color Code program)

11N40031N4003 1N4003Axial
24.7 uF 10% 16V X7R RAD475 475Ceramic
1shunt wire (cut-off lead)Cutoff
1SR Lite II (Combined) PCBMain Board
10.1 uF(smt) black stripe (smt) black stripeSMT 1206
1LM78L05 voltage regulatorLM78L05 LM78L05TO-92

Power Supply Summary Build Notes

Power Supply Detailed Build Notes

Bottom of the Board

Power Supply Bottom View

Install SMT cap

Install C13 SMT 0.1 uF cap

Take care to avoid solder "splashover" that could clog up the thru-holes above and to the right of C13 (see red dots in above graphic)

See hints on installing SMT Caps.

PCBSR Lite II (Combined) PCBMain Board 
C130.1 uF(smt) black stripe (smt) black stripeSMT 1206SMT

Top of the Board

Power Supply Top View

Install topside components

Install C1 and C2 (the two blue capacitors), U1, and D1. Install D1 such that the cathode end (the end with the band) is facing up and forms a hairpin. The hairpin lead will go into the square thru-hole (refer to the Completed Stage, Topside picture below).

See hints on identifying and installing Ceramic Capacitors

Take care to correctly identify the Voltage regulator, U1. It is a TO92 package, as are the 2N390x transistors. Some have had the misfortune of installing one of the 2N390x transistors in this stage, with strange results.

U1LM78L05 voltage regulatorLM78L05 LM78L05TO-92 
D11N40031N4003 1N4003AxialE-W
C014.7 uF 10% 16V X7R RAD475 475Ceramic 
C024.7 uF 10% 16V X7R RAD475 475Ceramic 

Install ground test loop

Using a short length of cut-off resistor or capacitor lead, fashion a short wire loop and solder it to the "ground" hole, such that the loop is available on the topside to provide a ground point for tests.

Install ground test loop photo
gndshunt wire (cut-off lead)Cutoff 

Power Supply Testing

Visual Inspection

Test Setup

Using very good lighting and magnification, carefully inspect the solder joints to identify bridges, cold joints, or poor contacts.

Current Draw

Test Setup

Test for current draw in 2 ways:

  • Use a 12 volt power supply
  • In one test there is also a 1k resistor in the series "chain" as well.
  • in the second test, the setup is the same except that the current-limiting resistor is removed
(measurements courtesy of Leonard KC0WOX) Current Draw

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
With 1k limiting resistormA< 94.1_______
Without current limiting resistormA3 - 64.4_______

Voltage Test

Test Setup

Once the current draw test is successfully passed:

  • Apply 12 Vdc (NO current limiting resistor) to the PWR + and - pads (upper right-hand corner of the board)
  • Measure the voltage with respect to ground at the testpoints below
  • Voltage Test

    Test Measurements

    TestpointUnitsNominal ValueAuthor'sYours
    R11 hairpin (5 Vdc point)Vdc54.93_______
    D1 cathode (square hole)Vdc11-1312.2_______
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