Power Supply Introduction


This stage provides the simple power supply for 12 and 5 volt busses.

Note that there are actually four power busses:

  1. The 12 Volt bus - for transmit
  2. The 5 volt bus on the "regular" ground plane (this stage) - for powering the Ics installed in the stages following the Local Oscillator stage)
  3. The "USB 5 Volt" bus - on its own, galvanically isolated "USB ground", for powering the Local Oscillator and the microcontroller. The PC provides the 5V dc via the USB cable. The USB jack is installed in the next stage.
  4. The 3.3V bus - used in the local oscillator stage and working against the "USB ground". (installed in the next stage)
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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)

(go directly to build notes)

Power Supply Bill of Materials

Stage Bill of Materials

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

122.1 k 1/4W 1%r-r-brn-r-br r-r-brn-r-br1/4W
168 1/6W 5%bl-gry-blk-gld bl-gry-blk-gld1/6W
11N40031N4003 1N4003Axial
24.7 uF 10% 16V X7R RAD475 475Ceramic
1010uF/16 VDC Electrolytic
1DC Power Jack PCB Mount (rt-angle) Jack-RA
1DC Power Plug 5.5/2.1mm Pos Ctr Plug
1LM78L05 voltage regulatorLM78L05 LM78L05TO-92

Power Supply Summary Build Notes

Power Supply Detailed Build Notes

Top of the Board

Power Supply Top View

Install Topside Components

Mount the 2 4.7 uF ceramics first and watch out for C38: the silkscreen could lead you to believe it is horizontally oriented when it actually is vertically oriented (N-S) on the board.

Pay careful attention to the polarity of the electrolytic capacitors. The positive lead is usually the longer lead; the negative lead is on the side with the grey stripe and a minus sign.

The diode's polarity is also critical. Mount it such that the cathode end of the diode (with the stripe on it) is a hairpin lead into the square hole.

Resistor R50, while not part of the power supply circuit, is installed in this stage so as to provide a convenient (regular) ground point at its hairpin lead for the tests at the end of this stage and subsequent stages.

P1DC Power Plug 5.5/2.1mm Pos Ctr Plug 
D41N40031N4003 1N4003Axial 
Mouint hairin style (barrel in round hole, hairpin lead in square hole)
C384.7 uF 10% 16V X7R RAD475 475Ceramicvert
C404.7 uF 10% 16V X7R RAD475 475Ceramichoriz
R5268 1/6W 5%bl-gry-blk-gld bl-gry-blk-gld1/6WN-S
(some kits may have 1/6W - this version is still OK)
U09LM78L05 voltage regulatorLM78L05 LM78L05TO-92 
C2810uF/16 VDC Electrolytic 
C2910uF/16 VDC Electrolytic 
C3010uF/16 VDC Electrolytic 
C3110uF/16 VDC Electrolytic 
C3210uF/16 VDC Electrolytic 
C3310uF/16 VDC Electrolytic 
C3410uF/16 VDC Electrolytic 
C3510uF/16 VDC Electrolytic 
C3610uF/16 VDC Electrolytic 
C3710uF/16 VDC Electrolytic 
R5022.1 k 1/4W 1%r-r-brn-r-br r-r-brn-r-br1/4WN-S
Installed and used as (regular) ground reference.
J5DC Power Jack PCB Mount (rt-angle) Jack-RA 

Power Supply Completed Stage

Top of the Board

View of Completed Top

Power Supply Testing

Current draw

Test Setup

Before applying power for the first time, test the resistance across the 12V power rail (positive lead at the point marked for the non-hairpin end of R52; negative lead on a convenient ground (see diagram below)). You should see a very high resistance (over 50 k ohms) and it should climb (as the electrolytic capacitors charge up with the ohmmeter's voltage). Ultimately, you should see an extremely high (or infinite) resistance.

Once you have successfully passed the resistance test, put your mA meter in series with the input voltage positive line and measure the current draw of the power supply.

See Tutorial on Measuring Current for an illustration of how to measure current in a circuit.

Current draw

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Current drawmA< 84.5_______

Voltage Tests

Test Setup

Power up the board with a 12V supply (no limiting resistor) and measure the voltage at the test points indicated.

(Author's measurements below were taken using power from a gel cel whose voltage measured 12.9V)

Voltage measurements are WRT regular ground. For this test you can use the "hairpin" of R50 as a convenient regular ground test point.

Voltage Tests

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
Hairpin lead (hole) of R61 (wrt ground)V dc5 (+/- 2%)4.94_______
Hairpin lead of D4 (wrt ground) - (with 12.9 V gelcell)V dc1212.3_______