1 Computer Power Supply Amateur Radio Conversion For Cheap +12 vdc Power March 17, 2003

2 Advantages Supplies are Readily Available They are Inexpensive ($1) 50 watt Radios Only Need Amps New Supplies Have Higher Wattage 100 Watt HF Rig Potential Or Parallel Supplies

3 Some Electronic Knowledge I = E/R P = E*I Series/Parallel Resistance Resistor Color Code Volt Ohm Meter Good vision!!! Persistence No Standard Design Requires

4 Resistor Color Bands

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6 With a Little Patience it’s a Lot of Fun No Rocket Science Required You Can Do as Much or as Little as You Feel Comfortable With Quick and Dirty… (Just 12 vdc) Moderate… (Try for 13.8 vdc) Overhaul… (13.8 vdc only) However!

7 Scope of Presentation Describe Quick and Dirty Demonstrate 13.8 vdc Modification Brief Description of In-depth Changes Build Comfort Level

8 Assumptions Power Supply Uses TL494 to Control Output TL494 Pin 2 is Reference Voltage Pin 1 is Sample of Output Seems to be the “Norm”

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10 Referenc e Voltage From Supply Output Sample Regulation Circuit

11 Voltage Regulation Lots of differences ( Ω)

12 KØGL SupplyWØKRB Supply

13 TL 494 Blk = GND Red = +5 vdc Yellow = +12 vdc Blue = - 12 vdc White = - 5 vdc Voltage Adjust Pot KØGL Supply

14 Adjust Voltage WØKRB Supply TL494

15 How To Start Take Supply Apart Blow Out Dirt Verify Lead Colors (Red +5 vdc etc.) Connect two 10 ohm 10 Watt Resistors in Parallel Across the +5 vdc Leads Separate Wires and Power Up Supply Measure Voltage and Check Fan

16 Measure Voltage & Check Fan Red Black 5 ohm Yellow Black I = 5v/5Ω = 1A P = 5v x 1A = 5 watts Caution: Heat Sinks May be Electrically Hot!

17 Once You Know It Works Find TL 494 Trace Foil From TL494 Pin 1 Measure Voltage on Pin 1 & 2 Draw Schematic of Resistors Compute Parallel/Series Resistance Does Voltage on TL494 Pin 1 Make Sense If so Decide How to Raise Voltage… If Necessary… BE CAREFUL !

18 Voltage Regulation Adjust Pot If Available Decrease Voltage On Pin 1 Increase This Resistance Or Decrease This One NOTE Only +5 used for regulation

19 Resistance Changes It’s Easier to Lower Resistance Than to Increase It… Just Put Another Resistance in Parallel Lowering Resistance Between Pin 1 and Ground Will Decrease Voltage (Pin 1). Regulation Will Immediately Adjust Voltage Back to Reference by Raising Output Voltage.

20 A Simple Spreadsheet Helps Determine Resistance Values for 13.5 volts Note: After the presentation an additional slide was added to further explain this spreadsheet (Next page). Also if you double click the picture of the spreadsheet it will open Excel so you can look at the formulas. Finally the spreadsheet was changed so you can enter a different Reference voltage and all values will change accordingly. R2 WHAT IF? R Yellow Indicates numbers that can be changed R1C R2 WHAT IF?

21 Additional Notes on the Resistance Spreadsheet This spreadsheet was developed for a single power supply that had a reference voltage on Pin 2 on the TL494 of volts. With the spreadsheet changes you can now change the reference voltage in the cell in the bottom left. The drawing at the left was the one that was used in the previous slide. The first section only used R1 the second section used R1 and R1a in parallel. The first part in each sections computes the current for R1 and gives the exact R2 that would be needed for 13.5 volts. The WHAT IF part just gives you the chance to put a standard value in for R2 to see what the voltage output would be. This was intended to just be a quick example of how a spreadsheet might help you compute resistance for one supply. The TL 494 will try and maintain the reference voltage by raising the output voltage. This “constant” is used to find the current that must flow through R! at this voltage. C = 1.254/R1 Only in the parallel example.

22 Temporary Wires Test Resistance Adjust Resistance for 13+ volts Solder in Place

23 What to Expect? Voltage on Pin 1 Will Stay the Same, Output Voltage Will Change. You May Not Get to Exactly 13.8 Volts Volts is Close Enough. Check Voltage/Regulation Under Load. Output Voltage Varies by Several Tenths

24 Note… You Don’t Have to Solder Resistor to the Bottom of the Board There Are Usually Two or Three Resistors in Parallel to Adjust Voltage. Just Replace One or Two of Them This is How I Modified Ken’s Supply for 12 volt Regulation 12 v

25 What Next? Mount 5 Ω Resistance Between Red and Black Lead (In Front of Fan if Possible) Install Binding Posts Use Several Yellow and Black Leads AC Switch Installation Note: Ken’s Supply was Modified for 12 volt Regulation Only and the Supply Wouldn’t Output Until the 5 Ω Resistance was Added to the 5 volt Output

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27 Lessons Learned Watch What You Touch Just Because You can Increase the Voltage Doesn’t Mean You Have Regulation Make Sure You Know How to Power Up the Supply Getting the Cover Off of the Supply Can be a Challenge ¼ Watt Resistors are Hard to Read… If You Can Find the Resistors From Pin 1 to Ground You Can Just Put a Resistor in Parallel With Them Most of the Time Some Supplies Are Almost Too Full to Modify

28 Overhaul Ideas… Convert for vdc Only Change Regulation to Monitor Volts Move High Power Diodes from +5 volt Side to 12 volt Side of Supply Add a Volt Meter Move Fuse to Front Panel

29 Parallel Operation I Put Two of the Supplies in Parallel and Tried My Old HT and the 60 Watt Amp The Voltage Only Dropped.1 Volt Under Full Load It Dropped Almost One Volt With Just One Supply Turned On You Should Monitor Voltage During Use Note: With Only One Supply Turned On (In Parallel), Both Fans Run

30 Created by KØGL