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Computer Power Supply

Amateur Radio ConversionFor

Cheap +12 vdc PowerMarch 17, 2003

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AdvantagesSupplies are Readily AvailableThey are Inexpensive ($1)50 watt Radios Only Need 10-15 AmpsNew Supplies Have Higher Wattage

100 Watt HF Rig PotentialOr Parallel Supplies

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Some Electronic KnowledgeI = E/R P = E*ISeries/Parallel ResistanceResistor Color Code

Volt Ohm MeterGood vision!!!Persistence

No Standard Design

Requires

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Resistor Color Bands

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With a Little Patience it’s a Lot of FunNo Rocket Science RequiredYou 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!

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Scope of PresentationDescribe Quick and DirtyDemonstrate 13.8 vdc ModificationBrief Description of In-depth ChangesBuild Comfort Level

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AssumptionsPower Supply Uses TL494 to Control OutputTL494

Pin 2 is Reference VoltagePin 1 is Sample of Output

Seems to be the “Norm”

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Reference Voltage

From Supply Output

Sample Regulation Circuit

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+ 12 vdc + 5 vdc

39 K 5600

1 K POT

3300

a11

a22

3a3

4a4

b1

b2

b3

b4

5

6

7

8

Vcc1

0

+ 5 vdc

4300

a11

a22

3a3

4a4

b1

b2

b3

b4

5

6

7

8

Vcc1

0

1450100 K 30 K

+ 1.254 vdc

+ 2.58 vdc

KØGL Supply

TL 494

TL 494

WØKRB Supply

Voltage Regulation Lots of d

ifferences

(1364.28 Ω)

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KØGL Supply WØKRB Supply

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TL 494

Blk = GNDRed = +5 vdcYellow = +12 vdcBlue = - 12 vdcWhite = - 5 vdc

VoltageAdjust

Pot

KØGL Supply

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Adjust Voltage

WØKRB SupplyTL494

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How To StartTake Supply ApartBlow Out DirtVerify Lead Colors (Red +5 vdc etc.)Connect two 10 ohm 10 Watt Resistors in Parallel Across the +5 vdc LeadsSeparate Wires and Power Up SupplyMeasure Voltage and Check Fan

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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!

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Once You Know It WorksFind TL 494Trace Foil From TL494 Pin 1Measure Voltage on Pin 1 & 2Draw Schematic of ResistorsCompute Parallel/Series ResistanceDoes Voltage on TL494 Pin 1 Make SenseIf so Decide How to Raise Voltage…

If Necessary…

BE

CAREFUL!

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+ 12 vdc + 5 vdc

39 K 5600

1 K POT

3300

a11

a22

3a3

4a4

b1

b2

b3

b4

5

6

7

8

Vcc1

0

+ 5 vdc

4300

a11

a22

3a3

4a4

b1

b2

b3

b4

5

6

7

8

Vcc1

0

1450100 K 30 K

+ 1.254 vdc

+ 2.58 vdc

KØGL Supply

TL 494

TL 494

WØKRB Supply

Voltage Regulation

Adjust PotIf

AvailableDecrease Voltage On Pin 1

Increase This

Resistance

Or Decrease

ThisOne

NOTEOnly +5 used for

regulation

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Resistance ChangesIt’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.

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A Simple Spreadsheet Helps Determine Resistance Values for 13.5 volts

3350 0.000374328 32714.59 33000 13.606841330 0.000942857 12988.18 13000 13.511142200 0.00057 21484.21 20000 12.6543300 0.00038 32226.32 33000 13.7944700 0.000266809 45898.09 33000 10.058685100 0.000245882 49804.31 1.2545600 0.000223929 54687.08 56000 13.7946800 0.000184412 66405.74 1.2548200 0.000152927 80077.51 1.25410000 0.0001254 97655.5 91000 12.665412000 0.0001045 117186.6 1.254

R1 R1a Parallel C R2 13.55100 82000 4801.378 0.000261 46888.1 47000 13.52923

Pin 2 REF Voltage: 1.254

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?R2 13.5

Yellow Indicates numbers that can be changed

R1 C

R2 WHAT IF?

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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 1.254 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.

+ 13.5 vdc

R2

a11

a22

3a3

4a4

b1

b2

b3

b4

5

6

7

8

Vcc1

0

R1aR1

+ 1.254 vdc

TL 494

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.

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TemporaryWires

Test Resistance

Adjust Resistance for 13+ volts

Solder inPlace

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What to Expect?Voltage on Pin 1 Will Stay the Same, Output Voltage Will Change.You May Not Get to Exactly 13.8 Volts.13.5 Volts is Close Enough.Check Voltage/Regulation Under Load.

Output Voltage Varies by Several Tenths

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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 IModified Ken’sSupply for 12 voltRegulation 12 v

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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 InstallationNote: 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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Lessons LearnedWatch What You TouchJust Because You can Increase the Voltage Doesn’t Mean You Have RegulationMake Sure You Know How to Power Up the SupplyGetting 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 TimeSome Supplies Are Almost Too Full to Modify

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Overhaul Ideas…Convert for +13.8 vdc OnlyChange Regulation to Monitor +13.8 VoltsMove High Power Diodes from +5 volt Side to 12 volt Side of SupplyAdd a Volt MeterMove Fuse to Front Panel

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Parallel OperationI 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 LoadIt Dropped Almost One Volt With Just One Supply Turned OnYou Should Monitor Voltage During Use

Note: With Only One Supply Turned On (In Parallel), Both Fans Run

30Created by KØGL