Universal D.C/D.C Inverter;   

  Warning: This circuit can produce very high                         
         Several years ago friend requested an inverter design for
         use in old tube car radios, This individual collects
         Nash Metropolitans and likes to keep the cars as stock as
         possible, this includes retaining the tube radios. These
         radios usually fail due to a shorted buffer capacitor, this
         part failure burns out the power transformer and vibrator
         All that is necessary to revive these radios is a working
         high voltage power supply.
         An inverter using mostly Radio Shack parts was designed.
         This inverter circuit is called "universal" because  it can
         used with almost any transformer with a center tapped
         winding.  No modifications to the transformers are necessary
         Many of us remember spending hours filing slots in the cores
         of transformers to provide room for feedback windings to
         turn filament transformers into flux oscillators.
         Because the frequency of oscillation of this circuit is
         controled by an RC time constant and not by core saturation
         as in flux oscillators there are no large current spikes       
         imposed upon the transistors and supply bus at switching times.
         Filament transformers and audio output transformers
         work well with this circuit, A 2 amp 18 volt to 120 volt
         transformer was used for the car radio power supply.
         With 14 volts in and a bridge rectifier and filter
         capacitor on the 120 volt side, the inverter output was
         148 volts at 75 ma. with a 1962 ohm load.

                                CIRCUIT DESCRIPTION
         Transistors Q1 and Q3 operate as a classical RC coupled
         multivibarator.  The frequency of oscillation is controlled
         by the RC time constants of C1  R2 and C2  R1.
         Resistors R4 and R6 provide a discharge path for capacitors
         C1 and C2 during the off cycle and limit the reverse
         voltage across the base emitter junctions of Q1 and Q3.
         thus preventing avalanche of the junctions.
         Resistors R4 and R6 also form voltage dividers with
         resistors R3 and R5. This voltage divider provides a small
         amount of bias at turn on to start the inverter.
         A small amount of power would be produced if a center
         tapped transformer were connected to the collectors of
         Q1 and Q3 The power output in this configuration would be limited
         by the small amount of base drive provided by the timing
         components C1 R2 and C2  R1.   Transistors Q2 and Q4
         operate as emitter followers, multiplying the collector
         current of Q1 and Q3 by the beta or current gain of Q2 and
         Q4. Resistors R7 and R8 pull the base emitter voltages
         of Q2 and Q4 to zero during the off cycle, this speeds up
         turn off and prevents excessive leakage. The Zener diodes
         DI and D2 prevent voltage spikes from killing the transistors.
         The transformer in an inverter can and will act
         just like an automotive ignition coil.  When a transistor
         in an inverter switches off it is just the same as opening the 
         points on an ignition system. The magnetic field will rapidly
         collapse producing a high voltage spike.

         This high voltage spike must be clamped at a voltage below
         the break down voltage of the transistors or the transistors will
         be destroyed. This voltage spike will not be clamped by a load 
         on the secondary of the transformer due to leakage inductance.
         The Zener diodes cause  the transistors to turn on at a  voltage                                                                           below the transistor break down voltage. This will clamp the                                                                                voltage until the energy in the spike is dissipated. A36 volt zener
         chosen for use in this 12 volt system.
         Zener diodes Dl and D2 should be soldered directly to the
         collector/base leads of Q2 and Q4.
         Transistors Q2 and Q4 must be mounted on a heat sink. The
         collectors of these transistors are connected to ground so
         they may be screwed directly to a heat sink with no
         insulating hardware. The collector of Q1 may be soldered
         directly to the base of Q2 and the emitter of Q1 to the
         collector of Q2. These leads match up if one transistor is
         placed with the collector tab down and one with the collector
         tab up. A Radio Shack filter kit was used on the D.C. input
         to the inverter. This filter performs several functions, it
         attenuates switching noise on the power supply line, it
         protects the inverter from high voltage spikes on the power
         supply line ( this is very important in automotive applications),
         and the filter capacitor exhibits a low A.C. impedance to the inverter.       
         The component values given are for an automotive application. 
         The voltages in a cars electrical system can vary from 12.6 volts
         with the engine off to over 15 volts with it running.  With the part
         values given in the parts list, the inverter will at start at 11 volts and
         is safe for operation up to 16 volts.
         If this circuit is to be powered by voltages higher than 16 volts
         some changes should be made. The voltage appearing across the
         off transistors will be twice the supply voltage so the voltage rating of
         all transistors should be at least twice the supply voltage plus 10 volts.
         The protective Zeners voltages ( D1 & D2 ) should be a
         minimum of 5 volts greater than twice the supply voltage and
         10 volts less than the transistors voltage rating. The value
         of resistors R3 and R5 should match the supply voltage.
         Calculate the value of R3 and R5 using the following : R=(V-.6)/.0029                                                                                    R=(V .6).0029 where R equals the value of R3 and R5 and                                                                                V equals the input voltage to the inverter.
         Where R equals the value of R3 and R5 and V equals the input Voltage.        
         This circuit may be used in a vehicle with a positive ground or with       
         the heat sink positive by making the following changes:
            Exchange the PNP and NPN transistors..
            Reverse the polarity of the Zener diodes..
            Reverse the polarity of the input filter capacitor..
            Reverse the polarity of capacitors labeled C5

                      ADDITIONAL INFORMATION

         Please note that with all transformers listed, the usual
         primary and secondary roles are reversed. The low voltage
         winding is connected to the inverter circuit thus becoming the primary.
         This circuit inputs much more voltage across the primary windings of T1
         than the transformers rating at its design frequency.
         A good example of this is the Radio Shack 2731352A and
         the Halldorson B5859.
         If the inverter is powered from 14 volts then almost 14
         volts will appear across one half of the primary.
         The oscillation frequency of the inverter must be raised to
         prevent saturation of the transformer core.
         The operating frequency of the circuit may be changed by
         changinq the value of C1 and C2. See the parts list.
         The component values listed in the parts list will provide
         a switchinq frequency high enough so none of the transformers
         listed will be saturated when running with a 15 volt input.
         Power may be pulled from the primary side of the transformer.
         The primary side will provide plus 21.9. 25 and minus 22.3
         volts if connected as shown in figures 2D, 2E, and 2F.
         The total power output of the inverter should be limited to       
         50 watts or the wattage rating of T1         

         This inverter drives the transformer with a square wave which
         has very fast rise and fall times.  Some old 60 Hz transformers
         may run hot when driven with a square wave due to iron losses.
         All Radio Shack transformers listed and audio transformers seem
         to have " fast enough iron " to run at their rated current with out
         Figures 2A through 2F show the many output circuits which may be used.
         Figures 2A through 2C take the output from the secondary
         winding of the transformer and will produce some high voltages.
         The outputs from circuits 2A through 2C are  isolated from the inverter                                                               and battery circuit so that either the plus or minus output may be grounded. The
         ground. Outputs from circuits 2D, 2E, and 2F are referenced to inverter ground                                                              or heat  sink.   When used as a power transformer an audio transformer
         Audio transformers may be operated at a power rating twice its audio rating.
         Capacitor C3 should have a rating of 8 to 47 uf and a working
         voltage of at least 10% greater than the expected output.
         The diodes used in Figures 2A through 2C should have a
         rating of 1 amp and a piv of triple the expected output voltage
         A bridge with similar ratings may also be used.
         The diodes used in Figures 2D through 2F should have a rating of 4 amps                                                                          of at least 6 amps and a piv of 100 volts.  Capacitors C4 and C5
         and a piv of at least 100v. Capacitors C4 and C5 should be 1000uf at 50v.

              Test results with various transformers.       Tests performed at 14 volts.  

 Radio Shack           273-1515C            Figure 2C 120v   to        18v     CT          2 amps    1uf 1962   ohms     148v         75ma 1.25 amps
 Radio Shack           273-1515C             Figure 2D 120v   to        18v     CT          2 amps    1uf 25        ohms 25v          1000ma 2.39 amps
 Radio Shack         273-1515C            Figure 2E 120v   to        18v    CT           2 amps    1uf 25        ohms -22.3v      892ma 2.14 amps
 Radio Shack            273-1515C             Figure 2F 120v   to         18v    CT           2 amps    1uf 25         ohms 21.9v       876ma 2.1 amps
 Radio Shack          273-1512B             Figure 2C 120v   to         25.2v  CT          2 amps    1uf 650       ohms 97.5v       150ma 1.5 amps
 Radio Shack           273-1352A            Figure 2C 120v   to        12.6v CT         1.2 amp                   .22uf 1960   ohms 166v       85ma 1.71 amps
 Triad                     F-40-X                 Figure 2C 115v   to          26.8    CT         1 amp    1uf 650      ohms 79v        121ma 1 amp
 Halldorson             B5-859                   Figure 2C 117v   to           5v       CT          3 amps    .68uf 9740    ohms 536v        55ma 3.13 amps
 Harmon Kardon      60+ watt                 audio output            Figure 2B 6K plate to plate to 4,8,and 16 ohms    1uf 1966    ohms 282v        143ma 4.1 amps
 Harmon Kardon     60+ watt                  audio output           Figure 2A 6k plate to plate to 4,8,and 16    ohms    1uf 491      ohms 133v       270ma 3.83 amps

                                                                               Electronic Parts List

Part Value Radio Shack # Comments
C1 & C2  .22uf 272-1070 Use for transformer 273-1352A
C1 & C2  .68uf No RS # Use for transformer B5-859
C1 & C2  1uf 272-1055 Use for all other transformers
D1 & D2 36v 1watt Zener 2ea 276-564 15v Zener, 1ea276-561 6.2v Zener Connect all 3 in series
F1 4amp power filter 270-030A  
Q1 & Q3 TIP31 276-2017  
Q2 & Q4 TIP42 276-2027  
R1 & R2 820ohms 1watt    
R4,R6,R7, & R8 220ohms 1/4watt    
R3 & R5 3900ohms 1/4watt    
T1     Select to provide desired output
Output rectifiers     See text
C4 & C5 1000uf 50v 272-1047  
C3     Capacity and voltage determined by use



                                                                                   Additional Notes:                                                                                  

Circuit 2F was intended to be a voltage doubler but it did not work that way because transformer terminals X and Z do not go negative only to ground.  To get more voltage from this circuit reverse the polarity of capacitors  C4 and connect the anodes of the two grounded diodes to transformer terminal  Y This should produce greater than 32 volts.  This has not been tested so I cannot say for sure it will work.

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