Universal D.C./D.C. Inverter

Warning: This circuit can produce very high Voltages

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 controlled 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 multivibrator. 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 273—1352A and the Halldorson B5—859. 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 overheating. 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 2C120v to 18v CT
2 amps
1uf1962 ohms148v
1.25 amps
Radio Shack 273-1515C Figure 2D120v to 18v CT
2 amps
1uf25 ohms25v
2.39 amps
Radio Shack 273-1515C Figure 2E20v to 18v CT
2 amps
1uf25 ohms-22.3v
2.14 amps
Radio Shack 273-1515C Figure 2F120v to 18v CT
2 amps
1uf25 ohms21.9v
2.1 amps
Radio Shack 273-1512B Figure 2C120v to 25.2v CT
2 amps
1uf650 ohms97.5v
1.5 amps
Radio Shack 273-1352A Figure 2C120v to 12.6v CT
1.2 amp
.22uf1960 ohms166v
1.71 amps
Radio Shack 273-1352A Figure 2C120v to 12.6v CT
1.2 amp
.22uf1960 ohms166v 85ma1.71 amps
Triad F-40-X Figure 2C115v to 26.8 CT
1 amp
1uf650 ohms79v
1 amp
Halldorson B5-859 Figure 2C117v to 5v CT
3 amps
.68uf9740 ohms536v 55ma3.13 amps
Harmon Kardon 60+ watt audio output Figure 2B6K plate to plate to 4,8,and 16 ohms1uf1966 ohms282v 143ma4.1 amps
Harmon Kardon 60+ watt audio output Figure 2A6k plate to plate to 4,8,and 16 ohms1uf491 ohms133v 270ma3.83 amps

Electronic Parts List

PartValueRadio Shack #Comments
C1 & C2.22uf272-1070Use for transformer 273-1352A
C1 & C2.68ufNo RS #Use for transformer B5-859
C1 & C21uf272-1055Use for all other transformers
D1 & D236v 1watt Zener2ea 276-564 15v Zener, 1ea276-561 6.2v ZenerConnect all 3 in series
F14amp power filter270-030A
Q1 & Q3 TIP31276-2017
Q2 & Q4 TIP42276-2027
R1 & R2820ohms 1watt
R4,R6,R7, & R8220ohms 1/4watt
R3 & R53900ohms 1/4watt
T1Select to provide desired output
Output rectifiersSee text
C4 & C5 1000uf 50v 272-1047
C3Capacity 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.