Old tube appliances use transformers usually with two windings at least. One sourcing a high voltage in the range of 125 to 350 V, with current rates between 10 mA and 300 mA, and a low voltage winding to power the tube filaments, sourcing 6 or 12 V and currents up to 5 A.
Our purpose is to assemble a power supply using one of these transformers. The power supply has the following characteristics:
- High voltage output between 100 V and 400 V, depending on the transformer.
- High voltage current between 10 and 300 mA, depending on the transformer
- Output voltage adjusted from zero to the nominal value
- Low voltage output: 8 to 15 VDC, depending on the transformer
-Low voltage adjusted in the range of 0V and the nominal value.
Depending on the appliance where the transformer is found, other components can be removed and used in this power supply. It is also possible to find transformers with more than two windings and then more voltages can be sourced by the supply.
We have to observe that today it is very difficult to find these transformers in common dealers, since they are not used anymore. If you find one, probably it will be sold for a very high price. Figure 1 shows a typical power transformer found in tube appliances.
The Circuit
The secondary windings of the transformers are used for sourcing AC currents to rectifying stages by using common diodes. Then, for the filtering, high value electrolytic capacitors are used. Figure 94 shows how a transformer can be used in this project.
The high voltage is rectified by two silicon diodes. These diodes should be rated at least to voltages two times the nominal voltage of the winding. If you have doubts about the voltage found in a transformer, power it and take a measurement using a Multimeter.
Filtering is done by an electrolytic capacitor. We do not recommend the use of an electrolytic removed from a very old appliance. The electrolytic capacitors loose their electric characteristics with time. Some years are enough to take out of any possibility of use for this kind of capacitor.
The capacitor charges with the peak of the rectified voltage, around 41% above the nominal voltage of the wound. So, a 350 V transformer can charge the filter capacitor with 490 V. After that, we have a resistor and another capacitor to increase the filtering. At this point, we find the first fixed output AT-1. This voltage has a maximum value of 1,4 times the voltage of the transformer.
For the variable output we use a 10 k ? linear wire wounded Potentiometer. Higher values can be used, like 22 k? or even 47 k?, but they must be wire wounded types. For the low voltage winding we use a diode bridge for rectification. The output of the rectifier stage is applied to a 1,000 ?F electrolytic capacitor.
The first low voltage output—B1 – is connected to this point. The value of the voltage found at this point depends on the transformer.
The next stage is a voltage regulator with a variable output, controlled by P1. With this Potentiometer we can adjust the output voltage in the range between 0 and a maximum given by the transformer‘s characteristics. We have a voltage selector plugged to the primary to select the input voltage according to the AC power line voltage. Figure 2 shows the complete diagram of the power supply.
The circuit is housed in a box where the terminal strips are fixed by screws. The components placement inside the box is shown by figure 3.
The heaviest component, the transformer, is kept in position by screws with nuts. The power transistor must be assembled on a heatsink. The other components are very common. You only have to follow the parts list indications. For the output, isolated terminals are recommended.
Testing and Using
Power the circuit on and, using a Multimeter, verify the voltages in the outputs. Move the knob of the Potentiometer to see how the voltages change.
When using the power supply, remember to follow the position of the connectors of polarized devices. This power supply is not protected against shorts or thermal runaways. Do not use this supply to power sensitive or critical devices that do not allow changes in voltages.
Q1—TIP41 or equivalent—Silicon power NPN transistor
D1 to D4—1N4002 or equivalent—silicon rectifier diodes
D5, D6—1N4007 or equivalent—silicon rectifier diodes
T1—Power transformer—valve equipments—see previous text
F1—1 A—fuse with holder
S1—On/Off switch
S2—Voltage selector switch (1 pole x 2 throw)
C1, C2—8 ?F—electrolytic capacitor—for voltage rates, see previous text
C3—1,000 ?F x 25 V—electrolytic capacitor
C4—100 nF—ceramic capacitor
C5—100 ?F x 12 V—electrolytic capacitor
P1—10 k?—wire wound Potentiometer
P2—1 k?—wire wound Potentiometer
R1—1 k ? x 10 W—wire wound resistor
R2—330 ? x 1/2 W—resistor (orange, orange, brown)
Other:
Terminal strip, box to install the device, power cord, output terminals, knobs for the Potentiometers, wires, solder, etc.
