Printed Circuit Board # 5
AWA Universal Battery Eliminator (UBE) PCB
Price: $38 including shipping to USA addresses
This printed circuit board component and the resulting battery eliminator are intended for experienced persons. The completed unit and associated components produce potentially dangerous voltages.
A three page column in the 2021 Spring AWA Journal reviews this item’s capabilities. The story is extracted into a PDF from the AWA Journal pages here.
The UBE printed circuit board (PCB) measures 6 ¼ by 6 ¾ inches with a maximum height of 2 ¾ inches. Three PCB-mounted transformers along with rectifier and regulator circuitry develop the “A”, “B” and “C” supply voltages. The transformers can be configured for either 120 or 240 VAC operation by installing the appropriate on-board jumpers. As an option, the builder can use off-board chassis mounted transformers if desired. AC mains wiring, fusing and switching are left as an exercise to the builder.
Each supply is isolated electrically from the others as well as chassis ground, to allow maximum flexibility in connecting to a wide array of vintage radio sets. Each supply is also protected against momentary short circuits to its negative rail. However, care must be taken to prevent voltage-to-voltage shorts.
Another important feature of the AWA UBE PCB is the ability to customize the power supply to fit individual needs. The UBE has a number of “B” and “C” output voltages in multiples of +22.5 and -4.5 volts. However, some early radios required only +90 volts plate and 1.25 V filament supplies. For such cases the “C” supply components can be left off, and the “A” and “B” transformers downsized to reduce cost and weight. A simple resistor change in the “B” regulator can be made to lower the +180 V to +90 volts. The rest of the “B” output components associated with the +135 V and lower output voltages can be left off of the board.
The PCB can be mounted inside a project enclosure with terminal strip connections or within the radio cabinet itself, with wires connected directly to the appropriate PCB pads. A line cord, fuse and on-off switch are needed to complete the installation.
The “A” supply can provide 1.25 to 6.0 volts DC at up to 4.0 amperes continuous for the set’s filament supply. The voltage can be adjusted to any value between 1.25 and 6.0 by means of a precision multi-turn potentiometer.
The maximum continuous current stated above is by the temperature of the LM1084 regulator using the on-board heat sink. The current value should be derated if the supply is mounted inside the radio cabinet. Table 1 shows the maximum recommended filament current versus temperature for the most popular filament voltages.
Table 1. Maximum recommended filament current versus temperature.
The continuous current value could be increased by using an off-board series pass transistor. Connection to the transistor can be made via terminals E7, E8 and E28, with a trace cut between E8 and E28 necessary to complete the change. The builder should be advised that further current limitations will be encountered, namely T1, C1 and the circuit wiring itself.
A transzorb diode can be connected across the “A” supply output to protect 6 volt tube filaments against a regulator failure. For situations where only 1 V filament tubes are used, three 1N4007 diodes can be installed for protection. Short circuit limiting is built into the regulator IC.
The “B” supply provides voltages of 22, 45, 67, 90, 135 and 180 volts DC. Triad’s workhorse VPS230-110 transformer powers the bridge rectifier for this circuit and is connected to the PCB using hookup wire. The +180 volt source is developed using a LR8N regulator with series pass transistor, while the lower voltages are derived from a string of Zener diodes.
To minimize power consumption, TIP50 emitter followers on the Zener regulator string provide a very low impedance source of current only when a load is connected. With this configuration the total Zener dissipation is about 0.7 watts as contrasted with other designs, where dissipation ranges from 6 to 8 watts.
With the exception of the 180 V supply, each individual supply can provide up to 50 mA in a standalone configuration. Where multiple voltages are needed, the total combined current drawn from the 22/45/67/90/135 supplies must be limited to 50 mA maximum. If the 180 V supply is used, it can handle up to 80 mA by itself but the total current draw from all B+ supplies cannot exceed 80 mA. The limit is the heat sinking of the pass transistors, particularly Q1.
The “C” supply is derived directly from a Zener diode regulator string and provides voltages of -4.5, -9, -16.5 and -22.5 volts, all referred to terminal C+. Total current draw of 10 to 20 mA can be handled without significantly changing the output voltage.
A representative parts list and fabrication drawing will be sent with the purchase of the PCB. The parts cost for a complete battery eliminator power supply is currently about $80, half of which is the cost of the transformers, if all of the parts are purchased brand new. A reduced feature supply, such as the 90V/1.25V case mentioned above, will have a parts cost of about $50, again with half of the cost in the new “iron.” A well-stocked junkbox will lower the cost appreciably. Please note that the cost of the PCB is additional to these estimates. Your purchase of AWA PCBs helps to support the organization and its important mission.
As mentioned previously, the builder must provide a line cord, fuse, and on/off switch. A filtered power-entry module will help minimize interference from noise appearing on the AC mains. Choice of mounting in a separate enclosure or radio cabinet is also left to the builder.
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