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R.E.E.Robinson VK2ILV
7 Roland Ave. Wahroonga 2076
Sydney, N.S.W. AustraliaAn inverter power supply suitable for valve radios is described.
This inverter uses a small single sided Printed Circuit Board (PCB). Use the overlay to position the components. Test all the resistors, diodes, transistors, and capacitors before using them. The transformer and Integrated Circuit (IC) are difficult to test, so just use them and hope for the best.
These instructions assume that the inverter will use a 9 volt battery and be intended to supply B+ in a radio. The inverter is activated by the filament current, which is controlled by the radio’s ON/OFF switch.
CIRCUIT:
The inverter is based on the TL494 integrated circuit. This is a switching regulator that will operate with an input voltage of between 7 and 40 volts DC. This input voltage is supplied to the IC through a reed relay. The reed in normally open. When filament current from a valve radio passes through a coil, the magnetic field makes the reed close and energises the inverter. The oscillator starts slowly (over several milliseconds) as C1 charges through R4. If this is not required, omit C1 and replace R4 with a wire link. The oscillator frequency is set by C2 and R5 and runs at 100 khz, for efficiency. The oscillator saw tooth waveform can be seen at pin 5 on the IC using an oscilloscope. The 2 internal switching transistors drive 2 external switching transistors which switch the battery voltage into a transformer. The secondary of the transformer is rectified and provides a DC voltage. The output frequency is high, so ordinary silicon diodes are not suitable and schottky diodes which switch fast and have a low voltage drop are preferred. Similarly, the output capacitors have high frequency currents, and so special capacitors are required also.
The output voltage is fed back to the error amplifiers and is compared to a 5 volt reference. This is done with a simple voltage divider consisting of R1, R2 and R3. The 5 volt reference is applied to the error amplifier, and the output voltage divider is also applied to the error amplifier. The regulator adjusts until the inputs match, and so regulation occurs. It regulates by adjusting the time that the output transistors are turned ON. If the voltage is too low, they are turned ON longer, and if the output voltage is too low, they are turned ON, but for a shorter duration. The voltage divider is used to set the output voltage. It must be between 0 and the maximum output voltage of the transformer. The step up ratio of the transformer determines the maximum output voltage. I have used R1 and R2 as a series combination to set the voltage, as I could not get exactly the desired output voltage using preferred values.
Read more source :
http://www.shlrc.mq.edu.au/~robinson/Information/Inverter_4W.html
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