"Battery Radio Power Supply"
|Battery sets from the late 30's through the 40's have not been very popular with the majority of radio collectors because the batteries are no longer available, subsitute batteries are very expensive - and power transformers suitable for building a power supply are next to impossible to find these days. This is changing as AC powered sets of the era become more and more scarce - so a solution to the power supply problem prompted digging into the junk box - and seeing what could be found. |
This power supply is designed for battery sets of that era that use 1.5Volt filament tubes - and usually a 90V B+. The parts can be scrounged together out of a fairly average junk box - but one could also go to Radio Shack and buy just about everything needed (they don't carry the B+ filter caps).
|Looking at the schematic - you can see how we solved the transformer problem. Since the average battery set uses less than 20ma. B+ two filament transformers wired back to back (i. e. secondary to secondary) will provide somewhere between 90 and 110 volts at usual working current (after rectifier and filtering). Tapping off of the secondaries with a full-wave bridge, filters, and an LM-317 regulator IC - a very accurate and stable 1.5V filament supply is provided. As noted - you can get almost everything for this supply at Radio Shack- or a well-stocked junk box. Radio Shack doesn't have 6V transformers - so the parts list shows 12V units. But any voltage can be used between 3V up to 24V - higher might exceed the 317's maximum rating - just so the secondaries are the same voltage. There doesn't have to be a center tap - it just so happens the Radio Shack 1A transformer has a center-tap - so it's used just to improve efficiency.|
|Parts List for 1.5V filament and approximately 90V B+|
|T1||120V Primary 12VCT 1A secondary|
|T2||120V Primary 12V 300ma secondary|
|D1 - D5||400V 1A|
|C1, C2||30ufd 150WVDC|
|C4||.1 ceramic or 1ufd Tantalum|
|R1||250 Ohm 1/4W|
|R2||47 Ohm 1/4W|
|R3||240 Ohm 1/4W|
|Don't use a larger value cap for C5 - it might cause a turn-on spike that would be un-friendly to the filaments. In fact - you can leave it out all together - it improves the 317's ripple rejection by roughly 40db - which is a bit of overkill for a filament circuit. |
This circuit can be adapted - for use with older sets if desired. Change the value of R2 to say - 380 Ohms (330+51) - and it will power a UX/UV199 set. The calculation for R2 is the standard formula for the 317: Vout = 1.25(1+(R2/R3))+(.0001(R2)).
By using a beefier T1 - (say 12V at 6 Amps), beefier diodes and a different regulator (say an NTE 932) you now have a power supply that can supply a battery set using a flock of 01's... be sure and use the full 12V secondary - rather than the center tap.
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