Clock Radio
ICF-C713 Power Circuitry
I’ve been poking at the power circuitry on this clock radio. I’m not totally sure how it works but it’s different and interesting.
You would assume the whole thing works on one voltage, probably 5V, but this design actually produces three (or maybe four) voltages from a 6VAC input using a single regulator and a few other parts.
The first trick is it uses half wave rectification in both directions, producing both -8.5 and +8.5V DC. Remember, AC is measured RMS but the rectified DC voltage comes from the AC peak voltage, which you get by multiplying the RMS voltage by the square root of two.
Then it pulls a switcheroo - it uses -8.5V as the ground reference and regulates the neutral with a plain old linear regulator, producing a 5V rail. The neutral then becomes an 8.5V rail. And the forward rectified 8.5V becomes double that at 17V. The 4.3V comes from regulation of 8.5V through a cheap transistor acting as an emitter follower following a zener diode - basically a poor man's linear regulator. That’s a dirt simple method without a lot of accuracy so apparently that’s not an important rail. Maybe it's a holdover from an earlier radio design?
I’m not sure why it regulates a 4.3 and a 5V rail. The 8.5 and 17V rails are probably to drive the LCD.