Chris said:
Hi, The1930s. A bit of a problem here -- older transistor radios used
the low voltage AC secondary for the lamps, not DC. That means you'll
have to drive your LEDs with a series diode, too.
You can simply run two LEDs back-to back. If you have enough voltage,
you can run two strings of two diodes (4 total) back to back with a
smaller resistor, and get more light for the same power. LEDs are
current-sensitive devices - the resistor is there to limit the current,
but the voltage dropped by the LED may vary a bit - ideally, you might
want to measure the actual voltage dropped by the specific part in hand
when adjusted to the correct specified current - use a potentiometer and
a milliammeter, and start with a high resistance on the pot (too little
current is fine, too much current is a one-time magic-smoke-releasing
event. Then stuff in a resistor just a bit larger (for a safety factor)
than the reading on the pot when you have the pot adjusted for the rated
current. If you have 6 volts, you should certainly be able to run 2 red
or yellow LEDs in series.
You might get a better color using a red and a yellow or green. Depends
what color you want...
The ones I presently have in stock list a typical forward voltage at
20mA of 2V and 2.2V, with maximums of 2.5V and 2.6 V - so if you have
6.3V available, pick a resistor to drop ~2V at 20 mA and it should work
to drive 2 in series. ie, 100 ohms, for that example. Your parts may
vary - typical circuits are designed to underdrive the LEDs to keep them
safe with part variations (ie, 270 ohms from 5 volts in series with a
single LED.)
6.3VAC~--------
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\
/
\ 100, 110, 120 Ohms (progressively dimmer and safer)
/
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-------
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\/ -- Back-to-back LEDs
-- /\
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|------| (connection here keeps reverse voltage low)
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\/ --
-- /\ Back-to-back LEDs
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