- Jan 1, 1970
Winfield said:Fred Bloggs wrote...
Without an LED load the voltage soars to near Vin, which could be as
high as 45V. What happens the instant the LEDs are attached? First
a short high-current spike, followed by excessive LM317 voltage (if
only one LED is used), etc.? Hmm, I wonder how the LM317 responds
to a high voltage across its Vout and ADJ terminals for a short time?
I see there's a 6V zener in series with 50 ohms inside the chip, but
that resistor isn't a power resistor... Time for some experiments.
I am not getting that- ADJ terminal drops instantly to Vled, and the
42/2.7K voltage divider, through D2, makes for a small 0.016 x (48-Vled)
or 0.75V worst case drop across the 42 ohm or 0.75/42=18mA- so you have
the wideband emitter follower response in the '317. I'm pretty sure most
'317's can tolerate 48V for a few msecs to discharge Cf.