Re: Relay Arc Supression Circuit

<richard.bair@xxxxxxxxxxxxx> wrote in message
news:1142364280.496032.325420@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
The 3.3 is my punt plan for the current artwork or to populate a 0 ohm
resistor where the zener is.

Here's what I'm not understanding...with the 5.1 zener and per my
o'scope, it appears the worst case reverse bias on the LED is 7.5 as I
aforementioned and I see about 3V across the series resistor, Rs. So
this sums to ~10.5 Vdc. So does the 5V rail + 5.1 (zener) + 0.7
(diode) create a voltage of about 10.8 reverse bias? This seems wrong
to me as the 5V rail is fixed but empirically the 10.5 is close to the
10.8. So, in my stupidity, I changed the value of Rs (reduced it as I
could afford to have more current run through the LED in normal
operation) but duh, the I-V charactereistics of the LED in reverse bias
pretty much fix the voltage drop across Rs. So I think your statment
about 5.1 + the diode drop at worst current must be what is yielding
the ~10.5 Vdc I see across the LED and Rs. Is that correct? Will the
diode/zener deviate that much from the nominal 5.1 + 0.7? Also, in
your experience, if this relay swithces slowly, would a diode by itself
be adeuqate? It only runs the coild of an offboard realy and is
switching 24AC to this other realy. I know some of these are basic
questions but I appreciate your input.

Thanks again for your time.

Series resistor? 10.8V?
Your coil is in parallel with a back-to-back diode/zener pair with no series
resistor.

Your LED is in parallel with the coil which is charged to give nearly 5V
across your coil and across your LED/330ohm combination and discharged to
dump the coil energy and give you concerns about LED reverse bias. The
activity is all relative to your 5V rail, not ground.

If you see 3V across the 330 ohm resistor in series with the LED, this is
the forward biased conducting side of your situation, not the discharge of
the coil where the LED reverse bias would happen. When reverse biased, the
resistor voltage should be nearly zero (0 relative or the same voltage as
your 5V rail).


.

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