Reply to "New Thread (From Mtg itad) & Isolated Rail Detection Discussion"

stan2004OGR Forum Member

@barnun posted:
... The first iteration was using 1K resistors on the AC Line side (R1A & R2A) and failed the finger test (man, were they toasty). Subsequently tried 1.5K, 2K, 2.7K and wound up using 3k.

Right. With, say, 18V on the track, the opto diodes only require about 1V. So the resistor is dropping about 17V.

Power (Watts) = Volts x Volts / Ohms = 17 x 17 / 1000 = 0.3 Watts. Those look like 1/4W resistors? But whether 1/4W or 1/2W they're still burning 0.3 Watts which can make even a 1/2 Watt resistor toasty!

At 3K, the resistor is only burning 0.1 Watts which is fine for a 1/4W or 1/2W resistor.

12/31/214:51 PM

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