| ??? 03/28/05 20:06 Read: times |
#90565 - Why the transformer? Responding to: ???'s previous message |
Erik Malund said:
I'd use a small transformer simply because its pins put the 220V further away from the low voltage circuitry than an optoisolator does. Maybe not a valid point, but I have seen too many sparks between probes and opto pins (and sometimes the probe has shorted 220V to 5V circuitry BOOM). The very fact that optoisos still come in DIL to separate the pins makes my case.
Again no "electronic" reason, just a better feeling. Erik I must agree, a probing accident could definitely result in an unpleasant situation. However, if the current limiting resistor is strategically placed (i.e., not immediately adjacent to the optoisolator), that should keep the voltage at the inputs of the optoisolator pretty reasonable. You could probably further enhance safety in this type of situation by splitting the current-limiting input resistor into two series resistors, one placed on the line input to the optoisolator, and the other placed on the neutral input to the optoisolator. By doing so, you could also avoid problems associated with accidental shorting of the optoisolator's neutral input to logic ground. In an ideal world, neutral and logic ground might either be at the same potential or completely isolated from one another. However, this dual resistor approach gives you some protection against a probing accident should neutral and logic ground not be at the same potential; that could readily happen in the absence of power supply isolation if the relay-type device is driving a large load and potential of the neutral rises due to I2R losses in the neutral conductor. --Sasha Jevtic |
