Why we use PNP transistors in the power isolation circuit?

Why PNP transistors are used in the isolation circuit. The signal is given first optocoupler transistor PNP power before giving it to relay. Why is this so?

It depends the polarity of your power supply. If the voltage is reversed, then you will need to use NOS. Most people are positive thinking, so PNP whereas it should work with both voltages reversed. Positive thinking should be higher than negative (which is mostly the reference mass) has to think the Western culture to be positive and negative good to be bad. There is no law of physics who says you can not cancel any and everything works that way. In fact, as well as all telephone networks plants work is say in the negative voltages. For telephone network, which operates at -48 volts, the laws of physics apply. Big batteries have the positive the housing that contains acid, so a battery that is on the ground should have the positive terminal to ground. In fact, car batteries should be worked this way, but culture and Western thinking ill-logic prevailed. Also look for most batteries, especially button-type, the positive is the case of containers.

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