What do AC/DC coupling mean in oscilloscope?
When an oscilloscope is AC-coupled a big capacitor is connected in series with the input forming a high-pass filter; this effectively filters out any dc-component in the signal which can be very useful sometimes.
However, you need to a bit careful with AC-coupling if you are measuring low-frequency signals (a few Hz).
AC coupling means that the input to the scope is fed via a capacitor so any DC component on the signal being looked at will be removed.
E.G... If you are looking at a sine wave on the base and collector of a bipolar amplifier to measure the gain any DC sit on those signals will be removed and they will appear as if they sit equally above and below the zero line on the scope. It will be more convenient to view on AC coupling.
I many cases it doesn't really matter whether the scope is on AC or DC but the DC component might be a high voltage and would push the trace inconveniently well up or down. A high voltage may damage the scope.
You could use DC coupling to measure the DC sit whether there is a signal there or not.
When we need to measure the working voltage between primary and secondary windings of the transformer bridging the pri-sec circuits of power supply, which coupling shall we use? AC coupling or DC coupling?
You shouldn't be doing that at all when the mains power is applied to the primary. Mains power is very dangerous
If you want to measure leakage resistance, you can do that with a high voltage insulation tester.
You may be able to measure a small AC voltage from the open circuited secondary winding to ground. This is due to the capacitance between the primary and secondary windings.
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