Purpose of the resistance between primary and secondary

Thread Starter


Joined Sep 30, 2018

What is the purpose of the resistance (1MOhm) between the primary and the secondary of a transformer. Is this for better knowing the isolation between the primary and the secondary ? It is also common to see capacitance between them.


Thank you and have a nice day,


Joined Aug 21, 2008
Sometimes it is done to limit the DC difference between the primary and secondary of a power supply, particularly where the primary side is grounded and the secondary side is not known to be grounded. Otherwise large voltage differences can develop that can result in strange effects such as cyclic static charge build-up on one side followed by arcing or other discharge means which can be undesirable, particularly in audio systems. A large value resistor is used to minimize the chance of ground loops.


Joined Oct 2, 2009
What you probably saw in a schematic diagram is not a resistor between the primary and secondary windings of the transformer but a large value resistor between the secondary winding (signal common) and the chassis ground (earth connection).