EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: electricar on April 24, 2016, 03:32:28 pm
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Hi folks,
I made a little dynamic load based on Jay_Diddy_B's design:
https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/ (https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/)
My schematic is in the attachment. Today I wanted to add a BNC connector in order to be able to connect the dummy load to my scope. The inner pin of the BNC connector is connected through 2x 100R in parallel (50R) in series to the shunt (source of the FET) and the outer case is connected to ground.
The funny thing now is that if I connect my right part of the case (on which the BNC connector is mounted) to the rest of my case (which is also connected to ground) I get a much higher and unstable lowest current (the pot is set to minimum). You can see on the pictures that I made the connection simply through a solderwire.
Does anybody know how this can be? I mean ground is already connected to the case...
Thank you very much in advance!
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Despite both units have a common mains ground connection, small ground loop currents will create offset voltages that will upset circuit operation.
Your best bet is to isolate the dummy load power line ground connection and this will fix the problem.
One solid earthed ground is enough.
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Thank you for your answer! They don't have the same ground! The +12V supply input comes from an ac/dc adapter, the DUT voltage is an isolated power supply. Even if I connect the negative rail of the DUT to ground, the problem still exists off course.
You mean to isolate the ground connection of the supply input (+12V, in the upper left corner) from the case?
In this case it would still be connected through the rest of the circuit to the case (shunt ground, bnc ground...).
Even if I could manage to do this, what would be with an isolated power supply? In this case the case wouldn't be grounded or am I totally wrong?
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Anybody?