test fixture ground

[wang]

Dear Xseadog,

Thanks for your help!

Here is the diagram that provide the information?

I look forward to getting your answer

[xseadog]

Hi

Pt1: Current meter:in present setup is this not a short circuit to ground? Current meters usually in line on positive terminal

so you can actually measure current. Can you not read this from you power supply?

Can you adjust current level on your power supply ?

Where on diagram is actual  power supply connection ?

You should connect power supply negative terminal to ground point. Check that you have a common ground  ie conductivity  test between ground points and terminal.

NB ENSURE THAT POWER IS NOT TURNED ON  IN FACT DISCONNECT NEGATIVE  TERMINAL FROM POWER SUPPLY.

Chow

xseadog

[tbob]

Actually, in this fixture, the current meter will read the current going through the UUT if the relay is opened.  Also, since the output of the diode bridge has one side connected to the ground symbol, and all voltage regulators also are connected to the ground symbol, and the current measurement is also connected to the ground symbol, then there is nothing wrong with this fixture.

Mr. Wang:  This post is directly related to your other post entitled "Ground".  Please stick to one post with the same title to avoid confusion, and to help you get the best responses.

[xseadog]

Hi Tbob
regarding your response, one is assuming that UUT does not have any 'splitting circuitry 'which draws currents.
So in present position possible to record 'false' value.
By having meter at positive terminal all current drawn is noted.

Also, is it a common ground if ammeter is on negative terminal ? You have a small voltage potential.Thus it can not be
a'common ' earth path.

chow
xseadog

[tbob]

Hi xseadog.  From your name, I would guess you used to be in the Navy?  Or maybe a shrimp boat captain (lots of those where I come from).

The ground does not have to be earth ground.  Many test systems use the power supply negative terminal for ground.  As long as all measurements are made with respect to the chosen ground, there will be no error.  When measuring UUT current, both of the following methods work:

V+ ----> Ammeter ---> UUT ---> Ground.

V+ ----> UUT ----> Ammeter ----> Ground.

The circuit shown by Wong is an example of the second method.  I did not see any connections to the UUT that would cause any current to go anywhere else in the ciruit besides ground.  His method will produce an accurate measurement.  I admit it is a lot more common to put the Ammeter before the UUT as in method 1 above.  That way, if some part of the UUT is shorted to ground other than at the point of connection to the Ammeter, you will see the extra current.  With method 2, you won't.  So method 1 is the preferred method, as you have noted.  If possible, I would suggest that Wong change the schematic as such.  The relay could be moved to bypass the Ammeter, but this is hardly necessary since the voltage drop across and Ammeter is miniscule (uV or even nano Volts).  The best practice of course is to use a power supply that allows you to read the current drain from the supply itself.  Then no ammeter is needed.

[LV_Pro]

The relay on the left is already positioned to bypass the ammeter, for what reason I can't guess. As tbob says, other than the parallel path provided by the relay, putting the ammeter in the ground side will give the same reading as putting it in the "hot" side.

P.M.

[tbob]

Seems to me that the reason for the relay is if the ammeter were ever removed, then you could provide a path to ground.