Ground

[wang]

Hi,

Thank you for all of your suggestion.

In order to make my schmatic clear, I have provide my clear schmatic again.

My quesitons are:

1) Is my ground in my test fixture just a ground, I mean not reference to the earth? Is that right?

2) If my power supply is connected to the earth, then my ground in my test fixture is whether refer to the earth?

I look forward to seeing someone replying this letter soon

[tbob]

Where is the ground symbol shown in your drawing connected?  To the metal chassis of the test fixture?  Is the negative terminal of the power supply output connected to the metal chassis also?  Instead fo the metal chassis, is the ground symbol and negative terminal connected to a ground plane on the fixture's PCB?  Your schematic is incomplete because it does not show what the ground symbol is connected to.  Normally, the negative output of a power supply is connected to a ground plane on the PCB, and all other components needing ground are connected to the PCB ground plane.  The ground symbol means a connection to the PCB ground plane.  Your AC outlet shows only two prongs, so earth ground is not present, and your system is not connected to earth ground, unless you have a wire going from your fixture to a metal plumbing pipe or other earth ground connection.  Forget the earth ground thing, you're beating this horse into the ground (pun?).  Take a DMM, measure the resistance from the negative terminal of the power supply output to a ground point on the fixture PCB or the fixture metal case.  If you get close to 0 ohms, your circuit is using the power supply negative lead as the ground point.  Earth grounds are usually used on the AC side for safety.

[wang]

Hi,

I took a DMM, measure the resistance from the negative terminal of the power supply output to a ground point on the fixture PCB or the fixture metal case.  I did not get close to 0 ohms, is my circuit is using what as a  the ground point? earth. but i did not connected earth

to my power supply

[xseadog]

Hi Wang

What actual resistance value did you note ? What value do you et if you short circuit probes on negative terminal?

Point 1: Connect your power supply to your mains.

Point 2: Do not switch on. I repeat -DO not switch on.

Point 3: Measure resistance between negative terminal and ground point on PCB.

Point 4: If you do not get zero or close to zero then goto point 5

Ponit 5: Proceed if point 4  not zero. Connect both probes to negative terminal !! Now it should be zero!!! Trace your negative circuit to discover where fault lies.( leave one probe on negative terminal).

chow

xseadog