power supply interference?

[jim]

I'm hoping someone can clear up some confusion I have regarding the
behavior of my power supply. I have a instek power supply,and it has
in addition to the plus,minus terminals,a ground terminal between
those two terminals. Now I notice that when I only connect the pos
probe of my oscope to the pos terminal of the supply ,I get a fairly
large 60 cycle signal,similaly when I connect the ground probe of the
scope to the ground terminal and the pos probe to the pos supply I get
the same signal, when I put pos to pos and neg to neg ,no
interference. The issue is a nuisance because I'm using these
terminals to run an opamp, with the ground running to pin 8 on the
741. Any idea what's going on here? thanks for any help. jim

 

[Greg Neill]

I'm hoping someone can clear up some confusion I have regarding the
behavior of my power supply. I have a instek power supply,and it has
in addition to the plus,minus terminals,a ground terminal between
those two terminals. Now I notice that when I only connect the pos
probe of my oscope to the pos terminal of the supply ,I get a fairly
large 60 cycle signal,similaly when I connect the ground probe of the
scope to the ground terminal and the pos probe to the pos supply I get
the same signal, when I put pos to pos and neg to neg ,no
interference. The issue is a nuisance because I'm using these
terminals to run an opamp, with the ground running to pin 8 on the
741. Any idea what's going on here? thanks for any help. jim

Sounds like the ground terminal is floating. Look
for a break in the path from the terminal to the
"true" ground of the circuit.

What do you get when you put the ground probe on the
ground terminal and the positive probe on the negative
terminal?

 

[Peter Bennett]

I'm hoping someone can clear up some confusion I have regarding the
behavior of my power supply. I have a instek power supply,and it has
in addition to the plus,minus terminals,a ground terminal between
those two terminals. Now I notice that when I only connect the pos
probe of my oscope to the pos terminal of the supply ,I get a fairly
large 60 cycle signal,similaly when I connect the ground probe of the
scope to the ground terminal and the pos probe to the pos supply I get
the same signal, when I put pos to pos and neg to neg ,no
interference. The issue is a nuisance because I'm using these
terminals to run an opamp, with the ground running to pin 8 on the
741. Any idea what's going on here? thanks for any help. jim

I expect you have a single "floating" supply - the output of the
supply is not connected to ground. The "Ground" terminal of the
supply will be connected to the case, and to the safety ground wire in
the power cord, but not to the power supply output.

 

[jim]

Sounds like the ground terminal is floating. Look
for a break in the path from the terminal to the
"true" ground of the circuit.

What do you get when you put the ground probe on the
ground terminal and the positive probe on the negative
terminal?

Same thing,60 cycle. I get 60 cycle even when I just put the pos probe
on either pos or neg terminal without even hooking up the oscopes
ground probe. jim

 

[Greg Neill]

Same thing,60 cycle. I get 60 cycle even when I just put the pos probe
on either pos or neg terminal without even hooking up the oscopes
ground probe. jim

Your ground terminal is floating. That is, it's not
connected to the circuit.

 

[John Fields]

I'm hoping someone can clear up some confusion I have regarding the
behavior of my power supply. I have a instek power supply,and it has
in addition to the plus,minus terminals,a ground terminal between
those two terminals. Now I notice that when I only connect the pos
probe of my oscope to the pos terminal of the supply ,I get a fairly
large 60 cycle signal,similaly when I connect the ground probe of the
scope to the ground terminal and the pos probe to the pos supply I get
the same signal, when I put pos to pos and neg to neg ,no
interference. The issue is a nuisance because I'm using these
terminals to run an opamp, with the ground running to pin 8 on the
741. Any idea what's going on here? thanks for any help. jim

---
What you have is not a dual supply with a common ground, but a single
floating supply, either end of which you can connect to ground (the
ground connection). This will allow you to have a power supply with a
positive output referred to ground if you connect the minus terminal to
the ground terminal, a negative output referred to ground if you connect
the positive output to ground, or a fully floating supply if you connect
neither to ground. The ground terminal is connected to the power supply
chassis and also (probably) to mains ground/neutral. Use your ohmmeter
to find out for sure. If you want to use this supply to run a 741 what
you'll need to do is connect the power supply + to the the 741's + power
input, the supply - to the 741's - supply input, and make a
pseudo-ground for the signal input. You can do that by connecting a
couple of equal-valued resistors across the supply + and - inputs and
connecting the junction of the resistors to to the - signal input of the
741. It would also probably be a good idea to connect the junction of
the two resistors to the supply's ground terminal so all the rest of the
equipment you'll be playing with will be on the same page.