Can I use a Lab Bench PSU and a Function Generator at the same time?

[codingwithethanol]

james_s,

"It doesn't seem like you're listening to what anyone is saying"

I followed xavier60's design to the tee after he made clear the circuit design. This whole thread was really the result of me not wanting to blow things up by shorting my equipment through earth ground as I have learned from EEVBlog and other enthusiasts. The posts here have been quite helpful in elaborating on the nature of the connections in the equipment.

"you're just flailing around and then reporting that it doesn't work"

Let me specify. My PSU went into overcurrent protection mode as I tested the circuit, and so I shut it down. I noticed that after I had, I was still seeing output from the opamp on my oscilloscope. This means the PSU was never powering it in the first place, and the oscilloscope was simply seeing the Function Generator output through the feedback resistors. Examining my PSU, I found that Channel 2 had been shorted and was reporting a fluctuating 4-5mA current when nothing was connected. I presume this to be an indicator that something had gone wrong with the protection circuitry, and so I am now limited to one channel until I can figure out what got burnt and replace it. In the meantime, I will be working on other things until I have a better understanding of what went wrong.

[xavier60]

It's not easy to damage a well designed power supply as that one appears to be.  Have you had it for long?
It may have already had a fault.

[codingwithethanol]

Yeah i've had for only a few weeks or so, didn't have a reason to use the second channel until recently but I thought I had tested it earlier.  It's actually quite likely it was already damaged or simply waiting to fail since I purchased it second hand for quite a reasonable price.

[codingwithethanol]

As an addendum, I recently opened up my function generator to see what was inside and found that the BNC port grounds were also earth referenced. I checked a few other threads here and here are my main concerns.

1. Removing or simply taping over the ground pin on the power cable on the Function Generator would solve the issue of blowing up my oscilloscope but some people said it would probably lead to inaccurate operation or HV developing across the unused connectors which would obviously be a safety concern.

2. Always taking care to where the ground lead is placed is another solution but this limits what you can probe.

How can I make sure to protect my equipment and stay safe while still being able to probe around and experiment freely?

[Zero999]

No, don't remove the earth connection to the oscilloscope. It's very dangerous, as it could float at a high voltage and cause electrocution.

Most power supplies have an adjustable current limit. Check it isn't sat to a very low level. Disconnect your op-amp circuit and connect a 1k resistor. The power supply should be able to supply 15V to 1k without current limiting. Check the manual for the power supply or search the net for the make and model number.

[xavier60]

Having the function generator mains grounded and  the CRO mains grounded attached to the op-amp circuit won't cause any damage even with the 0V point on the PSU also mains grounded, assuming that everything is wired correctly.
It's best practice to leave the PSU totally un-grounded unless there is a good reason to do so.
Having more than one piece of equipment mains grounded does cause ground loops which usually adds a small amount of interference to displayed waveforms.

[David Hess]

As an addendum, I recently opened up my function generator to see what was inside and found that the BNC port grounds were also earth referenced. I checked a few other threads here and here are my main concerns.

1. Removing or simply taping over the ground pin on the power cable on the Function Generator would solve the issue of blowing up my oscilloscope but some people said it would probably lead to inaccurate operation or HV developing across the unused connectors which would obviously be a safety concern.

That can be a solution under some circumstances however the common mode capacitance between the BNC ground and Earth ground still exists.  This can be several hundred picofarads.

Some function generators have a fully isolated output just like some oscilloscopes have fully isolated inputs.

2. Always taking care to where the ground lead is placed is another solution but this limits what you can probe.

What I have done in some cases is to build the equivalent of an instrumentation amplifier into the circuit which will accept the grounded test signal and level shift it to the point where I need it applied.  Or I may do the opposite to provide a grounded signal to the oscilloscope.

[codingwithethanol]

I learned something fantastic recently, where you can probe voltages without even using the ground clip, completely solving the problem. The only con is that you cant display the output of 2 things at once, but I feel that that is far outweighed by the positive of worry free probing anywhere in a mains earth grounded circuit. I learned this fantastic trick from a youtube video and I havent seen this mentioned anywhere else, so here it is:

https://youtu.be/VWodjUSkYVE?t=222

[spiff72]

I don't know if you ever got this working, but in looking through your thread, I see that you are trying to do the same thing that I was just doing.

Please check this thread out:

https://www.eevblog.com/forum/beginners/agilent-e3646-power-supply-with-opamps-(for-dual-rail-power)/

My thought is that you need to take the "earth" ground connections out of your circuit.  These are what may be confusing you here.

I posted a schematic of how I connected this up on my Agilent power supply (which doesn't have any earth ground connections on the front panel - but does have them on the back).

I turned on tracking mode on my supply, which lets you adjust voltage for both channels at the same time, and then connected a jumper from the "-" terminal of Channel 1 to the "+" terminal of Channel 2.  This puts connects them in series providing a total of 20v, and I connected another jumper from that series jumper to my breadboard to serve as my "common" reference point.  This is the zero voltage point that some have called ground (including me).  Calling it "common" is better because it doesn't introduce confusion about earth ground.  Since the voltage from each channel is 10v, the common voltage can be considered 0, Channel 1's "+" terminal serves as the +10v rail, and Channel 2's "-" terminal serves as the -10v rail.

The only difference is that I have the op amp wired up as an inverting amplifier.

Also note that the ground clip of each scope probe I used, AND the ground clip of my BNC-to-alligator cable from the Function Generator are all connected to that zero reference point. 

Finally, when you set up the power supply, even if you don't have tracking mode set up, just set both channels to 10v, with reasonable current limits (I think mine was set at 0.5 amps, but the circuit only draws about a milliamp or two from each power rail for my 741 op amp).

[codingwithethanol]

Yes I had in fact wired it up this way before, but I discovered that my second channel was shorted. I had gotten it recently and someone said that it could have already been in a state of disrepair when I purchased it which is highly likely. Anyways, I'll soon open it up and do a thorough teardown and replacement of any malfunctioning components.