Can I use a Lab Bench PSU and a Function Generator at the same time? - Page 1
Forum section: electronics / beginners

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 25, 2019, 01:20:50 am
content: Long story short, I want to experiment with Op Amps; can I feed the rails with my PSU and the inputs with my function generator? I've read enough to know that hooking equip up wrong can lead to explosions and fire and all sorts of nasty stuff, so I just want to be sure that one wont back feed the other and fry it. Also how would you probe the output with an O-scope?

author: xrunner
author_rank: Super Contributor
timestamp: May 25, 2019, 01:23:32 am
content: codingwithethanol,Yes you sure can do that. Don't hook the signal generator directly to the power supply though.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 25, 2019, 01:25:47 am
content: Ok cool, just one question about the output, what would be its ground reference? I want to view the output on my oscilloscope. I would think it would just be the ground of the psu but theres feedback so im not sure.

author: xavier60
author_rank: Super Contributor
timestamp: May 25, 2019, 01:50:10 am
content: Quote from: codingwithethanol on May 25, 2019, 01:20:50 amYou are uncertain as to how to power the op-amp and what to use as the ground reference.Research "op amp single supply vs dual supply"I suggest single rail for starters. You could wire a resistor in series with the output of the function generator to protect it, 1KΩ or higher.More: I see that the PSU is dual channel, I still suggest staying with single rail for now.

author: xavier60
author_rank: Super Contributor
timestamp: May 25, 2019, 02:04:03 am
content: I'm starting to change my mind. Although dual rail can be confusing for a beginner, single rail has its own messy complications.Anyways, show us a diagram of what you want to try.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 25, 2019, 02:19:14 am
content: xavier60,Front page:http://prntscr.com/nt2afuPinout:http://prntscr.com/nt2alqThis is the data sheet for the op amp I haveSketch:http://prntscr.com/nt2dbgThis is a badly drawn sketch of the circuit I want to create, so I can verify signal gain on my oscilloscope I changed the sketch to incorporate the suggested 1k ohm resistor

author: xavier60
author_rank: Super Contributor
timestamp: May 25, 2019, 03:03:18 am
content: Some of the pin numbers on the op-amp symbol are wrong. Don't use the offset pins.Where the positive and negative of the 2 PSU channels connect together, becomes 0V and also the ground reference for all measurements, also the ground connection for the function generator.Make certain that any output DC offset adjustment on the function generator is set to zero volts for now. Use a lower output level for now.I used Diptrace to draw the schematic. The limited use version is free to get. Or if someone knows of a more suitable drawing application.

author: xavier60
author_rank: Super Contributor
timestamp: May 25, 2019, 03:17:04 am
content: ±5V is the minimum supply voltage for the UA741. After the circuit is working, increase the supply to ±10V.

author: David Hess
author_rank: Super Contributor
timestamp: May 25, 2019, 03:48:35 am
content: Yes, sort of.Just be aware that the output of the function generator is not isolated so the BNC ground is the same as the chassis ground of the power supply. This is not a problem if the power supply output is floating which is common but there will still be considerable capacitance between the power supply output and chassis ground.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 25, 2019, 03:51:58 am
content: Ok, im going to run to the store and grab some snacks before it closes, and then im going to breadboard the circuit and test it. Results coming up in ~20 minutes

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 25, 2019, 04:51:22 am
content: xavier60,I apologize, that was a bit longer than 20 minutes (alot longer)The max my unit can do is 18V so im gonna keep it at 16V p-p so 8VACHere is the breadboarded circuit:http://prntscr.com/nt39g1I've set my function generator to ~1kHz, 3V p-p (1.5VAC), I am viewing the output on my oscilloscope without turning on the PSU and it looks rather unusual (top is function generator output, bottom is opamp output):http://prntscr.com/nt39kt Just so you know, I discovered that my opamps were actually produced by ST, and checked their datasheethttp://prntscr.com/nt3anqAnyways, ready to flip the switch, hope nothing dies

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 25, 2019, 05:13:11 am
content: Ok this is very unexpected:I have no clue what could be causing thisAn even bigger insult is that it doesnt even produce any gain

author: xavier60
author_rank: Super Contributor
timestamp: May 25, 2019, 05:36:38 am
content: You need to follow my diagram exactly. Both PSU channels are used, they are PS1 and PS2 in the diagram, wired in series just like two batteries might be wired in series. Use the red and black terminals, not the green ones.Set each channel to 5V for now.The op-amp wont do anything meaningful until powered up correctly.I'll add more detail if I spot specific problems.Start with a much lower signal input level like 100mV p-p.Correction: I see that they are white/black terminals for +/- on the PSU.Extra: The op-amp will appear to function oddly if the Pot is at a particular one of its extreme settings.Leave it set near the middle until things stat to make sense.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 25, 2019, 06:14:48 am
content: Here is how i have the circuit wired to the PSU:http://prntscr.com/nt3mjgIve seen people run both channels in series, I can probably do so if neededI followed the schematic when breadboardingHeres an annotated picture:http://prntscr.com/nt3qdrAs I was annotating, I realized I had switched the inputs by accident, and switched them backThe output is relatively the same as before: forgot to paste image linkhttp://prntscr.com/nt3rzuAnyways, the minimum my function generator puts out is 2V p-p (1VAC), so ill make a voltage divider to get it lower I just sweep the pot to test, the only thing that changes is the amplitude

author: xavier60
author_rank: Super Contributor
timestamp: May 25, 2019, 06:28:07 am
content: Quote from: codingwithethanol on May 25, 2019, 06:14:48 amAlthough that's not the correct way, it can be made to work, Connect a resistor from the + terminal to the green terminal. Connect another resistor of the same value from the - terminal to the green terminal. 1KΩ should do.The idea is to balance the + and - rails. Measure the voltage of each rail with respect to ground, they should be about the same as each other.More: Take the rail measurements at the pins of the op-amp. Also both input pins and output should be 0V.

author: xavier60
author_rank: Super Contributor
timestamp: May 25, 2019, 06:40:35 am
content: The ground leads for the function generator and CRO must connect to the point that has the ground symbol in my diagram, no to the op-amp's inverting input.

author: Brumby
author_rank: Unknown
timestamp: May 25, 2019, 07:14:06 am
content: This is a rough block diagram showing the basics of your supply and how it might typically be used for op-amp circuits.Note:* The earth terminal is not connected to any of the output terminals -unless you make that connection yourself.* Each output should be set to the same voltage. If your supply has dual tracking capability (I haven't checked the manual), then you can set both at the same time.* Many supplies also allow for both outputs to be paralleled internally for greater current output. Make SURE this is not engaged!!!

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 25, 2019, 07:42:24 am
content: Quote from: xavier60 on May 25, 2019, 06:40:35 amBut its AC? I dont understand how that would work

author: xavier60
author_rank: Super Contributor
timestamp: May 25, 2019, 07:50:19 am
content: Quote from: codingwithethanol on May 25, 2019, 07:42:24 amYes, the signal from the function generator is usually AC, not always. Is that what you mean?Everything has to be wired exactly as shown in my diagram to work properly.On second thoughts about adding the 1KΩ resistors. If you haven't done it yet, It's best to use both PSU channels as explained. It is the preferred way to implement dual rails.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 25, 2019, 07:51:50 am
content: Quote from: Brumby on May 25, 2019, 07:14:06 amI found this video of a guy testing a very similar supply with slightly different specs, it does have a tracking mode, and ill try this next(Tomorrow, because its nearly 4am over here)

author: Brumby
author_rank: Unknown
timestamp: May 25, 2019, 07:55:09 am
content: The+and-inputs to the Op amp are different to the positive and negative markings on your power supply.For an Op amp:- any signal applied to the+will cause the output to move in thesamedirection. When the signal goes up, the output goes up and when the signal goes down, the output goes down. The input signal usually goes in here.- any signal applied to the-will cause the output to move in theoppositedirection. When the signal goes up, the output goes down and when the signal goes down, the output goes up. The feedback loop usually runs back into here.The output from the signal generator will typically go to the+input and the 0V point in the diagram I posted above.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 25, 2019, 07:58:11 am
content: Yeah I pulled V+ and - to ground as well as the non-inverting output as in the schematic and im getting a flatline on my scopeIts getting late where I am so im going to drift off to sleep but ill be around tomorrow at the same time to wrap this little experiment up.Thanks for your help so far, I appreciate it. I had been doing EE study before, but never really had enough equip to do anything beyond arduino gimmicks and blinking leds, so now that i do have a mini lab set up, im going to delve into more advanced stuff

author: xavier60
author_rank: Super Contributor
timestamp: May 25, 2019, 08:43:14 am
content: This diagram should remove all ambiguity.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 30, 2019, 05:32:41 am
content: Ok here's the scoop:Never got it to work, as a matter of fact, while trying to figure out why the gain was literally negative, i turned off my PSU while looking at my oscilloscope and the output didn't change! Something else I discovered is that the second channel on my PSU is acting funny, I set it to 10V and it shows 4-5V with 4mA current with no load! Anyways, i've moved on to other things for the time being but ill come back to this issue eventually. Thanks to everyone who volunteered info, I learned quite a bit

author: james_s
author_rank: Super Contributor
timestamp: May 30, 2019, 05:55:50 am
content: It doesn't seem like you're listening to what anyone is saying, you're just flailing around and then reporting that it doesn't work. This is a very, very simple circuit, if you wire it up exactly as shown it will work. If your power supply is not working properly it may have been damaged but do make sure you're using it correctly first. Slow down and read the manual, follow instructions, if you just blindly wire stuff up without understanding the most basic aspects you are not going to learn much.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 30, 2019, 06:48:16 am
content: 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.

author: xavier60
author_rank: Super Contributor
timestamp: May 30, 2019, 08:17:28 am
content: 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.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 30, 2019, 08:33:09 am
content: 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.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: May 30, 2019, 09:21:33 am
content: 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?

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2019, 10:25:18 am
content: 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.

author: xavier60
author_rank: Super Contributor
timestamp: May 30, 2019, 10:54:38 am
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2019, 03:47:50 pm
content: Quote from: codingwithethanol on May 30, 2019, 09:21:33 amThat 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.QuoteWhat 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.

author: codingwithethanol
author_rank: Regular Contributor
timestamp: June 03, 2019, 03:48:20 am
content: 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

author: spiff72
author_rank: Regular Contributor
timestamp: June 05, 2019, 02:13:37 pm
content: 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).

author: codingwithethanol
author_rank: Regular Contributor
timestamp: June 05, 2019, 07:51:06 pm
content: 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.