Worried about ground with lab power supply, oscilloscope and function generator - Page 1
Forum section: electronics / beginners

author: blurpy
author_rank: Regular Contributor
timestamp: October 24, 2019, 06:00:34 pm
content: Dave's video "How NOT To Blow Up Your Oscilloscope" scared me a bit.I've been playing around with battery powered stuff so far, but I've now got a lab power supply, proper oscilloscope and function generator, so I need to think about ground.This is the power supply:https://www.velleman.eu/products/view/?id=439082(Velleman LABPS3005DN)It has a separate ground terminal, so I guess it's "floating" unless I connect ground with minus (or plus), right?The oscilloscope and function generator (both Siglent) are properly grounded, I think.I've attached an image of the current state of my experiment. It's a breadboard where power to a servo is provided by a USB power bank, and a PWM signal is provided by the function generator (via the BNC cable there). The servo is functioning as expected, and I can adjust its position from the generator. The only grounded item so far in the setup is the function generator, so according to Dave I should be safe.Next I could replace the USB power bank with the lab power supply, and I'm still safe, right? No possibilities of a ground loop?But what about if I connect the oscilloscope to measure the signal sent to the servo? My initial thought would be to connect oscilloscope ground to the minus rail on the breadboard and the probe to either the plus rail or the (orange) signal, or maybe one probe on each. Suddenly I have 2 grounded items connected through the breadboard. Can I now blow everything up if I accidentally short something?

author: rstofer
author_rank: Super Contributor
timestamp: October 24, 2019, 07:05:45 pm
content: At these voltage levels and the fact that everything is earth referenced through the signal generator, you shouldn't be worried about 'ground loops'. They simply don't apply in this case.Tie ALL of your grounds together through the test leads and call it a day.Why is this ok? Well, the signal generator is, by definition, earth referenced if it is line powered and has a BNC jack. Same with the scope. You can connect an earth ground at the power supply if you wish but it's optional.What Dave is talking about is a circuit that has no convenient earth reference and won't allow one. Like when you are taking the voltage across a resistor on a high voltage circuit and neither end is 'grounded'. Just as soon as you connect the ground clip from the probe, you have an 'oopsie' because the point you connected wasn't at earth potential and can't be pulled to earth potential.The type of thing you are doing is done every day by thousands of hobbyists without a bit of concern. Just make sure you connect the scope ground lead to the ground side of your circuit (the black wire of the servo, for example) and everything will be fine. That is likely to be the same place you connected your PS black wire and your signal generator ground.Think about this: I have been playing with uCs since the mid '70s and FPGAs since 2003. I have NEVER had a need to float my scope or use an isolation transformer. I have always tied my scope into circuit ground whether it was earth grounded or nor. The simple explanation: I don't work on switching power supplies. Watch Dave's video again and pay particular attention to what he is trying to measure.Ground loops can be a problem for audio systems. They are not likely to be much of a concern for robotics. I don't consider them at all. I don't deliberately ground my PS output but I don't avoid grounding the signal generator or scope either.For the truly paranoid, put a 1k resistor between the scope probe ground clip and the circuit ground. It won't hurt anything (compared to 1M scope resistor) and it will limit current through the ground lead. Then, measure the voltage drop across the resistor with a DMM. I'll bet it's near zero and, if so, it can be eiiminated.For the circuits you are working on and the voltage levels involved, this concern about grounding is unwarranted.

author: rstofer
author_rank: Super Contributor
timestamp: October 24, 2019, 07:16:51 pm
content: Quote from: blurpy on October 24, 2019, 06:00:34 pmAbsolutely! In fact, you are at more risk with a battery pack than with a decent power supply. The battery pack likely has a very low internal impedance and thus can deliver a lot of current and there is no control over this current. With a decent lab supply, you can set the current limit to some reasonable number and limit the fault current.This is one of the great advantages to a lab supply over wall warts and batteries.A few months ago, I was working on a Z80 project and I had a bus address conflict with two sets of bus drivers opposing each other. No worries, I had set the current limit very low and the only thing that happened was the voltage rolled off to about 0.2V instead of 5V and the system was limited to about 50 mA (IIRC). No magic smoke escaped.You can do the same thing. Set the current limit low and keep increasing it until the voltage reaches the setpoint. You can add a little more current limit just for a margin. Then hook up your test equipment! You will find a sweet spot where the servo runs fast and the power supply doesn't trip on overcurrent.The Rigol DP832 has two limits. The common current limit that causes the voltage to roll off on over-current and the current trip (higher) that causes the output to shut off.

author: blurpy
author_rank: Regular Contributor
timestamp: October 24, 2019, 07:47:40 pm
content: Thank you very much for detailed explanations rstofer! I'm a lot less worried now.I think I'm going to need to reread your posts a few more times to understand it all though. And I'm going to watch Dave's video again.The current limit is one of the reasons I want to use the power supply more. I did some experiments with a cheap tiny servo tester, powered by the power supply, and I had to increase the current limit more than I expected to get the servo and tester to initialize. But during use it was much less power hungry. That was interesting.It's going to be fun connecting and measuring everything at once!

author: rstofer
author_rank: Super Contributor
timestamp: October 24, 2019, 08:44:25 pm
content: Just remember: Everything you are working on has a "common" - the black wire of the servo, the (-) side of the power supply and the "Gnd" pin from the uC. There is no reason you can't tie these together (in fact, you have to) and also tie them to earth ground through your test equipment.I so much hate the word "ground". Are we talking about a circuit "common", a chassis ground tied to a circuit "common", the "Gnd" pin of a uC which may or may not actually be grounded or an actual earth "ground". It all gets very confusing.

author: blurpy
author_rank: Regular Contributor
timestamp: October 26, 2019, 11:12:08 am
content: Yeah, it's a bit confusing how DC minus suddenly turns into ground. "Common" seems perhaps a bit more intuitive.

author: rstofer
author_rank: Super Contributor
timestamp: October 26, 2019, 04:21:24 pm
content: The whole thing comes unhinged when there are (+) and (-) supplies referenced to a 0V "common" which will likely be called "ground" without an earth reference in sight. We have "circuit ground", "chassis ground" and "earth ground" and, when done correctly, the schematic will indicate the difference by using appropriate symbols but, usually, the symbol will be an arrowhead which, to me, indicates "circuit ground" or "signal ground" or "common".I wish the symbols were used correctly but they are often incorrect:Then there is the 'virtual ground' of op amp inputs - a topic for later.

author: Johnboy
author_rank: Regular Contributor
timestamp: October 31, 2019, 12:27:46 am
content: Quote from: rstofer on October 26, 2019, 04:21:24 pmThis was one of the main puzzles (along with "conventional current vs. electron flow") when I first began studying electricity. The word "ground" has multiple meanings... and schematics often establish the convention of which symbols are selected ( for "chassis" and "earth", say) by whomever it is who is drawing it. This is notable in the "How Not to Blow Up Your Scope" video, but it's something that really threw me when I realized that some drawing schematics willonlyuse the earth ground symbol, and others will useonlythe chassis ground symbol, no matter what it's actually connected to. This was confusing to me at first.I think that if someone had gone to the trouble to write a history of "ground" and explained the conventions, as well as safety applications with some practical examples like those in this thread along with proper use of symbols in the schematics, there would be a niche for such a book, even as a standalone primer. Ground is additionally confusing because of the nomenclature aspect. This topic is rarely covered in depth in the primers I've looked at.

author: tggzzz
author_rank: Super Contributor
timestamp: October 31, 2019, 12:43:09 am
content: Quote from: rstofer on October 24, 2019, 08:44:25 pmJust so.And then you can add in the, ahem, imprecise nature of the "actual earth ground"