Probe Alligator Clip Worries  - Page 1
Forum section: electronics / beginners

author: tony3d
author_rank: Frequent Contributor
timestamp: November 25, 2013, 07:06:54 am
content: I'm still confused at to the trouble I could get myself into with improper use of the probe ground clip on my scope. If I'm using my scope on simple bread board circuits powered by a bench varriable power supply, typically using well under 30 volts, can I still accidentally damage my scope? If I make sure that the probes clip is always on the negative side of the circuit, and the tip on the positive side, will I be ok?

author: Avotronics
author_rank: Regular Contributor
timestamp: November 25, 2013, 08:59:03 am
content: Quote from: tony3d on November 25, 2013, 07:06:54 amIt sounds like you do not understand the very basics, I would therefore advise you put your scope probes n a drawer and go do a bit of reading first to understand. You need to know 'why' and understand 'why' you can damage your scope, its no good us just telling you yes that is safe, no that isn't safe etc.If you haven't watched Dave's video in your other post, watch it, it's explained simply.

author: tony3d
author_rank: Frequent Contributor
timestamp: November 25, 2013, 01:03:51 pm
content: Quote from: Avotronics on November 25, 2013, 08:59:03 amOk, I just watched Dave's excellent video, and now I fully understand the dangers involved. Seeing how my power supply's negative terminal is not shorted to the earth ground terminal, any circuit I power with it should remain floating. I would certainly be extremely careful were I ever to probe a wall powered product however. Thanks.

author: ConKbot
author_rank: Super Contributor
timestamp: November 25, 2013, 01:26:09 pm
content: In addition to safety of you and your scopes ground leads, you also have to be mindful of where youre putting the ground clip when you are probing something. The scope is earth referenced, but your DUT will be floating, but the scope ground lead will add capacitance that will load the circuit. Think of the loop formed by the scope ground clip. You have a solid ground from the scope lead to the power strip, and from your power strip to your bench power supply. That provides isolation, however there will still be capacitance between the ground and the PS - output. The PS - connects to your circuits floating 'ground' reference. So any parasitic capacitance between your DUT and ground, and between your power supply input and output, will then end up attached to wherever that ground clip is.For lower speed stuff this wouldnt matter at all, but if you have an op-amp circuit which misbehaves or only works when the ground clip is hooked up, there's a clue

author: tony3d
author_rank: Frequent Contributor
timestamp: November 25, 2013, 01:41:18 pm
content: Quote from: ConKbot on November 25, 2013, 01:26:09 pmWow! Never thought about that. Man I'm surprised that half of the new comers to electronics haven't blown up their scopes or worse themselves! You really must make sure that little clip stays on the negative side of things. I will be learning with this scope on very low power simple circuits powered either by battery. or my bench power supply so I should be fine. I don't think I'll ever try probing a live earth mains referenced board for some time though.

author: tony3d
author_rank: Frequent Contributor
timestamp: November 25, 2013, 02:07:57 pm
content: This whole thing really put the fear in me. I am going to order my scope probably today. I am so happy I found this info out, and took the time to look at Dave's to the point video. Really, beginners must be sending a lot of scopes back days after getting them. Now I'm so paranoid that I took my DMM and checked the negative terminal of my Korad power supply just to make sure there was no reference to it's mains earth ground! Thankfully not! Almost afraid to turn it on, and actually connect it to anything when it arrives!

author: Avotronics
author_rank: Regular Contributor
timestamp: November 25, 2013, 07:28:15 pm
content: Quote from: tony3d on November 25, 2013, 02:07:57 pmIt should be fear into you, at least enough to stop you jumping in without thinking. It takes very little current to put your heart into cardiac arrest. No one wants you to NOT work with this stuff because you fear it, our intent is purely to slow you down and make sure you understand it first. Theory has its place.If you're working with LV stuff, you should still practice safety but you needn't let it put you off.

author: Rigby
author_rank: Super Contributor
timestamp: November 25, 2013, 09:08:31 pm
content: If you ever do need to measure a circuit that is grounded to earth, and you don't want to roast your scope, use the tips of ch1 and ch2 and have the display add the measurements (or subtract if ch2 is inverted) to see the signal. The tips aren't grounded, and there is no path between ch1 and ch2 tips, provided you stay within the probe and scope current & voltage ratings.and always (ALWAYS) use the 10X mode on scope probes. Tek has a pamphlet called "ABCs of probes" which explains why, and Linear Technologies App Note 47 by Jim Williams also has a more succinct explanation if you don't mind reading the entire app note to find it. It's a good read, even for a newbie like me.In fact, this whole library is probably great reading:http://www.tek.com/learning/oscilloscope-tutorialYour scope will take a few days to arrive. Use that time to absorb as much information as you can. It's easy to fry equipment if you do the wrong things.

author: Nerull
author_rank: Frequent Contributor
timestamp: November 26, 2013, 01:49:09 am
content: Quote from: tony3d on November 25, 2013, 01:03:51 pmYou can connect the clip to any part of the circuit if it's powered by an isolated supply. Keep in mind, however, that once you clip the scope to it, it is no longer isolated.Whatever you clip the scope to becomes earth ground and all other voltages on the DUT - including the circuit ground and/or chassis - become live, potentially dangerous, voltages. If you clip your scope to the positive rail of your 30V supply your negative rail will be at -30V to earth. On a mains powered device with an isolation transformer you can put 120/240V on an exposed case if you're not careful.You can think of it this way - an isolated supply gets you a single point of safety. You can touch (in theory) any single voltage on the device, no matter what it is, without danger. (In reality there is always risk, and you should never put yourself in a situation where your safety depends on it) Negative rail, positive rail, high voltage, whatever. Touching two different voltages can still get you killed. When you're hooking up the scope, you're using that single point to protect the scope, and it can't protect you anymore.

author: BravoV
author_rank: Super Contributor
timestamp: November 26, 2013, 01:58:27 am
content: Isolation transformer also has it's limitations, other alternative is using a "fully isolated" and battery powered scope.But becareful though, watch the words "fully isolated", cause not every battery powered scope has this safety feature designed and implemented in it which may be dangerous too if not used wisely.

author: tony3d
author_rank: Frequent Contributor
timestamp: November 26, 2013, 02:45:28 am
content: Has to make me wonder how anyone actually enjoys this hobby. I think I'm really wondering if perhaps this hobby is to risky for me. I was going along pretty well till I got this idea to buy a scope. Now I'm beginning to doubt everything I though I knew. I delayed buying my scope today till I rehash this whole thing. This went from a fun new world of exploration, to a very dangerous one false move, and your dead nightmare. I understand the need to be cautious, but I'm feeling very vulnerable. I don't want to be afraid to try new things that's half the fun, but knowing that one false move can have these kind of consequences makes me believe that perhaps my dream of owning a scope may be just that a dream. I really want to be able to see the charging of a cap, and the clipping of an amplifier, but is it really worth all the stress? I don't know.

author: Rory
author_rank: Frequent Contributor
timestamp: November 26, 2013, 02:50:07 am
content: Quote from: tony3d on November 25, 2013, 01:41:18 pmNot sure you're getting it quite right.In some cases you'll find the "ground" is at a positive potential compared with the part of the circuit you want to measure. A good example is the old fashioned transistor radio with PNP transistors. 9v battery red wire going to the common and black through the switch... It's perfectly acceptable to attach scope probe ground to the circuit common in this case, as long as it is at earth ground potential or completely floating independent of mains AC, either through transformer isolation or battery power. Another case would be a magnetron or klystron power supply where the cathode is driven at a high negative voltage while the anode is at earth potential. Seems counterintuitive at first, with the anode at 0v.For the times I want to look at switch mode PS waveforms and not sure about whether the common is above earth potential I dig the Fluke 97 Scopemeter out of the cabinet and run it on batteries. No worries, then.

author: Rory
author_rank: Frequent Contributor
timestamp: November 26, 2013, 03:04:38 am
content: Quote from: tony3d on November 26, 2013, 02:45:28 amDon't sweat it too much. If you're breadboarding, you first want to draw stuff out on a schematic diagram and determine where the common side of the power supply actually is.And if you're using a bench power supply, in most cases there's a green binding post right next to the black one which you can put a little jumper in and be sure that your B- is at earth ground potential... and for split supply op-amp circuits you can attach the connection between the two supplies B+ and B- common to the green earth terminal, then just be certain that when you take your measurements you attach your scope ground to the 0V common rail.The nice thing about all this is that there is a logic to it, and once you understand the simple basics as Dave put out in his video, you won't have to worry about it.

author: Rigby
author_rank: Super Contributor
timestamp: November 26, 2013, 04:45:36 am
content: Its plenty fun, just don't dive in without knowing what you're doing to a small degree. This kind of thing is where an old, uncalibrated scope will show you all you need for less than $100, via eBay, and probably even be fixable if you make a mistake.

author: casinada
author_rank: Frequent Contributor
timestamp: November 26, 2013, 06:13:37 am
content: If you want to learn about scopes just go to w2aew's YouTube channel.If you work on microelectronics and only play with small voltages there is nothing to worry about. Just follow some precautions as suggested in previous posts. Enjoy your Hobby

author: kenshi
author_rank: Contributor
timestamp: November 26, 2013, 06:56:07 am
content: If I may ask a related question... I understand the stuff in the video. However, I live in an old house that doesn't have true grounding. The ground terminal is either connected to neutral or floating. The one where I typically do my circuitry is floating. I'm getting about 10 VAC in the ground compared to neutral. Is this going to cause poor readings from the scope? Since running a ground wire isn't a practical solution, would it be better to connect the ground to neutral?

author: Avotronics
author_rank: Regular Contributor
timestamp: November 26, 2013, 08:33:59 am
content: Quote from: Nerull on November 26, 2013, 01:49:09 amYou quoted Tony there, not me. ;-)

author: Avotronics
author_rank: Regular Contributor
timestamp: November 26, 2013, 08:37:34 am
content: Quote from: Rory on November 26, 2013, 02:50:07 amYou're right, a lot of posts here sound misguided or just plain wrong.

author: Nerull
author_rank: Frequent Contributor
timestamp: November 26, 2013, 11:25:25 am
content: Quote from: Avotronics on November 26, 2013, 08:33:59 amThat's because I was replying to Tony, not you.

author: Rory
author_rank: Frequent Contributor
timestamp: November 26, 2013, 12:59:18 pm
content: Quote from: Avotronics on November 26, 2013, 08:37:34 amIs my post an example?