Safety first with oscilloscope & Co. - Page 1
Forum section: electronics / beginners

author: chrisstra84
author_rank: Contributor
timestamp: February 28, 2015, 06:47:35 pm
content: Hello guys,I have a question about mains earth with an oscilloscope and a function generator and co. I have seen the videos about “Oscilloscope & Function Generator Measurement” and “How NOT to blow up your Oscilloscope”.So, my question is: What should I do? Can an isolation transformer help me? Should I isolate the scope or the function generator or another test object, or both, or all of them? I want to make my working place more safety. I do a lot of electronic stuff just for fun and so I often use a scope to learn more about these things.But safety first!So, can someone give me an advice?Thank you in advance.Christian

author: Neilm
author_rank: Super Contributor
timestamp: February 28, 2015, 09:07:37 pm
content: What are you trying to do? If you are just wanting to inject signals into a floating test piece, then connect the reference of the function generator and the reference of the scope lead to the same point and there should not be a problem.

author: Meshka7
author_rank: Contributor
timestamp: February 28, 2015, 10:08:58 pm
content: I think you only need an isolation transformer if you are working with high voltages. If you are working with only low voltages, you just need to be sure that you don't connect your grounds arbitrarily to the circuit. Identify one point in your circuit as ground and make sure all the grounds connect to that point.Disclaimer: I'm a noob myself

author: max666
author_rank: Frequent Contributor
timestamp: March 01, 2015, 12:43:43 am
content: I think the opinions on isolation transformers and safety are divided.On one hand if you isolate a circuit you can touch any single point of your circuit safely (one hand in your pocket rule), but as soon as you touch two different points you can be in trouble.You are from Germany so households nowadays should have Residual-Current Circuit Breakers (FI-Schutzschalter), which means as soon as you touch something hot and more than 30 mA are flowing back via earth it should trip.Keep in mind if you use an isolation transformer the protection provided from the Residual-Current Circuit Breakers is nullified.Personally I prefer the Residual-Current Circuit Breaker, but I'm not saying you can do whatever you want as long as you have a Residual-Current Circuit Breakers.

author: KerryW
author_rank: Regular Contributor
timestamp: March 01, 2015, 04:52:10 am
content: You don't need an isolation transformer. Almost nobody EVER needs an isolation transfom=rmer. Virtually every piece of equipment you'll ever use has a transformer that isolates the output from the line at the same time it shifts the voltage. Some pieces of equipment (scope, PC power supply) tie 1 leg of the secondary to the chassis and thus to the ground pin of the plug. Most (lab supply, wall wart, meter, function generator) leave the outputs floating. Some (most? all?) lab supplies allow you to optionally connect their outputs to chassis ground.As long as you do not work directly with mains wiring, leaving the scope ground in place is perfectly safe. Just be careful if you are using a PC supply. If you connect the scope ground lead to a voltage, you will (hopefully) pop a circuit breaker.As they told us in the Navy, "Never disconnect the safety ground. lest your buddies console your widow by buying her beers, and in other ways."

author: homebrew
author_rank: Frequent Contributor
timestamp: March 01, 2015, 10:29:34 am
content: I never ran into these problems, as my scope is not mains earth referenced (Philips PM3216 - 35MHz).But of course you have to be careful as the potential of the entire scope depends on where you connect your ground clamp.As long as every device is connected to the same ground, there should not be much of a problem...

author: VK5RC
author_rank: Unknown
timestamp: March 01, 2015, 11:41:31 am
content: A bit off topic but related to NOT blowing up your oscilloscope, I only have one probe lead that I leave the earth clip on, next to my oscilloscope, to stop me from doing dumb things (again). I have the three others nearby "de-clipped". I seem to have a one track mind and invariably when I am problem solving I can stuff up really easily. I also leave a note attached to the scope to remind myself about earth loops/ ?isolation transformer etc.So far no magic smoke from the scope but I have stuffed a few DUTs. (Made the repair job quite a bit harder).

author: chrisstra84
author_rank: Contributor
timestamp: March 01, 2015, 01:06:35 pm
content: Hello together,At first I thank you all.At the moment I’m just using voltages up to 30V and so is no danger. Up until now my scope was not mains earth referenced because the wire was broken. Now I have fixed it. My scope (old Telequipment S51B) is now mains earth referenced and my function generator (a very new OWON AG 2062F, up to 60MHz, 250 MSa/s) is it too. Fortunately our house has a Residual-Current Circuit Breaker (RCCB) (30 mA). For my experiments I use a dual output-power supply with have a connection jack to mains earth, if I want.But if I want to use higher voltages, I don’t sure to use an isolation transformer, you all told me advantages and disadvantages. But my normal option is, that the mains earth connection has it right to exist. But isolation transformers, too?So, if I use just low voltages, what can I do to protect my devices? If I make an electrical short, can it damage my function generator or my scope?I’m still learning electric stuff day by day, it’s fun. But it should be save. So I have many ideas to do. I think it’s really important to ask other people and their experience. So you don’t have to be shy to ask, or?Christian

author: max666
author_rank: Frequent Contributor
timestamp: March 01, 2015, 03:55:23 pm
content: Keeping you safe (from destruction) and keeping your devices save from destruction, are two different questions, which is why my first post only addressed your safety.If we're talking about your devices, I would say as long as only one of them has it's ground mains earth referenced, you're quite safe not to short anything out. I think the OWON AG 2062F has it's outputs floating (not sure), but you can easily find out and do a continuity check between the output ground and the mains earth pin on the back.If you would connect two devices to your circuit, that have mains earth referenced grounds, and the grounds aren't connected to the same potential in the circuit, then you would have to isolate one of the devices with an isolation transformer, so that it's ground isn't earth referenced any more.In case you would short something out due to mains earth reference, I think it's fair to say that most likely your circuit gets damaged, then scope probes, and least likely your scope or function gen.Your scope only has one channel, so at the moment you don't have to worry about multiple ground connections from your scope, but your function gen has two outputs and their grounds are connected together, so if you're connection both outputs you have to make sure not to connect the grounds to different potential. But as you have already watched “How NOT to blow up your Oscilloscope” this should be already clear to you.

author: chrisstra84
author_rank: Contributor
timestamp: March 01, 2015, 04:48:47 pm
content: Hello,The outputs of my OWON AG 2062F are mains earth referenced. I have tested it with my multimeter. Yes, I have already watched the video and I think I have understood what David is talking about. But I’m not really sure about mains earth isolation. But I won’t to do a mistake if I can avoid it.At first you can just use your mind and be always careful. After I watched the video, my mind was going around and around. So I have found out the damage in my device.I have read any books about electronic stuff. So, when you have started to do something in this way, you’re thinking more and more about it and want really to understand everything. So in practice it is always a little bit difference than in theory. I really want to understand these things and so I hope avoid stupid mistakes.Best wishesChristian