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| robint:
Hi Guys, can you help this board in clearing up the rubbish spouted about making differential measurements in circuits using a scope :blah: :rant: So this topic doesnt get convoluted - let us dispense here with concerns about electrical safety - that is probably well covered elsewhere. OBTW I deplore the cheap charlie way of just disconnecting the earth at the mains plug :rant: What we are concerned with here is protection of your scope and also the viability of making differential measurement (where the gnd croc cant be attached to PE) I have upgraded to a new DSO 4ch gear and (from my old school CRO scope bg) its quite a revelation and requires repurposing my habits so I can meddle with digital ccts safely. I spent several weeks re-learning this new hi tech quantum leap stuff so bear with me if I stumble. First I was made firmly aware that my DSO is a sensitive instrument and easily damaged by ignorant use. Fair play. So I came to the area of Differential probes and was shocked to discover that they can cost nearly as much as my new DSO. Back to basics so many rubbish vids out there were informercials pushing Diff probe kit yet there is a simple way and that is to use 2 channels (with the croc gnd leads connected) so the probes are then free to use on ccts without danger of shorting out or blowing up your scope by careless application of the croc gnd lead. It seems another way might be to use a cheap audio xformer 1:1 (£5) and use the primary as the device under test probes and the secondary for the scope leads. Total isolation. Of course this is no good for dc measurements (so use a DMM instead!) and will become inaccurate at mf-hf, but should be ok to poke around SWM PSUs. Anything wrong with that? Pls advise Any Techs out there care to share wisdom on this :-BROKE |
| modoran:
I haven't seen anyone using audio transformers as probe input, this will load the circuit under test and get unexpected results, even blown it up. Cheaper option will be to use an isolation transformer for the DUT power supply. |
| tautech:
--- Quote from: robint on October 13, 2023, 05:37:21 am ---Back to basics so many rubbish vids out there were informercials pushing Diff probe kit yet there is a simple way and that is to use 2 channels (with the croc gnd leads connected removed) so the probes are then free to use on ccts without danger of shorting out or blowing up your scope by careless application of the croc gnd lead. --- End quote --- Correction made. You should also fit the probe condoms to further isolate the reference (gnd) rings near the probe tip from accidental short circuits when the grabbers are removed. |
| tggzzz:
--- Quote from: robint on October 13, 2023, 05:37:21 am ---So I came to the area of Differential probes and was shocked to discover that they can cost nearly as much as my new DSO. ... Any Techs out there care to share wisdom on this :-BROKE --- End quote --- If you think probes only cost as much as a cheap DSO, you haven't been looking :) Or is that :( There are many types of differential probe. You need to characterise your requirements before this can be answered. What is are the differential and common mode voltages? What are the edge rates of digital waveforms or the bandwidth of analogue waveforms? What are the impedances where the waveforms are being measured? You will probably find the references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ helpful. |
| robint:
@tautech (with the croc gnd leads connected removed) I think the tiptster's purpose was that the croc leads didnt flop around - as they do and short to something bad |
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