A trick I've used for years to look at low frequency ac power is to use a small transformer to couple a single-ended scope probe. Connect the primary to whatever you want to look at and clip the probe to the secondary (usually 12 to 24 V). Avoids letting the smoke out.
Scott B. Lacey On 10 Mar 2009 at 21:42, Derek Walton wrote: > Hi Doug, > > of course you know I'd reply... I'd wager, confidently, that there is > more error using single ended scope probes. As you yourself write about > adding beads and minimizing loops etc etc etc. > > If you can't use a scope and probe properly, you shouldn't be using one: > Period. For those people that can, and I include myself in this group, > Diff probes are a godsend. > > BTW, when I talk probes, I'm not talking about ones made from chewing > gum, scotch tape and string... I'm talking credible probes. > > Cheers, > > Derek. > > Douglas Smith wrote: > > Hi Everyone, > > > > Differential probes are one of the most misused pieces of equipment > > and their output should always be verified to be true. It is very easy > > for a differential measurement to result in garbage output. From the > > discussion, the posters so far seem to be referring to very low > > frequency probes, below one megahertz. Using two probes and channel > > subtraction does not work above 20-30 MHz, or at least the common mode > > rejection can actually be gain instead of rejection. I present data on > > this subject in my classes. If there are two probe cables used, they > > should be in close contact for most of their length. Very bad things > > can happen if this is not done, including becoming sensitive to every > > noise source in the room. > > > > The best way to know if your differential probes are working is to > > short the two tips together and touch them to each node, one at a > > time, to be measured. Both results, I call it a "null experiment," > > should be small compared to what you are trying to measure. An > > alternate method is to reverse the probe tips. The waveform on the > > scope should be a mirror image. This alternate method only works on a > > scope. > > > > Except at low frequencies, 20 dB of common mode rejection can be hard > > to get. > > > > Doug > > > > Cortland Richmond wrote: > >> Hi, Derek. > >> > >> I know how it is. I also know there was no way was I going to take a chance > >> on a reversed ground to a floating 'scope when I could get rid of the > >> problem with one extra probe. FWIW, at 60 Hz it's easy to match probe and > >> channel gain, and since we were only looking at decay time there was no > >> need to buy a diff. probe. Where I work now we have some. HV ones, too! > >> > >> On a recent assignment, a place I was helping was cited by their State OSHA > >> equivalent for not having the power terminals on LISN's covered to prevent > >> contact. There's a lot of stuff in a lab that can bite. Hard. > >> > >> Cheers, > >> > >> > >> Cortland Richmond > >> > >> > >> > >>> [Original Message] > >>> From: Derek Walton <[email protected]> > >>> Subject: Re: scope probe gnd > >>> > >>> Hi Folks, > >>> > >>> I was keeping quiet.... but you know sometimes it's just too much. > >>> > >>> Almost all measurements I take in my lab use a differential scope probe. > >>> The exception is when High voltages are involved. And I dont mean the > >>> rinky dink two probes/scope math function either.... > >>> > >>> With diff probes I can connect anywhere to anywhere and measure it. I > >>> never cringe hooking up the 0 volt reference side.... never sparks.... > >>> > >>> Give one a try, you will never go back. > >>> > >>> Derek Walton > >>> > >>> Cortland Richmond wrote: > >>> > >>>> In a previous life -- at a company no longer around -- when our Safety > >>>> Engineer quit we EMC types had to step in, and one of the very first > >>>> > >> things > >> > >>>> I did was get rid of a lethally ungrounded scope setup. It costs little > >>>> > >> to > >> > >>>> use probes rated for line voltage or better and dual channel math > >>>> > >> functions > >> > >>>> to allow proper safety grounds. > >>>> > >>>> Cortland Richmond > >>>> > >>>> > >>>> > >>>> > >>>>> [Original Message] > >>>>> From: Pettit, Ghery <[email protected]> > >>>>> To: [email protected] <[email protected]> > >>>>> Date: 3/10/2009 12:43:37 PM > >>>>> Subject: RE: scope probe gnd > >>>>> > >>>>> Or there were the old scopes where you used patch cords and banana > >>>>> > >> jacks. > >> > >>>>> > >>>>> > >>>> We had what was left of the power cord for one such scope hanging in the > >>>> front of the motors lab in college that showed the importance of having > >>>> your setup checked before applying power. Seems the hot and ground > >>>> > >> leads > >> > >>>> to the scope from the motor being measured got reversed and the 120 VAC > >>>> supply was bypassed to ground through the green wire in the power cord. > >>>> For a short period of time. Then the green wire acted as a fuse. That > >>>> cord was NOT repairable. And, NO, I was not the culprit. Didn't > >>>> > >> happen on > >> > >>>> my watch. > >>>> > >>>> > >>>>> Ghery Pettit > >>>>> > >>>>> > >>>>> -----Original Message----- > >>>>> From: [email protected] [mailto:[email protected]] On Behalf Of Ralph > >>>>> > >>>>> > >>>> McDiarmid > >>>> > >>>> > >>>>> Sent: Tuesday, March 10, 2009 9:35 AM > >>>>> To: [email protected] > >>>>> Subject: RE: scope probe gnd > >>>>> > >>>>> Once for me as well. I never forgot that lesson. > >>>>> > >>>>> > >>>>> Ralph McDiarmid, AScT > >>>>> Compliance Engineering Group > >>>>> Xantrex Technology Inc > >>>>> > >>>>> > >>>>> -----Original Message----- > >>>>> From: [email protected] [mailto:[email protected]] On Behalf Of John > >>>>> Woodgate > >>>>> Sent: Tuesday, March 10, 2009 5:47 AM > >>>>> To: [email protected] > >>>>> Subject: Re: Surge testing Questions > >>>>> > >>>>> In message > >>>>> <9d04b979323dcd428297dda95108893e0120c...@bb-corp-ex2.corp.cubic.cub>, > >>>>> dated Tue, 10 Mar 2009, "Price, Edward" <[email protected]> writes: > >>>>> > >>>>> > >>>>> > >>>>>> What, you never had that happen?! > >>>>>> > >>>>>> > >>>>> Only once - so far! > >>>>> -- > >>>>> > >>>>> > >>>> - > >>>> ---------------------------------------------------------------- > >>>> This message is from the IEEE Product Safety Engineering Society > >>>> > >> emc-pstc discussion list. To post a message to the list, send your e-mail > >> to <[email protected]> > >> > >>>> All emc-pstc postings are archived and searchable on the web at: > >>>> http://www.ieeecommunities.org/emc-pstc > >>>> Graphics (in well-used formats), large files, etc. can be posted to > >>>> > >> that URL. > >> > >>>> Website: http://www.ieee-pses.org/ > >>>> Instructions: http://listserv.ieee.org/request/user-guide.html > >>>> List rules: http://www.ieee-pses.org/listrules.html > >>>> > >>>> For help, send mail to the list administrators: > >>>> Scott Douglas <[email protected]> > >>>> Mike Cantwell <[email protected]> > >>>> > >>>> For policy questions, send mail to: > >>>> Jim Bacher: <[email protected]> > >>>> David Heald: <[email protected]> > >>>> > >>>> > >> > >> - > >> ---------------------------------------------------------------- > >> This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> > >> > >> All emc-pstc postings are archived and searchable on the web at: > >> http://www.ieeecommunities.org/emc-pstc > >> Graphics (in well-used formats), large files, etc. can be posted to that URL. > >> > >> Website: http://www.ieee-pses.org/ > >> Instructions: http://listserv.ieee.org/request/user-guide.html > >> List rules: http://www.ieee-pses.org/listrules.html > >> > >> For help, send mail to the list administrators: > >> Scott Douglas <[email protected]> > >> Mike Cantwell <[email protected]> > >> > >> For policy questions, send mail to: > >> Jim Bacher: <[email protected]> > >> David Heald: <[email protected]> > >> > >> > > > > -- > > -------------------------------------------------------------- > > ___ _ Doug Smith > > \ / ) P.O. Box 1457 > > ========= Los Gatos, CA 95031-1457 > > _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 > > / /\ \ ] / /\ \ Mobile: 408-858-4528 > > | q-----( ) | o | Email: [email protected] > > \ _ / ] \ _ / Web: http://www.dsmith.org > > -------------------------------------------------------------- > > > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > Graphics (in well-used formats), large files, etc. can be posted to that URL. > > Website: http://www.ieee-pses.org/ > Instructions: http://listserv.ieee.org/request/user-guide.html > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <[email protected]> > Mike Cantwell <[email protected]> > > For policy questions, send mail to: > Jim Bacher: <[email protected]> > David Heald: <[email protected]> > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
