OK, I have done it this way. One side of your device under test is tied=20 to Neutral (earth) and your scope ground is (supposedly) tied to the same=20 (supposedly) neutral. Carefully take a meter and measure the potential=20 between these two grounds. If there is less than a volt or two, they are=20 OK to hook together. But add a 250ma fuse in there anyway to protect your = scope ground lead in case you made a mistake. Also, make sure that the=20 ground connection also measures low voltage to a handy water pipe or=20 something else that is earthed. Electricians randomly switch the neutral=20 and hot wires on electrical plugs just to keep us guessing.=20 -- Lawrence Lile Jan-erik S=F6derholm (QAC) Sent by: pic microcontroller discussion list 03/17/2003 03:28 AM Please respond to pic microcontroller discussion list =20 To: PICLIST@MITVMA.MIT.EDU cc:=20 Subject: Re: [EE]: Measuring AC mains with a O-scope. Now, lets take a second round on this issue... I think it's clear that I need a isolation transformer. It also looks like it would be best to connect the IT between the mains and the device under test, not to the scope, right ? I have checked a few electronics catalogs, and the IT seems to be from $150 USD and up starting with aprox 300VA units. Now, my device under test is realy low power. It's a "transformerless power supply" with a "secondary" voltage of 5V. Maybe 5-10 mA in average on the 5V rail. Just a 400V "X2" cap, a few resistors, a low voltage smothing-cap, a zener and a 78L05. I does work, a get 5V as messured with a battery powered voltmeter, but I'd like to "see" what what the 5V rail "looks like" under load. So, would it be possible to put together something inexpensive either with some ready made (smaller) transformar, or buy handwinding something ? Any design tips ? Jan-Erik. llile@SALTONUSA.COM wrote: >No Isolation Trasformer =3D BOOM! or Yeeoooooow! >Isolation transformer =3D OK >Hook the isolation transformer to your device under test, not your scope. >If you hook the isolation transformer to your scope, the scope ground=20 will >become live when you hook it to your circuit. -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu