Isolation Transformers for Oscilloscope vs Power Supply - Page 1
Forum section: electronics / testgear

author: SharpEars
author_rank: Unknown
timestamp: November 16, 2014, 02:47:52 am
content: Most decent power supplies offer the ability to disconnect the negative terminal from earth ground. So, if I do this, does it still make sense to use an isolation transformer to for example plug my oscilloscope into in order to perform measurements on the DUT which then is not itself in any way connected to earth ground.In other words, if the DUT has no connection to earth ground, do I still need an isolation transformer at all?

author: Nerull
author_rank: Frequent Contributor
timestamp: November 16, 2014, 04:40:03 am
content: No.When you are using an isolation transformer, its generally better to power the DUT with it, since the scope has controls you will be using which may suddenly be at dangerous voltages they were not designed to isolate you from.

author: SharpEars
author_rank: Unknown
timestamp: November 16, 2014, 02:47:33 pm
content: Quote from: Nerull on November 16, 2014, 04:40:03 amBut using a power supply for the DUT which decouples its ground out from its negative terminal means that an isolation transformer is not needed, correct?

author: janengelbrecht
author_rank: Regular Contributor
timestamp: November 16, 2014, 03:25:29 pm
content: I use isolation transformers for isolating the mains phase/live from my circuit...not ground ? My scope is grounded...and all other equipment but not...D.U.T. in general..D.U.T...That i supply by an isolation transformer... I dont know how electrical systems are outside of my country ( Denmark) ... but here we have phase/live, neutral (return line) and protective ground (all current going through here are measured by a HFPI circuit and if it rises above 30 mA the relay in the HFPI circuit breaks the phase/live. If i use an isolation transformer for my D.U.T. i no longer has phase/live and neutral....in general that means that power wont return to earth/ground.....thats what i've been told anyway at Uni many decades agoSo it shouldnt it be an issue that my oscilloscope is grounded / connected to earth terminal at its Chassis/ground terminal. But as i said..can only say how it is here in Denmark

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 16, 2014, 03:36:34 pm
content: Yes, theoretically speaking, using an isolated power supply accomplishes the same thing as operating the DUT from an isolation transformer. However, if the DUT operates from some DC voltage (presuming that is what is coming from your unidentified power supply), then it is not clear WHY you need it isolated at all?And if you are talking about AC voltage (as from a "variac" or similar), then are you REALLY sure that it is truly isolated? Most variacs are auto-transformers and DO NOT isolate.Your question raises more questions that must be answered before anybody can give you an authoritative answer.What is the DUT? Exactly what power does it require? (Volts, Amps, AC/DC)? WHY does it need to be isolated at all?

author: janengelbrecht
author_rank: Regular Contributor
timestamp: November 16, 2014, 03:49:19 pm
content: Quote from: Richard Crowley on November 16, 2014, 03:36:34 pmThese autotransformers ought to be illegal!

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 16, 2014, 04:04:23 pm
content: Quote from: janengelbrecht on November 16, 2014, 03:49:19 pmNo! Not at all.There is a legitimate place for variable AC sources.There is a legitimate place for autotransformers (where isolation is not needed).There is a legitimate place for full isolating transformers.As with most other things, it is a matter of properly educating yourself to understand the differences, and analyzing each situation to see what meets the needs and what does not.

author: LukeW
author_rank: Frequent Contributor
timestamp: November 16, 2014, 04:25:26 pm
content: Can you actually get variable "variac" transformers whichareisolated and arenotautotransformers? I've never actually seen one.

author: nctnico
author_rank: Super Contributor
timestamp: November 16, 2014, 04:56:34 pm
content: Quote from: SharpEars on November 16, 2014, 02:47:52 amNever disconnect your test equipment and DUT from ground AND make sure to have a GFI. For working on dangerous voltages a differential probe is the best way to make measurements. Isolation transformers are a thing from the past (before GFIs became common) and only offer safety in a very controlled test environment where people know what they are doing.

author: janengelbrecht
author_rank: Regular Contributor
timestamp: November 16, 2014, 05:02:46 pm
content: Quote from: nctnico on November 16, 2014, 04:56:34 pmEngineers ought to know what they are doing ?

author: atsthng
author_rank: Contributor
timestamp: November 16, 2014, 05:17:01 pm
content: Grundig RT5A as an example but no longer made...(http://www.amplifier.cd/Technische_Berichte/Amplifier_Trenntrafo/isolating_transformer.html)Quote from: LukeW on November 16, 2014, 04:25:26 pm

author: Chryseus
author_rank: Regular Contributor
timestamp: November 16, 2014, 05:21:14 pm
content: Quote from: nctnico on November 16, 2014, 04:56:34 pmIsolation transformers are a must have if you're working on a mains circuit and don't have a differential probe, which is frankly out of the budget of most hobbyists by a large degree unless you build your own, every one of course should be using a GFI, in the UK at least most houses are fitted with one.As for floating the equipment, yes it carries risks but as long as you're well aware of those it's a perfectly valid technique if you don't have an isolation transformer on hand.

author: janengelbrecht
author_rank: Regular Contributor
timestamp: November 16, 2014, 05:35:18 pm
content: Quote from: Richard Crowley on November 16, 2014, 04:04:23 pmBut isolating variable transformers existIn essence isolating transformer AND auto transformer...security and variable outputOkay...if you still need the phase and zero outputs in your circuit in a circuit with need for a voltage other than the mains then an autotransformer is still a must...agree

author: nctnico
author_rank: Super Contributor
timestamp: November 18, 2014, 11:02:45 am
content: Quote from: Chryseus on November 16, 2014, 05:21:14 pmIf you use an isolation transformer the GFI will no longer protect you. Besides a proper isolation transformer is just as expensive as a differential probe so it's better to get the differential probe.