Is My Transformer Isolated? - Page 1
Forum section: electronics / beginners

author: NovumLux
author_rank: Newbie
timestamp: May 21, 2014, 06:42:25 am
content: I am pretty much a novice but getting better.I see this Sentrex PCLC-110 Power Supply, used, in a marine shop for $42.00. I need an isolation transformer, so I whip out my phone, get on line, get the specs. and on the front page it says "Super Isolation", i thought great bought it went home.The "Super Isolation" should have been a tip off because either its isolated or not, there is nothing super about it! I get home, print the specs. and on page two it says "Isolation: Complete From Line; OUTPUT NEUTRAL BONDED TO GROUND" (emphasis added). I performed Dave's test using meter to check resistance on earth grounded plug side and the PCLC earth ground and of coarse - no resistance because its bonded to earth ground and the neutral wire.When I opened the chasy, sure enough, the green earth ground on the PCLC plug goes straight into the chasy ground where it joins the earth ground from the power cord and the green wire coming from the secondary side of the transformer (same place as neutral wire on the transformer).I assume this is not the isolation I wanted for safely probing AC mains supplied equipment. But now I'm not sure. Its mainly a line conditioner and after looking at examples of isolated supplies in a reference books I don't know if its isolated or not. And if its not, can I just simply remove the green earth ground wire from the PCLC plug so it is no longer bonded to earth ground and the neutral wire. Totally confused any help would be very much appreciated. Included are the specifications and a picture of the unit circuitry that allows you to see the secondary wiring and how it is all boned together if that helps.

author: IanB
author_rank: Super Contributor
timestamp: May 21, 2014, 06:58:56 am
content: It's a line conditioner, not a test isolation transformer. The isolation it describes is isolation of the protected device from input surges, noise, spikes and other disturbances. The connection of the earth wire to the output socket and transformer neutral helps to provide this isolation.Rather than modify it for a purpose it was not intended I think I would continue to look for a genuine test isolation transformer.

author: planet12
author_rank: Regular Contributor
timestamp: May 21, 2014, 02:24:38 pm
content: Although I'd recommend following IanB's advice in the first instance, if you aren't willing or able to ("if you were me"), then it may be possible - with modifications - to safely use it as an isolation device, but this depends greatly on how the transformer has been wound.It appears to be a "split bobbin" construction, with the mains only on the farthest winding in your picture - however I can't tell for sure how things are connected just from your picture. If that us the case, the isolation is likely going to be good enough, but you'd need to very carefully verify that this is the case before proceeding to any modifications.As you consider yourself a novice, you may not wish to be fiddling with something that is going to be responsible for you Not Being Dead.

author: johansen
author_rank: Super Contributor
timestamp: May 21, 2014, 02:44:47 pm
content: edit:it can be isolated.read the manualhttp://www.mdspower.com/wiremold/pclc_specs.pdfoutput neutral is bonded to ground.clip that green wire and it will be isolated.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 21, 2014, 02:48:53 pm
content: "Isolation" has multiple meanings. As IanB said, in this case "isolation" means blocking mains noise to some extent (nothing is perfect.) It does NOT mean galvanic/voltage isolation. Furthermore, "isolating" the green-wire safety ground gets into risky territory, not recommended for amateurs.The triple-section construction of the transformer, and the presence of that large capacitor make me think this is a "constant-voltage transformer" that uses magnetic resonance between the capacitor and the third winding.http://en.wikipedia.org/wiki/Transformer_types#Constant_voltage_transformer.You may want to re-consider WHY you think you need mains power isolation at all? Exactly what are you doing that you think you need isolation even from green-wire safety ground?

author: retrolefty
author_rank: Super Contributor
timestamp: May 21, 2014, 03:15:35 pm
content: I agree with Richard, it's almost certainly a constant voltage transformer, we called them Sola transformers at work as it was a main product line of that company. As far as needing to 'isolate' mains powered equipment for troubleshooting it depends on what test gear you have available to you and what you are trying to measure or test.In field service troubleshooting I often was limited to having a mains powered scope and a DMM only. If I needed to determine what amount of current was flowing through a specific component, like a resistor, and didn't want to have to lift a lead and insert the DMM in series, I would be forced to isolate the scope by using a 3 terminal to 2 terminal AC adapter on the scope (the scope did not tie it's neutral to it's case ground) and I could just connect the scope's probe and ground clip across the resistor in circuit and get a voltage drop reading representing the current value flowing through the resistor. So it has value to sometimes 'isolate' the scope from AC ground, but I never found a reason or need to isolate the equipment being powered from AC ground. Others may have come across other needs I didn't come across?

author: NovumLux
author_rank: Newbie
timestamp: May 21, 2014, 10:58:50 pm
content: Thank you very much for your replies I really appreciate it.I think Ian is right. I'm not qualified yet to make modifications safely and it wasn't really intended for that purpose. Don't have a lot to spend right now, that's why I was trying to make this cheap. But, I know more than enough to kill myself so I will leave it to the professionals unless I can find an understandable circuit to build.Also, I wanted to be able to probe with my oscilloscope circuits that are plug into the mains without accidentally blowing something up by placing the ground lead in the wrong place and then creating a short with my probe; that is the purpose for the isolation transformer I will build or buy.So, Bob is definitely not my uncle on this used unit, as I now like to say here in California.Thanks for your replies.

author: NovumLux
author_rank: Newbie
timestamp: May 22, 2014, 06:10:20 am
content: Part of the problem I keep running into is the ambiguous specifications of "isolation transformers." Included as an attachment is a Hammond isolation transformer I might buy and the specifications show that earth ground on one side is tide to earth ground on the other side. I keep seeing this example. I must be reading the schematic incorrectly or 'am I. Shouldn't the circuit just show the floating secondary without any earth ground?

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 22, 2014, 06:24:25 am
content: Quote from: NovumLux on May 22, 2014, 06:10:20 amIt depends on what you are trying to "isolate". Why does everyone want to disconnect the green-wire safety ground?I sincerely don't get it.

author: NovumLux
author_rank: Newbie
timestamp: May 22, 2014, 07:07:42 am
content: If your using an oscilloscope and you are probing a circuit powered by ac mains and there is no isolation there is a possibility if you incorrectly attach your ground on the probe of creating a serious short that blows your scope and the circuit is made through the scope probe ground to earth ground mains back through the earth grounded mains of the circuit being tested. The isolation wanted is a floating secondary for the circuit being tested without the earth ground circuit return possibility as a safety feature.

author: Kremmen
author_rank: Super Contributor
timestamp: May 22, 2014, 11:18:29 am
content: Quote from: NovumLux on May 22, 2014, 06:10:20 amIt would be OK and good from EMI point of view to have the transformer core grounded and a static shield between primary and secondary. For an isolated system the secondary should be floating with no ground and what is more, the transformer itself should be encased in an insulator housing so as to guard against touch or connections with the grounded parts. Only in this case it is assured that no spurious connections between neutral and ground can happen. Should that be the case the isolation fails and the system is unsafe.Quote from: Richard Crowley on May 22, 2014, 06:24:25 amJust because of the above reason. There are numerous ways to create accidental connections between neutral and ground in complicated test arrangements and then you have lost the protection of the isolation transformer - maybe without realizing it which is worse than not having it in the first place.

author: IanB
author_rank: Super Contributor
timestamp: May 22, 2014, 02:01:01 pm
content: Quote from: NovumLux on May 22, 2014, 07:07:42 amFor which the short answer is, "Don't Do That". Seriously.If you probe low voltage circuits which are powered from the isolated low voltage side of a step down transformer or power supply you will be fine.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 22, 2014, 03:26:13 pm
content: Quote from: NovumLux on May 22, 2014, 07:07:42 amIf you whole experiment is at some dangerous voltage above ground, then that is exactly the OPPOSITE of a "safety feature". It means you are doing something seriously dangerous and you should re-think what you are doing and how you are doing it.Back before most of you were born we had "hot chassis" radio and TV receivers which ran directly off the mains power with no transformer. The rectified the mains power directly for the B+ voltage (typically a few 100 volts), and in the typical 5-tube radio, the heater filaments were all connected in series and designed to run on 110V total. THOSE were the days when isolation transformers actually made things SAFER to work on. But I would like to see some examples of where people think they need to isolate the mains power, INCLUDING the green-wire safety ground. That is just inherently dangerous, and I cannot conceive of how circumventing the green-wire safety ground makes anything "safer"?

author: N2IXK
author_rank: Frequent Contributor
timestamp: May 22, 2014, 04:10:33 pm
content: A true isolation transformer is useful for doing any testing on the primary side of a SMPS, or any other equipment that directly rectifies the incoming line voltage. For the same reasons it was useful for working on "hot chassis" TV and radio sets.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 22, 2014, 05:48:53 pm
content: Quote from: N2IXK on May 22, 2014, 04:10:33 pmAgreed. But the ability to work on a SMPS would seem to be well beyond the skill level of the people who are posting questins in these forums about probing dangerous things with their new o'scopes and using mains iso transformers. Something just seems incongruous here. If you actually know how to work on a SMPS, you wouldn't be asking these kinds of questions in the first place.