Help understanding isolation transformer circuit - Page 1
Forum section: electronics / beginners

author: prestonian
author_rank: Newbie
timestamp: December 05, 2022, 05:31:37 pm
content: I have a Tripp-Lite IS1000 isolation transformer. I want to use this to electrically isolate devices from mains and earth ground (during service/repair procedures), and aim to modify this transformer to achieve that.However, this isolation transformer's circuit is confusing me (hand-drawn circuit diagram attached). My concerns:1) I cannot identify the primary and secondary coils because mains hot/neutral are connected to both coils. Why?2) The outputs also tied together on both coils, at the outlets. Why?3) This circuit seems to deviate from my expectations. I expected both coils to possibly share earth ground, but not couple mains (input) together, and couple the outputs together. A general analysis of what's going on here would be appreciated.* edit - schematic now attached

author: bdunham7
author_rank: Super Contributor
timestamp: December 05, 2022, 05:39:26 pm
content: Both 'coils' are both primary and secondary--they actually have two separate windings. What it looks like is they have two 120V transformers (each 'coil') and they are operating them in parallel. Presumably the 240V version would use them in series. Since they don't have a shared core, you wouldn't be able to use them in a step up/down configuration like some other dual-voltage isolation transformers.

author: langwadt
author_rank: Super Contributor
timestamp: December 05, 2022, 05:47:12 pm
content: Quote from: bdunham7 on December 05, 2022, 05:39:26 pmwhy not? you could run the primary in series and secondary in parallel or vice versa

author: Stray Electron
author_rank: Super Contributor
timestamp: December 05, 2022, 06:35:43 pm
content: It's not considered good practice to tie transformer outputs in parallel since no two outputs are EXACTLY equal and you could have one transformer trying to fight the other one. But it's done all of the time.Unless one outlet draws more power than a single transformer can supply (and therefore you need the 2nd transformer to assist it), I would disconnect the L and N links between the two outlets. That would prevent the two transformers from fighting each other and it would give you two AC circuits that are not only isolated from the AC power line but also from each other.*Earth ground doesn't really come into play in these transformer circuits. E should be tied to the E outlet on both outlets and to E pin of the incoming power circuit and to the metal case and nowhere else. (Well possibly to any RIFA filters.) E is strictly for personnel safety and is not part of the power circuits and the circuit should run perfectly fine without any E connections at all. In higher frequency circuits E may be connected to some RF shielding but in this circuit (60Hz) that shouldn't be an issue.* If you work on something like an old radio or TV that has a HOT chassis, then you need to electrically isolate the AC power on many scopes and meters or you can get a direct AC current path from the radio chassis back to the AC Neutral an/or AC Ground through the scope probe or the probe's shield.

author: bdunham7
author_rank: Super Contributor
timestamp: December 05, 2022, 08:24:57 pm
content: Quote from: langwadt on December 05, 2022, 05:47:12 pmHmmm, I think you're right--I didn't think that through. If the primaries are in series the current has to balance because it's the same current.

author: prestonian
author_rank: Newbie
timestamp: December 05, 2022, 09:56:10 pm
content: It's all making sense now. I didn't realize that each "coil" is a whole transformer.

author: Stray Electron
author_rank: Super Contributor
timestamp: December 05, 2022, 10:16:27 pm
content: Quote from: bdunham7 on December 05, 2022, 08:24:57 pmAccording to the schematic that you drew, your primaries are in parallel. Therefore there is 110 VAC applied to both of them. And their Input to Output ratio is 1 to 1 so you're going to get 110 VAC (More Or Less!) out of each one. That's why the manufacturer was able to get away with tying the outputs of both transformers together in parallel.The problem occurs when one transformer puts out say 110.0 volts and the other 110.5 VAC. Now you have 0.5 volts forced into the output of first transformer and VERY little resistance in the connecting wiring and two windings.

author: Stray Electron
author_rank: Super Contributor
timestamp: December 05, 2022, 10:17:16 pm
content: Quote from: prestonian on December 05, 2022, 09:56:10 pmBINGO!