Isolation transformer, wiring question - Page 1
Forum section: electronics / projects

author: cs.dk
author_rank: Unknown
timestamp: August 31, 2018, 11:09:29 am
content: Hi,Just got my self a 5A isolation transformer for cheap.However, something looks wrong to me. The blue and yellow are the primary (in), why are they joined like the picture shows?Can a thermal fuse be potted? The label says it has a 5A thermal fuse on the primary, but i can't see one.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2018, 12:30:49 pm
content: If it has a thermal fuse, it's most likely embedded into the windings. The primary should still be fused anyway, as the thermal fuse is non-replaceable.

author: cs.dk
author_rank: Unknown
timestamp: August 31, 2018, 02:14:28 pm
content: ThanksAny idea why the other wires are soldered together?

author: ArthurDent
author_rank: Super Contributor
timestamp: August 31, 2018, 04:00:44 pm
content: I would hazard a wild guess that is a used transformer and that the embedded thermal fuse died (it happens) and the previous owner bypassed it. Not a very neat solder job but apparently it worked. A bypassed thermal fuse isn't the worst thing in the world and if that is what was done and you want to restore that function you could get a replacement with an appropriate thermal rating and epoxy it in the donut hole of the transformer. One reason for potting the fuse is so there is good thermal transfer between the transformer and fuse.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2018, 04:07:00 pm
content: Quote from: ArthurDent on August 31, 2018, 04:00:44 pmI would have thought the fuse would be embedded in the windings, making that unlikely.I think it's more likely there are two primary or secondary windings for operation at either 120V or 240V. The blue and yellow wires are probably where the two windings are connected in series, for 240V operation. To find out, power it up and measure the voltage between the soldered blue and yellow wires and the blue and yellow wire. If it's 120V, then it's as I thought. If it measures close to 0V to one of the wires and the full mains voltage to the other wire, then it is indeed a fuse connection.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 31, 2018, 09:49:02 pm
content: The original post says there is a label on the transformer that states there is a thermal fuse on the primary. If it has this information the label must also indicate what the voltage rating is as well. If there were 2 isolated primaries and only one thermal fuse there is only one way to make this work and that would be to have 5 leads going to the primary side. One line lead would have to go through the thermal fuse to one side of one primary and then the 4 remaining leads from the two primaries would go to a voltage selector. This is the only way that a thermal fuse could work for two primaries if they could be wired in series or parallel. attached is a traced out circuit that a commercially made bench supply used.If there is a single primary, which is what I think the wiring indicates, then the 2 jumpered leads could be directly across the thermal fuse and could go to a blown fuse indicator. The thermal fuse is probably in series with a lead wired as I described above and it would make little difference if the thermal fuse is against the outside of the transformer or potted in the epoxy. All that is required is close thermal coupling between the two. If the jumpered leads are opened and checked for continuity and the thermal fuse has failed then one of the leads should read open circuit to the other leads. If there are two identical windings then the transformer could only be used on 240 for the thermal fuse to be in series with both primaries.Maybe cs.dk could show a photo of the label and/or supply more information.

author: cs.dk
author_rank: Unknown
timestamp: September 01, 2018, 04:57:00 am
content: Quote from: Hero999 on August 31, 2018, 04:07:00 pmNow thats weird.. From blue to the soldered wires theres 4.xxV and from yellow the the soldered wires theres 16.xxV - Whats going on there?The output is a little bit higher then the input. 229V in and 239V out, but thats unloaded. I think with a load it will equalize.Arthur, I have attached the label. (Danish translation: Ind = In - Ud = Out)To my surprise when I powered it up, this thing hums relative loud in my opinion.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 01, 2018, 01:23:25 pm
content: The wiring diagram corresponds to the physical layout on the connector so it is a 1:1 220V isolation transformer but there are still those two soldered leads and the other larger white wire that aren't mentioned.I still would like to see the jumpered leads opened and continuity checked for all leads. The voltage readings to the jumpered leads from the primary would indicate that they aren't connected to the primary but have capacitive coupling that gives meaningless readings. It is curious that the jumpered leads aren't shown on the drawing.

author: cs.dk
author_rank: Unknown
timestamp: September 01, 2018, 01:34:52 pm
content: Quote from: ArthurDent on September 01, 2018, 01:23:25 pmThe larger white wire does not go to the potting (I see what you mean on the image) - The white is the one feeding the connector-block. (Both in and out is white on their outer insulation). They just go out of the plasticbox.I'll try to open the soldered wires.. I think they'll get a screw terminal block for isolation anyway. And the soldering job looks really shitty IMO.

author: cs.dk
author_rank: Unknown
timestamp: September 01, 2018, 01:41:43 pm
content: Desoldered the wires.Yellow to yellow is a dead short. 0.0 ohms. Blue to blue jumps a little from 0,3 to 0,5 ohms. Thats with a BM257 multimeter. Maybe a LCR meter is better suited for these low values?EDIT: Blue (soldered end) to yellow is open, and yellow (soldered end) to blue is open.Big thanks to all these knowledgeable people on the forum

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2018, 02:04:02 pm
content: Quote from: cs.dk on September 01, 2018, 04:57:00 amThe output voltage being slightly higher than the input is normal. It will normally have a slightly higher turns ration thant 1:1 to compensate for the voltage dropping under load. 239/229 = 1.04, so the turns ration will be 1:1.04 which seems reasonable for a transformer this size.I don't know about the wires which are soldered together, which is very odd.

author: cs.dk
author_rank: Unknown
timestamp: September 02, 2018, 07:38:23 am
content: Quote from: Hero999 on September 01, 2018, 02:04:02 pmSeems resonable that it will drop voltage as the load rises.When measuring on the terminalblock, there's no continuity from blue to yellow, when the soldered pair is open.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 02, 2018, 03:01:09 pm
content: With the added information I'm still going with my initial guess in posts #3 and #5 that the jumpered leads originally went to a blown fuse indicator and when the thermal fuse opened for whatever reason someone cut the leads short and jumpered them to allow the transformer to be used despite the open fuse. The following two quotes from cs.dk support this theory and if you check my drawing, it makes sense. With the wires jumpered the resistance of the primary and secondary at the terminal block should be perhaps several ohms with the primary maybe being ever so slightly lower. It still would be a wise idea to add a replacement thermal fuse in series with the jumpered leads as I described in my previous post."EDIT: Blue (soldered end) to yellow is open, and yellow (soldered end) to blue is open.""When measuring on the terminalblock, there's no continuity from blue to yellow, when the soldered pair is open."Note: I just took another good look at the soldered connection and I can't be sure not having the transformer right in front of me but it looks under the yellow insulation there appears to be one solid insulated wire just like the winding wire. That yellow wire is soldered to the short blue wire that looks like stranded tinned wire. The other yellow wire that goes to the connection block may also be solid wire. If this is correct then that would also support that the two yellow solid wires are just the primary.

author: cs.dk
author_rank: Unknown
timestamp: September 09, 2018, 06:44:25 am
content: Sorry for the late reply, been busy in the week.I think your right in your theory. The blue is a stranded wire, where all others are solid. It begins to make sense now.I think i'll add a 5A fuse in there, just for good measure. Is a fuse normally on the input or the output of the transformer?

author: SeanB
author_rank: Super Contributor
timestamp: September 09, 2018, 08:10:50 am
content: Normally on input to protect the transformer, though you can certainly add a separate fuse as well to the output to disconnect in case of a overload on the output side.

author: cs.dk
author_rank: Unknown
timestamp: September 09, 2018, 12:30:40 pm
content: Would it be OK, with a 5A fuse on the output to protect the transformer from overload, and then a larger ie. 10A on the input in case the transformer shorts out? Or it that bad practise?

author: ArthurDent
author_rank: Super Contributor
timestamp: September 09, 2018, 01:01:41 pm
content: With a 1:1 transformer any current in the secondary will be the same (actually slightly greater) in the primary. Because of inefficiency if there is a 5A load on the secondary of the transformer there will be perhaps 5.2A flowing through the primary. A single 5A fuse on the primary should be all you need but this fuse will be for over current and not serve the same function as the open thermal fuse that is now bypassed. As I mentioned before I would get a new thermal fuse and wire it in place of the jumpered leads. It can be epoxyed in the center of the toroidal transformer and will sense the heat the same as the original thermal fuse.Any regular fuse you put in the primary side should never be greater than the transformer current rating.

author: cs.dk
author_rank: Unknown
timestamp: September 15, 2018, 09:49:21 am
content: Thanks - I'll get a fuse in there again

author: cs.dk
author_rank: Unknown
timestamp: January 05, 2019, 08:55:26 pm
content: So, had to use the transformer today.. Installed a fusesocket long time ago, and put it away with the box closed..No outputHmm, open it again - Forgot to join the two wiresBut now i know why the soldering looks like it did. The yellow wire is enamelled, and it was quite hard to get rid of. I did though that soldering iron temps would melt it, but no.How ever, it works again - Only annoying thing is the mains hum. I'm surprised it's so noisy.