Transformer substitution - Sanyo DP42849-01

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

I don't like to give up, as we can see from how long I've been working on this project. I've been doing a lot of research on ferrites and I can find a lot of references to them being permanently damaged from overheating, but these are not even getting half the temp that permanent damage would occur. So, I'm led to believe it is something I'm doing wrong. And, I actually think I've figured it out. In the ones I've rebuilt, the ferrites aren't touching each other. There is a very thin layer of adhesive which separates them. I had originally believed this was plastic, part of the primary bobbin. Then I pushed through one with a screwdriver because I had no other way of getting a broken ferrite out. What I found, is that this layer broke out way too clean to be part of the bobbin. If it is part of the bobbin, it must be a defect because it significantly affects the transformer's inductance. I experimented a bit and found that separating the primary and secondary ferrites by as little as 1mm reduces their inductance by almost 40%. I'd bet there's at least that space between the ones I've rebuilt because I've added my own adhesive on top of that. The piece I broke out of one was .52mm thick. I have the last 2 I rebuilt soaking in acetone to loosen the epoxy. I'll try one more time when I get them apart. Probably have to soak them at least 24-36 hours.

I have not failed, I have simply learned several ways not to rebuild a transformer. And, what I lack in success, I have gained in knowledge.

[Agent24]

Re: Transformer substitution - Sanyo DP42849-01

Bummer... assuming everything else is the same, including the winding technique, you may be right about the ferrite being overheated or damaged in some way. It's the only other thing I can think of that would affect inductance too.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Failed again. It does come on, and the temp is much lower, but once you turn it off it wont come back on until it cools. Inductance is very low on the new primaries. All I can think of is that the ferrites must be damaged from overheating. I guess I'm going back to the original plan, trying to find a suitable substitute and making an adapter board.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

If you ever plan on making litz wire, make certain you have a lot of patience. I wasted at least 800 feet of wire before finding a way that works. This stuff is just way too fine to work with 1 strand at a time. I had originally planned on trying to make some complex jig which would twist and wrap this stuff onto a spool, but then I found a YouTube video where some guy was doing it the way I'm now doing it, but he was working with much bigger wire. Well anyway, I had bought 50 plastic sewing machine bobbins for that project. So, what I just did that worked, is I wound 5 bobbins with the wire, put them all on a screwdriver, soldered the ends together, and then I was able to work with 5 wires at a time. And, before I twisted them, and after, I coated them with insulating varnish so they will not unwrap. So far, so good. I'm waiting on the first one to dry now.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Well, that litz wire I ordered was too small so I decided to try my luck at making my own. I made a jig which involves me stretching the wire from a hook in my living room wall, to another hook in my shop doorway, about 20 feet away. That hook can be spun with a screw gun equipped with a phillips bit. It's 25 strands of 39AWG magnet wire I got from Ebay. First attempt, I'll call fair. I'd say it went well, but as soon as I released tension, it unwound quite a bit. I wrapped 1 primary bobbin with it and my resistance is .84Ω. That's right on target with what I was expecting the original windings should have been. The two issues I came across were, 1, that it didn't stay twisted as tightly as I wanted it. And 2, that each strand wasn't equally tensioned so I noticed in a few areas that I had individual strands with a bit of slack. So I'm not going to use this first one. My next attempt, I'm going to try to maintain equal tension between strands, and I'm going to coat the wire with insulating varnish before I release the tension.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

I'd guess the quality of the litz wire is probably not even close to that of my datasheets. So that may be a factor.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

I'm do believe bad design plays a role. Or rather, substandard materials. But they typically last a few years. I unwrapped 2 and tested them with my LCR meter. The first tested at .89 ohms. But I forgot what it tested on the bobbin. Chalk that up to my frustration with unwrapping it without removing the ferrite. The second measured .70 on and off the bobbin, but there was no ferrite when I tested it. I don't think that makes much difference though. Increased resistance I can understand. They have a bondable coating so I could see the possibility of damage while unwrapping. But reduced resistance while unwrapped, that has me baffled.

I don't have a ring tester.

[Agent24]

Re: Transformer substitution - Sanyo DP42849-01

I wonder if the resistance dropped due to failure, maybe shorted turns? Did you ever try a ring tester on these?

If the resistance was lower from the factory, I guess it would have to have less turns or thicker wire. Less turns might be plausible if they changed the core material I suppose but I am not an expert...

In any case a lower resistance should mean higher current assuming nothing else is controlling it. Maybe that leads to overheating and breakdown?

Don't overlook the possibility of bad design, of course.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

I no longer have a good board with a stock transformer. I couldn't keep the money tied up for too long. I can say, just from memory, that I usually get above .4 Ohms on a good transformer tested in board. They are connected in parallel so the actual resistance on each side would be double that. So, about .8 Ohms.

[Agent24]

Re: Transformer substitution - Sanyo DP42849-01

It is interesting that the primary winding has lower resistance than expected, but the question is why? Does a 'good' transformer have the same resistance or closer to the expected 0.82 Ohms?

I don't know if they can be damaged, but they do change with temperature: http://www.mag-inc.com/products/powd...r-powder-cores

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

I think I may have solved this. I tried again using 34AWG HPN. The "35" I'd used before was way under specs and was more likely 37AWG. So, using the 34AWG, the open air temp was around 170-180°F. Still hotter than a working stock transformer, but about 20-30°F lower than my last attempt. Resistance measurements were around 24.5Ω as opposed to 26.5Ω on a working stock transformer.

I found the housing for this TV in a pile in my basement so I decided to see what effect enclosing it would have on the temp. I used a wire probe for these tests, inserted between the two secondaries, rather than the laser. The temp gradually rose to 234°F and stayed there throughout the 12 hours of testing.

In previous tests, I'd determined that the board with a rebuilt transformer was drawing 1.68A and a stock one was drawing around 1.45A. About a 13% increase.

Up to this point, I've been carefully disassembling these so I could save the primaries. Now I'm led to believe the primaries are the cause for the increased current and for the secondaries burning out in the first place.

Going over datasheets for 25/40 litz wire, which is what the primaries are wound with, I've determined that I should have around .82Ω on each primary. I determined that by unwinding and measuring the wire, then calculating based on the resistance at 1000' from the datasheet. What I have on several bad primaries is around .70Ω. That's pretty close to 13% less resistance. 13% less resistance would mean 13% more current flow right? I no longer have a working stock board.

I've several times swapped good transformers from boards with bad PWMs or other bad components, when I didn't have the parts on hand, so I'm pretty confident that the sole problem is the transformers. I've lost count of how many of these I've encountered. I know I have 8 boards without transformers right now, and I've sold at least that many.

I'm probably on my final attempt now. I've ordered enough 25/40 litz wire to rebuild 1 transformer. I wasn't able to find it unserved for a reasonable price, so I'm going with served litz wire. For anyone wondering what that means, served litz wire has a sleeve over it to provide additional insulation.

If this doesn't work, then the only other possible explanation I could think of would be that the ferrites have been damaged from overheating. Is that even a possibility?

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

I thought that was how you got the desired thickness

[stj]

Re: Transformer substitution - Sanyo DP42849-01

i dont understand the need for double or tripple insulated, isnt it about the thickness/material rather than the number of layers?

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Heavy build is double insulated I guess.

[budm]

Re: Transformer substitution - Sanyo DP42849-01

"heavy build" I do not understand the heavy build. BTW, the winding are on its own bobbin too to the leakage between the windings will be very very low.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

So a heavy build would probably be OK then?

[budm]

Re: Transformer substitution - Sanyo DP42849-01

Only if you want it to go to compliance testing. I had to do that in our export products.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

I don't have any way to figure out the voltages in these secondaries. Do you think I really need triple insulated wire? It is incredibly expensive.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Thank you

[Agent24]

Re: Transformer substitution - Sanyo DP42849-01

You've done a good job so far, most people wouldn't even bother trying!