Transformer substitution - Sanyo DP42849-01

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

OK, laser temp probe arrived today.

Good board with stock transformer, after about an hour, secondaries were about 163°.

Board with rebuilt transformer, 190-199°.

And those are open air readings. I suppose they would be hotter if enclosed in the TV. Definitely not safe. These will not be sold. I have one more awaiting rebuild and I'm going to try a larger gauge wire. If that makes a difference, I will attempt to disassemble the ones I rebuilt already. To my advantage, I know what epoxy I used and have some left. So, I'll make some samples and see what solvents will work best to break it down. At least I'm learning from doing this.

[stj]

Re: Transformer substitution - Sanyo DP42849-01

was the heat on the primary side, or secondary?

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Secondary

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

The wire I used is .0005 smaller than the original. 0.0045 instead of 0.005. 35awg triple insulated. It was supposed to be 0.005. I should have sent it back when it was smaller than it should have been. I didn't think it would make this much difference though.

[stj]

Re: Transformer substitution - Sanyo DP42849-01

wouldnt it be better to oversize a little - given that the original design dies a lot?
your difference there is hardly small - 10% diameter, but the actual area is probably closer to 20

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

That would probably be a good idea. I hadn't thought of that. There isn't a whole lot of extra room on the bobbins though. Maybe I could go up a half gauge. Not much more though.

[Agent24]

Re: Transformer substitution - Sanyo DP42849-01

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

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Thank you

[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.

[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

So a heavy build would probably be OK then?

[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

Heavy build is double insulated I guess.

[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

I thought that was how you got the desired thickness

[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?

[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 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

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'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.