Transformer substitution - Sanyo DP42849-01

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Oh, the inductance was 36.4mh in the rebuilt, and 39.3mh in a working stock transformer. Both tested in board. The rebuilt sounds better than any I've encountered so far too. It is completely silent. A lot of the working ones I've encountered have a light buzz/squeal to them.

[budm]

Re: Transformer substitution - Sanyo DP42849-01

You sure do have lots of patience on this project, great job.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Thank you. I enjoy this stuff.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

After 24 hours of testing this, I think I may possibly have a problem. It seems like it's getting too hot. It doesn't smell hot, just feels hot. It's sitting right on the backlight with a plastic sheet as an insulator so that may be a contributing factor. I no longer have the housing for this, I wish I'd kept it now. I don't have a temp gauge and my stock board sold yesterday so I have no way to make any real comparison. What I do know is that I have 32.5 ohms resistance rather than 25.5 - 26.5 which was average on most working transformers I've encountered. Increased resistance is going to increase heat correct? Thinking about it, it's nearly 25% higher resistance than what I suspect it should be. So maybe I should use larger gauge conductor. I'm noticing too that in magnet wire, specifications seem to vary dependant upon manufacturer. I'm noticing now that if I give my caliper the slightest pinch, it reads 0.0045 instead of the 0.005 which I get from the old wire. I epoxied these things real good. I doubt I can disassemble them again. Advice?

[budm]

Re: Transformer substitution - Sanyo DP42849-01

You may have to check the circuit current draws and compare it to the working original to see if the circuit is drawing too much current.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Would I measure the current with a clamp ammeter on the wires connecting it to the backlight? Also, I do have a few other board waiting for transformers. Maybe, since I don't have a working original at this time, I should put the transformer on a different board and see if it is the same.

[budm]

Re: Transformer substitution - Sanyo DP42849-01

You will use your digital meter in DC A mode and connect it in series with power supply line that feeds the inverter circuit to see the circuit current draw, or if it has the fuse for each section of the transformer drive circuit then you can put your meter in place of the fuse to see current draw of that section and compare it to the section that has original transformer.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Only has the 1 fuse on it. So I would pull the fuse then and connect the meter in series. OK. Thanks.

[budm]

Re: Transformer substitution - Sanyo DP42849-01

Yes, I am surprised that it only has one fuse.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

This is the only fuse I see.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

I get 1.68 amps. And as stated, I don't currently have a board with an original transformer. I have 2 more coming though so I might have a good one within a week. I'll cross my fingers and hope one is good. I've worked on at least 10 of these so far though and only 3 had good transformers, so the odds are not in my favor.

[budm]

Re: Transformer substitution - Sanyo DP42849-01

There may be a fusible resistor for feeding the Voltage to that inverter circuit where the power MOSFETs are mounted on that heat sink to the right of the inverter transformer. You can then check the voltage drops on that fusible resistor to calculate the current draw.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Can you tell from these pics which one it would be?

[budm]

Re: Transformer substitution - Sanyo DP42849-01

It looks like you will be measuring the DC Voltage across fusible resistor R322 which is fed by the B+ from the positive pin of the main filter cap C821, this resistor will show the total current draw of the Inverter, standby power supply and the third supply I cannot clearly see from the picture, but this will give you some idea of the current draw, the inverter circuit will draw most of the current.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

OK, I will take the readings tomorrow and post back with the results and calculations.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

I finally came up with another working board with a stock transformer. That board draws 1.45 amps and both with rebuilt transformers draw 1.68 amps. It's also the same if I put them on different boards, as I have a few spares without transformers. I ordered a laser temperature probe. When it arrives, I'll see what the actual temp differential is. I have a feeling I am going to have to up the gauge on the magnet wire. Also, I got the exact same voltage on both sides of resistor R322. Maybe I didn't do it right. I measured from the negative pin of the main filter cap C821.

Got the digital proximity sensor counter installed and working in my winding jig too. Pic attached.

[budm]

Re: Transformer substitution - Sanyo DP42849-01

You have to measure across the resistor, not ref to the circuit ground (the negative leg of the cap). You will put one probe on each end of the R322 which is very low Ohm resistor to measure the Voltage drop on the resistor, then you can calculate the current draw, but since you already made the current measurement then you already know the re-built Xformer is drawing more current (16% higher) than the original, just have to figure out why.

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Could the wire gauge be the cause? Resistance is 12 ohms higher (6 per side) than the average working transformers.

Here's the readings.

Board with original transformer: 0.038vdc

Board with rebuilt transformer: 0.041vdc

[budm]

Re: Transformer substitution - Sanyo DP42849-01

It could be the inductance, the DCR, and the turn count, or the core is being saturated so the inductance will drop and cause more current draw. When they design the circuits, they take lots of values into account.
http://www.edaboard.com/thread199057.html

[lookimback]

Re: Transformer substitution - Sanyo DP42849-01

Turn count is definitely correct. I was meticulous with that. I get what you are saying about inductance. So if the core is saturating, that would mean the inductor never reaches it's resonant frequency. Correct? Would the impedance of the inductor affect it's resonant frequency and result in saturation? There's certainly more to this than rewinding the transformer. I'll put some more time into this tomorrow. Thanks again too.