Electronic – Reverse engineer a custom (failed) transformer

identificationreverse-engineeringtransformer

I am attempting to repair an aged air conditioner I have tracked the issue to a failed secondary on a current transformer.

Very helpfully schematics are available for the circuit board and inform me that the part is a ETQ19Z59BZ this appears to have no information available so I suspect it is custom.

I have closely inspected around the terminals of the transformer with a loupe and cannot see any broken wires.

With a open circuit secondary is there any easy way to identify the properties of the transformer to allow me to select an appropriate replacement.

Best Answer

Take the transformer apart and count the number of turns on both the primary and the secondary. Any direct replacement should have the same ratio.

Look at the schematic and see what the burden resistance is. You should also have some idea of the maximum current the primary has to be able to handle.

Now look around for something that can handle the primary current, has the same turns ratio, and is specified for close to the same burden resistance.

A alternative to the above would be to get what you can that has about the right size and primary current capability, then adjust the current sense circuit accordingly. You say you have the schematic, so there is probably a way you can tweak the gain between the current transformer and whatever uses the signal a little either way.

I would also help to know what the purpose of measuring the current is. You say this is for a air conditioner. It is probably to verify that the compressor is neither stalled nor open. There may be wide latitude in the sense levels.

We may be able to help more by seeing the schematic.

Related Solutions

Electronic – Is it possible to reverse engineer a chip design

ChipWorks has an excellent blog about doing exactly this, with lots of great pictures here.

FlyLogic also has an excellent blog. It is here.

The short answer is it is absolutely possible. IC DIEs are basically really small circuit boards. You can reverse engineer them pretty easily, it just takes a different tool set.

I want to particularly call attention to some posts flylogic did on reverse-engineering ICs (how topical!) here and here.

^{Image from flylogic website}

Electronic – Reverse Engineer Serial Signal

My first thought (based on the async decoded data only) was this could be Manchester encoded signal, instead of async serial. Now I think the data is encoded by supressing alternate clock pulses.

Here's why:

With the exception of the 0x80 (which may be a "start byte"), the async serial 8N1 claims that the messages are composed of only 10 distinct symbol bytes:

0x55 01010101 (139 occurrence) 
0x57 01010111 (32 occurrence) 
0x5d 01011101 (16 occurrence) 
0x75 01110101 (45 occurrence) 
0x77 01110111 (49 occurrence) 
0xd5 11010101 (5 occurrence) 
0xd7 11010111 (5 occurrence) 
0xdd 11011101 (24 occurrence) 
0xf5 11110101 (11 occurrence, only at end of message) 
0xfd 11111101 (2 occurrence, only at end of message)

Looking at the binary it looks like the message bitstream is composed of long and short pulses, and may not be correctly synced up with the start and stop bits of the async serial. The only bit patterns that have more than three 1's in a row are 0xFD and 0xF5, but that occurs only at the end of a message -- and remember, async serial is LSB-first. So those longer sequences of 1's in the MSB may be after the end of the underlying Manchester encoded signal. Similarly the 0x80 at the start of each message may precede the underlying signal.

Atmel's primer on Manchester Coding Basics gives a good overview of encoding and decoding: http://www.atmel.com/images/atmel-9164-manchester-coding-basics_application-note.pdf

waveform capture picture

Now that I see the waveform, it's definitely not Manchester coding... Looks like idle-high, with a long pulse as a header, followed by a very long string of clock pulses -- with occasional missing clock pulses. At this point I'd guess the pulses alternate between Clock pulses and Data pulses. So a 0 is encoded as two clock pulses, and a 1 is encoded as one clock pulse and one "missing" pulse. I'm not sure what this is called but I think I've seen it before, a long time ago...

Best Answer

Related Solutions

Electronic – Is it possible to reverse engineer a chip design

Electronic – Reverse Engineer Serial Signal

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