HVAC Question.

My AC has been blown a few transformers, so obviously there is something wrong in the units. I have tested all the control wires for shorts, and the fans, blowers, switches and coils for shorts. Nothing reads below 500 ohms, except for the 24v power leads to the board, that reads 9ohms. At 24v/9 ohms that about maxes out the transformer. So I guess my question is.. Is 9 ohms a bad reading at the control board and can there be a short in the board that still lets the system run but burns out a transformer in about 10 days? Oh and the control board does not show any bad error codes.
Thanks
Todd
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snipped-for-privacy@gmail.com wrote:

Boy, I guess that *does* overload the xfmr! Not the HVAC expert, but I wonder if the board has an MOV on the 24V bus and the MOV might be shorted.
Or a filter cap after the bridge rectifier shorted. Unsolder stuff like that and measure again?
Jim
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snipped-for-privacy@gmail.com wrote:

Are you sure the transformer is sized correctly? Maybe you need a bigger one.
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I am pretty sure it's sized right because one of the replacements that burnt out was the same part as the origonal which had been working for about 10 years.
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On 21 Jul 2006 15:44:59 -0700, snipped-for-privacy@gmail.com wrote:

What are the times between installation and blowing out, first one first, through last. Estimates at least. So far I don't know if you are talking about 3 months or 3 minutes.
If they last a month and the thing works, well something has almost failed, or maybe one of the 4 diodes in the bridge rectifier has failed.

Usually the first parts the AC current encounters in a transistor board is a bridge rectifier, either 4 diodes arranged in a square**, with input on 2 opposite corners and output on the other two corners. Besides the parts the other guys mentioned, if one of those diodes is shorted, that would be bad. Did you measure the resistance in both directions? If it is higher in the other direction, that would be a clue that the once symmetric bridge rectifier is now asymmetric, because one of the 4 diodes is shorted. When good they aren't zero and infinite in the two directions. They are low and high. When bad, they are probably low and low. You might ask for more help on sci.electronics.repair . They are nice there too.
You may have to do some unsoldering too measure these things, because the circuit is not a simple rectangle, like in first electric classes.
**If it's not 4 separate diodes, it could be diodes in one little box (with 4 leads.) I don't know how hard it is to find these, but you can just use 4 separate diodes to replace them. It gets confusing which direction each of 4 point
My transformer burned out the first July 4 weekend I lived here, when I had 3 guests.
I couldn't get just a replacement trasformer that fit the original place, so I had to buy a bigger one and mount it somewhere else. Has lasted 23 years. I didn't do any testing though, beyong testing the output of the xformer, which was dead. So I'm not saying a bigger one would work better. But maybe one would. ask on the other ng.

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had a similar problem..kept blowing fuses.The control wires had a bad spot with deteriorated insulation close to the compressor.When the fuse blew I would check and all was good.But after the unit ran a while a little condensation from the lineset would find its way to the bad spot on the control wires shorting them.I pulled a new 24vac control wire and fixed the problem.
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