Why does the high voltage side of transformers tend to burn out first?

[Rive]

You haven't noticed that everything built is basically becoming cheaper?

What matters here (by my opinion) is, that control circuits needs less and less power, so the originally robust (few dozen VA) transformers feeding it started to shrink (few VA). With that, they got thinner and thinner wire on the primary. So thin that a few second more soldering could thin them further: so thin, that a few mm in free air could work as a fuse.

The introduction of small scale SMPS circuits was a real blessing and had a definitive impact on reliability.

 

[sophiecentaur]

The transformer is also the front end interface to the power grid. The transformer helps to filter spikes protecting the rest of the circuitry on the low side.

It's not the size of the job that counts, it's how it's achieved. You can't really 'excuse' bad transformers once their requirements have been specified and they have been specified for a hundred years or more.
Where would we be if that was the attitude with modern solid state devices? One place we wouldn't be is talking to each other via PF.

 

[anorlunda]

It's not the size of the job that counts, it's how it's achieved.

I think you're misunderstanding. Suppose we have a voltage surge from the power system. A spike high enough to damage equipment. So given any complex electrical device, how would you guess which component fails? Wouldn't it be the component electrically closest to the mains?

Sometimes we use a MOV, metal oxide varistor, closest to the mains to make a sacrificial surge suppressor. It is shown as VAR in the circuit below. So if VAR is missing, what else would you expect to fail first?

 

[DaveE]

Also recognize that you can often avoid putting a fuse in your circuit if you design (and test) the transformer to fail safely. This is done by melting the primary windings. So, it's not always the transformers fault.

 

[sophiecentaur]

I think you're misunderstanding.

No. It is you who is misunderstanding my questions and comments. I am well aware of possible failure modes and possible causes and I think you know the engineering is not the issue.

It is possible to transport a take a drone to Mars and to fly it on the surface of Mars many times. If it failed to take off then would there be any use in explaining to me that a component failed because it had not been suitably specified and built? My question was about the choice of a manufacturing system that (so the story goes) delivers faulty / inadequate transformers for equipment. We all know that one could be made to last for hundreds of years under almost any conditions.

We all have the right to moan and complain about shortcomings in equipment and that's what I am doing. Could I do better? Yes - given time and a suitable price.

 

[Averagesupernova]

It's quite simple. Folks have gotten used to the fact that most consumer items are not built to last and take the abuse and continue to work like a drone flying to Mars. It is accepted that water heaters get tossed every few years and home heating systems are not far behind. We all know residential furnaces could be built to last a very very long time. The cost of such a furnace would be much higher than what we have now. I can't say I like it this way, but in some cases it has advantages. My high efficiency gas furnace has failed due to a leaky condensate pan and a plugged orifice that serves to carry the flame across to all four burners. None of that was electronics related and I cannot complain other than about the dummies that tried to sell me a new furnace because they were too stupid to realize what was happening when it went into safety lockout because the flame was not being carried from the first burner next to the igniter all the way across to the fourth burner where the flame sensor is mounted.
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If we want to have equipment that lasts for half a century, then we need people that understand the equipment better than the schmucks that attempt to service the equipment now and throw their hands up and say: I guess you need a new one. It doesn't matter how well something is built, when it has some mechanical parts, it will need service of some kind in 50 years.
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So to boil it down to why are these transformers failing, it can be simply said because the manufacturers can get away with it.

 

[sophiecentaur]

So to boil it down to why are these transformers failing, it can be simply said because the manufacturers can get away with it.

Maybe it's not as the repair man was implying. He may have found many other faulty components and the few transformers that he actually examined in detail had this particular problem. Not a regular enough fault for the makers to sort out the problem, perhaps.

 

[Averagesupernova]

Maybe it's not as the repair man was implying. He may have found many other faulty components and the few transformers that he actually examined in detail had this particular problem. Not a regular enough fault for the makers to sort out the problem, perhaps.

Could be. I will say that my parents oil burner has a blade style fuse on the circuit board. It has blown several times due to an intermittent problem on the outdoor unit (air to air heat pump). It is a dual fuel system. Prior to the intermittent issue, that circuit board was replaced due to an unrelated issue. The fuse is on the low voltage control side. It's a pretty efficient setup, but it has not been without trouble.