Author Topic: DC bias in transformer  (Read 15662 times)

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Offline AVGresponding

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Re: DC bias in transformer
« Reply #25 on: October 13, 2019, 07:13:38 am »
Indeed, and yet it's clearly apparent that the majority of cheap goods (some may come from places other than China I guess) don't meet any of the standards they claim to meet.

Seen loads of bigclivedotcom videos demonstrating complete lack of isolation in any number of line powered "low voltage" stuff, seen it myself in plenty of things too.

Then there's things like poor termination, undersized conductors, non-certified line fuses in plugtops, lack of sufficient insulation, and all that's just the safety stuff, not even got to the poor build quality, poor quality out of tolerance components, poor and inappropriate designs.

And THAT'S before we even get to the deliberately engineered to fail, deliberately disposable stuff!

I'm not naive enough to think these are new problems, but I would argue the magnitude of the problem is far greater now than ever before.

The CE mark and others are a complete joke, and make me wonder if anything ever gets checked for safety by the relevant authorities.
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Offline Simon

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Re: DC bias in transformer
« Reply #26 on: October 13, 2019, 08:12:42 am »
CE marks are a bit of a joke. The standards are not but a manufacturer is allowed to self certify. I blame the authorities on not carrying out random checks. Ultimately it is the responsability of the last person that made the sale. If i buy something from china that is CE marked and i sell it and there is a problem I am liable. Sure I can sue the chinese seller but good luck with that. On the other hand getting something CE marked can be daunting. Dave's µCurrent was being rejected by over zeleaous german customs because it was not CE marked when such a marking is hardly neccessary and Dave may be safe to just put one on and hope for the best.

Ultimately i blame consumers not demanding better and being willing to pay for better.
 

Offline AVGresponding

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Re: DC bias in transformer
« Reply #27 on: October 13, 2019, 02:22:54 pm »
I prefer to blame the shoddy education systems that allow people to think "shiny = good" and "cheap = best".

The lack of critical thinking these days is a huge problem, its effects are extremely visible everywhere you look.

I know otherwise moderately intelligent people that believe things purely on the basis they saw it on Farcebook or read it in a newspaper, without taking the slightest time to think logically about it, or even do the most basic fact checks.

If something is ridiculously cheap, there's going to be a good reason for that, which will be one or more of the following:
1. Cheap, substandard materials
2. Cheap, substandard design
3. Cheap, exploitative labour

It can be hard even for a well informed person to avoid falling into the trap of "ooh, shiny, ooh, cheap, me want!", but I personally do at least try...

It's sadly ironic that even premium consumer products can employ these elements, anyone who watches Louis Rossmann's YT channel will know what I mean.
It can't be a good thing when a major manufacturer has to install anti-suicide nets on their factories.


Anyway, back on-topic, if you're worried about dc bias on your transformer, stick a big-arsed cap in series with the primary. Problem solved   :horse:
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Offline Simon

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Re: DC bias in transformer
« Reply #28 on: October 13, 2019, 02:45:40 pm »
that is why we have standards because people will always buy the cheapest so you legislate on quality (warranty period) labour conditions and safety and these days enviromental standards. It's not a foolproof system but it's the best we have.
 

Offline MagicSmoker

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Re: DC bias in transformer
« Reply #29 on: October 13, 2019, 02:49:17 pm »
...
Anyway, back on-topic, if you're worried about dc bias on your transformer, stick a big-arsed cap in series with the primary. Problem solved   :horse:[/b]

No, that just prevents the mains from seeing the saturated transformer as an abnormally high load; put the capacitor in series with the secondary and restore balanced operation completely. In fact, pretty much all microwave ovens with a mains transformer instead of an inverter use this trick for the half-wave doubler.

Also note that most distribution transformers located closest to the point of use have a small gap in the core to prevent hard saturation from a little DC bias in the first place (at the expense of drawing a higher magnetizing current from having a reduced magnetizing inductance).

But at this point I bet Simon is wishing he'd locked the thread rather than responded to it...  :-DD
 

Offline Simon

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Re: DC bias in transformer
« Reply #30 on: October 13, 2019, 03:09:17 pm »

But at this point I bet Simon is wishing he'd locked the thread rather than responded to it...  :-DD

I don't mind a genuinely useful discussion and the thread was not that long so fine to carry on. the problem is people reviving threads that are pages and pages long and have not been posted in for years. The initial revival was for a near pointless post but if something comes of it all the better.
 

Offline AVGresponding

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Re: DC bias in transformer
« Reply #31 on: October 13, 2019, 05:21:57 pm »
...
Anyway, back on-topic, if you're worried about dc bias on your transformer, stick a big-arsed cap in series with the primary. Problem solved   :horse:[/b]

No, that just prevents the mains from seeing the saturated transformer as an abnormally high load; put the capacitor in series with the secondary and restore balanced operation completely. In fact, pretty much all microwave ovens with a mains transformer instead of an inverter use this trick for the half-wave doubler.

Also note that most distribution transformers located closest to the point of use have a small gap in the core to prevent hard saturation from a little DC bias in the first place (at the expense of drawing a higher magnetizing current from having a reduced magnetizing inductance).

But at this point I bet Simon is wishing he'd locked the thread rather than responded to it...  :-DD


I would argue that putting a big cap on the primary side prevents the transformer from becoming saturated and therefore an abnormal load, and that putting one on the secondary side is a compromise as you can use a smaller, and hence cheaper, cap...

Until the "Your momma" jokes start, I see no reason to close the thread   :popcorn:
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