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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 2nd Jan 2018, 2:03 pm   #21
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
Default Re: Mains isolation

Of course, the isolation transformer is only present to allow you to connect earthed test equipment like an oscilloscope to the primary side of the DUT. It's not really about safety - in terms of receiving an electric shock - but at least it means you don't have to do unsafe things like removing the earth from said oscilloscope and allowing it to float at "half-mains".

That said, I fix a lot of power supplies, and can't remember the last time I needed to perform such measurements on the primary side of a live PSU. Indeed, I've removed my rack-mounted isolation transformers from their prominent position on my bench to make way for gear that gets more use. Today, nearly all faults are caused by capacitors and start-up circuits (and the occasional non-dramatic failure of those 7 or 8 pin chips with an integrating switching transistor). By and large, the days of big blow-ups are happily behind us as designers have worked out how to make reliable designs that don't cascade catastrophically. And on the occasions that you do meet one of those, it's usually possible to fix them by powering the live side control circuitry safely from a bench power supply and checking the operation "cold". The hardest aspect of this is having to reverse-engineer the schematics, but luckily most of them are fairly generic in topology and layout.

And yes, there are PSUs out there that lack under-voltage lockout - they usually rely on sensing the primary-side current seen by the switch, which is a risky strategy IMHO. Luckily, most of those date from the '80s and before. But because you can't always be sure if you happen to have one of these, I much prefer to use a lamp limiter rather than a variac - this is a much better way of avoiding a silicon melt-down in my experience. But because of the large in-rush current, you need a larger lamp than you would for fixing radios. 150W halogen bulbs work well IME.

For interest, this is a fascinating device that seems to have everything you need, including an over-current cutout: http://thesignalpath.com/blogs/2017/...fety-analyzer/ - I've yet to see one of these in the flesh, and I imagine that they are quite expensive to buy, but there's plenty of good ideas that we can take from the design of it (manual available here).

Regarding the original question, I too wondered about the prospect of saturating one of the transformers. However, it does depend on what sort they are - in my experience, I've found that larger torroidal types are on the edge of saturation in normal operation, which is why many of them are mechanically noisy (when building big audio amplifiers, you need to specify special low-flux types, which are physically larger for the VA rating). I've found that frame types have a softer "knee" by comparison. It's easy to detect saturation by monitoring the current taken - a current transformer is a useful device here as you can safely observe the current waveform on a 'scope. That's something that is worth considering for your project anyway, as the current waveform contains lots of useful information about the DUT. I did that using a cheap Chinese voltage/current monitor as my starting point: http://www.markhennessy.co.uk/mdu/

As mentioned, this might feed into the order of items, assuming your variac can go above 230V. In this case, it's best to put the transformer(s) first so that it/they won't saturate should you turn the dial above 230V. But obviously this requires the variac to have excellent isolation to ground. It should do, naturally, but if it didn't, then in ordinary use it would "merely" trip an RCD. But when placed after the isolation transformer, it can no longer do that, and any leakage could cause confusion or worse. So perhaps you might prefer to keep the variac first in line (also giving you that extra LV supply at the same time). It's worth saying that most variacs can be re-wired to give 0-230 only.

Hope these thoughts are of interest
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Old 2nd Jan 2018, 4:06 pm   #22
kellys_eye
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Join Date: Jan 2014
Location: Oban, Scotland, UK.
Posts: 1,118
Default Re: Mains isolation

Quote:
Originally Posted by mhennessy View Post
For interest, this is a fascinating device that seems to have everything you need, including an over-current cutout: http://thesignalpath.com/blogs/2017/...fety-analyzer/ - I've yet to see one of these in the flesh, and I imagine that they are quite expensive to buy, but there's plenty of good ideas that we can take from the design of it (manual available here).
I'm glad you linked to that Youtube site....

"The Signal Path" was precisely what gave me the impetus to build this device and I have the Sencore manual for reference (I'm adopting the current trip circuitry from it).

Anyone who watches his repair videos will soon be struck by how often he uses it to power up defective equipment - safely - and I thought his methods were certainly worth replicating.

The only other criteria I'm limiting myself by is the availability of transformers - I already have the two mentioned, plus the variac and have a digital V/I meter (AC readout) on order (was considering one with VA capability too but I reckon my 'multiplication' skills can cope with that task!).

Going from simple 'isolation' to a unit that has multiple capabilities is just a matter of some switching and terminal posts and a shame to NOT include just because it's such an easy implementation.

Safety factors such as the aforementioned zero-setting interlock (auto reset when output has been switched off), preset current trip, fuses (where required), DC and AC metering of all outputs and thermal overload/trips for the low voltage output side should be enough for anyone!

As initially stated, this device isn't meant for continuous running purposes, only to establish safe parameters for repair/testing purposes as far as the isolation aspect is concerned - the low voltage AC/DC outputs won't even activate the 'step-back-up' side of the device so core saturation won't be an issue in low voltage applications.

Whilst a simple isolation transformer (240:240) is, perhaps, all that is required, the additional functions offered by my adaptation of back-to-back transformers plus their availability (nothing down, no more to pay!) makes it a sort of 'project developed from bits I have lying around' rather than a ground-up design.

I'll keep the thread updated. And thanks to all for the interesting input.
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