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Isolation transformer

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irishape

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As a novice, with a capitol N, I have been working with linear power supplies. I have now been hooked on smps. Two questions: is it necessary, considering this just a hobby, that a person invest in an isolation transformer? Also a tech gave me some smps boards from his bone yard and I have traced one down to a bad trans and was wondering if a person could get a trans for a smps? I have been warned about the danger of these systems. They are a very interesting part of electronics to me. Opinions?
 
An isolation transformer is a good thing to have on your workbench, especially if debugging SMPS power supplies. One slip with the ground clip on a scope probe can create a disaster with no isolation transformer between the wall-outlet and the exposed stuff in a SMPS (or other line-powered stuff like light-dimmers, speed-controls, etc).


Having the isolation transformer there makes it less likely that you will die because you inadvertently put one hand into the SMPS and touch the face of your oscilloscope with your other hand.
 
Mike, thank you for such a quick and informative reply. I wonder if you could explain one more thing. I would like to know just how an IT works. I can understand how it could isolate the earth ground but you are still working with, in my case anyhow, 120 volts. Get between +/_ you are still in a world of hurt, right?
 
In the USA and CAN, L1 (black wire, narrow prong) is 120Vac with respect to Neutral (white wire, wider prong). Green wire (round prong) is tied to grounding rod under your electrical panel. Neutral is also connected to the ground rod (in just one place, inside the panel).
It is ok to touch the white wire, or the green wire with impunity; not so much the black wire.

Your scope has a three wire cord, so its outside case is intrinsically grounded through the green wire in its power cord. You have a 120Vac-powered non-isolated project on your work bench. You are probing around with the scope probe to see voltages inside the project. Scope probe ground-clip has a direct path to the scope case, and back to earth through the green wire. Where do put the scope ground-clip? If you guess wrong, and select something with a path to L1 (black wire), instant flash as you vaporize the ground clip on the probe. Flash is big enough to burn you or flip molten metal into your eye.

Or with one hand you touch something in the project that has a path to L1 (black wire), and with your other hand touch the scope face. Scope face is grounded. Current path is from L1 in the exposed wiring on your bench project, through one arm, through your chest, across your heart, out the other arm, to the scope. Your wife gets to call the paramedics.

Either of these scenarios are effectively prevented by putting a low-leakage 1:1 120Vin/120Vout isolation transformer between the wall socket and your project.
 
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MM, that is one clear and helpful explanation and answers my question fully. Who knows, you may have even saved my life. Don't let it go to your head though, I am 82 years old. Still-----. Thanks a lot Mike.
 
MM, that is one clear and helpful explanation and answers my question fully. Who knows, you may have even saved my life. Don't let it go to your head though, I am 82 years old. Still-----. Thanks a lot Mike.

Only ten years behind you...
 
There is a good laptop in my scrap box, dead, it wishes I had a isolation trans at the time I blew it up.

Yes most certainly a good idea, in fact very important if you do anything off line sucj as smps's.

You can improvise a cheap version by using 2 low voltage transformers, say mains - 12vac then 12vac - mains, not as good as a proper isolation trans but ok for smaller smps projects.

Transformer design in smps's takes an amount of experince and calculation, I use a coupel of cad packages for assistance.
 
In your first post you said you had traced a failed supply to a bad trans.

I'm not sure whether you are referring to a transistor or a transformer.

Typically, the transformer is just about the most robust component in a SMPS. If they ever fail, it's usually because something else failed first.

And the transformers is SMPS are very difficult to find replacements for. Other then where designs have been copied, I've never seen the same transformer construction used in otherwise equivalent supplies. I think if you gave the same functional spec to 10 different power supply engineers, you'd end up with 20 different transformer designs. :)
 
Like I said I am new at this, in particular smps but that transformer has a couple of bubbles in the insulation and I have the blue ring tester and it showed bad. I am sure that is why the tech that gave it to me threw it in his bone pile as he called it. And as you said it would be impossible to get a new one. To bad because everything else checked good. The reason the landfills are filling I guess.
 
Yeah Dr pepper I am in agreement with you on the isolation transformer, I have one coming. I have been hit by electricity pretty hard a couple of times in my years and I don't need any more.
 
Typically, the transformer is just about the most robust component in a SMPS. If they ever fail, it's usually because something else failed first.

I've been repairing SMPSU's professionally since they first appeared in domestic electronics (in the Thorn 3000 series colour TV), and while the transformers are generally VERY reliable, there are a small number of models where transformer failure is common - and I used to keep about three different transformers in stock for the affected units. In all three of those models, there was nothing else that caused the transformers to fail, and replacing the transformer was usually all that was required - depending on exactly how the transformer failed, sometimes it blew other things as well (there wasn't just a single failure mode).

I've seen them O/C, shorted turns, and even short between windings.

But they are usually extremely reliable, and even in the cases where they did fail, it wasn't the first 'port of call' when fault finding.

So if the transformer is actually duff in his unit, then it's most probably a common fault on that model.
 
Thank you for the info Nigel it helps guys like me who are trying to learn about smps. I was told before that transformers in these units did not fail very often. That is why I would like to have found one just to see if I am correct about every thing else being ok. The TF has two what looks like blow outs I guess you would say, spots where the clear covering is melted. I have no use for the unit, I just wanted to see if it started with a new TF. Anyhow the guy that gave me this board also gave me two more so on to the next.
 
The only time I can recall a trans failing is when something happened that made it fail, even with ones I have wound.
 
Yeah Dr pepper I am in agreement with you on the isolation transformer, I have one coming. I have been hit by electricity pretty hard a couple of times in my years and I don't need any more.

If you wanted an Isolation Transformer, I could have send you a good one for FREE, Excluding the Shipping Cost.
 
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