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Digi-Key Sold Me 7 Year Old Capacitors
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Smoky:
Thanks Ed,

 Maybe Ive been calling my Tenma the wrong word? It does read "isolation" transformer near the dial.



edpalmer42:
Most isolation transformers are fixed, not variable.  So I was referring to the 'isolation' feature.  Yours has the additional feature of being variable.  Very nice!

But apparently, some switching supplies don't like having the input voltage ramped up slowly.  I heard that many years ago and I don't know if it was true then or if it's true now.  I've just gotten into the habit of using a dim bulb tester and a fixed isolation transformer when I work on a switching supply.  That way, the supply always sees normal line voltage on startup, but it's protected in case something goes wrong.

Ed
james_s:

--- Quote from: Smoky on August 29, 2019, 10:35:51 pm ---Yes, anytime I replace any electrolytic capacitors, the repaired project is brought up incrementally and slowly on the Variac.

In the case of this SMPS, I brought the AC volts up slowly to 25 and let it sit there for three to five minutes while I check for any hot spots.

I repeated that until I reached the full 120 VAC.

--- End quote ---

I've heard of guys doing that when they want to try to save the 70 year old in an antique radio but anything else and it's a waste of time. I've seen zero evidence that there is any benefit of re-forming modern as in less than about 40 years old electrolytics. I have loads of 70s and 80s gear running most of the original capacitors, I only replace the ones that test bad or commonly fail and that normally happens because they are heated either externally or experience high ripple current.
Brumby:
My understanding of the basic operating principle of switchmode power supplies is this:

* The input voltage is rectified and used to operate an oscillator
* The oscillator output is fed into the primary of a transformer, with the secondary producing the target voltage
* The operating point of the oscillator is controlled to generate and regulate the target voltage (via feedback)
* As such, when the input voltage is low, the oscillator will be driven harder, drawing more current so that it can deliver the required target voltage at the current required.

My (simplified) interpretation of this is something along the following lines...
If a SMPS requires 120VA to be able to deliver its target voltage (at the requisite current), then it will draw 1A from a supply voltage of 120V.  If, however, the supply voltage is 8V, it will try to draw 15A.

This level of current is likely going to be a big problem for a circuit that was designed for 1A typical.

But this is just a fundamental issue.  In addition to this, you will have the practical issues of the ability of the actual circuit to operate in such a situation.


On that basis, I would not be presenting any SMPS with notably low source voltages - unless it was a design meant to work with them.
Smoky:
James,

I recently acquired some vintage McIntosh stereo equipment from 1973 (the transistor type not tube). I have not turned them on yet.

I spoke with a technician at a local hi-fi shop and he said to bring the equipment up with a Variac slowly.

He gave me an example where one of his customer's power amplifier would blow the main fuse if the amplifier sat more than a couple months without being played. It was blamed on the old power supply capacitors.

Now in this SMPS, I replaced 7 large capacitors due to them physically leaking. 2200uf, 6800uf, and 680uf.

Since the new capacitors have never been brought up to working voltage that I know of, are you saying it is Ok to just hit the "On" button after such a capacitor swap?

I do have a Sencore LC53 Z meter, and before I install any capacitor (electrolytic or film), I test its "capacitance" for matching purposes and also the "leakage" at rated voltage. Does this classify as "reforming" the new capacitor? The leakage test is about one minute until the numbers stabilize or until the capacitor shows zero leakage (which can be less than a minute).
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