[R-390] Variacs and solas: additional thoughts.
Barry Hauser
barry at hausernet.com
Sun May 1 14:24:18 EDT 2005
Hi Cecil
I agree -- monitoring current is a good idea. My favorite variac "ac
supply" is a Staco I picked up a few years ago. I was told it was working
and maybe it was. I fired it up and it smoked. That's how I learned about
fouled winding contacts -- the hard way.
This was rated for 4.5 amps, though it had a standard 5 amp Staco
autotransformer in it -- cabinet about 12 inches wide with ammeter and
voltmeter. Has a shunt for the ammeter. I managed to match up the correct
raw autotransformer to get it going again. Both meters are pretty accurate
and the switching setup makes it very convenient.
Yup -- the ol' lightbulb trick. The first I came across that was part of
the initial testing instructions for a 3-tube Lafayette KT-135 regenerative
radio kit many years ago. They also advised using a neon bulb tester to
manually polarize the ac plug so the front panel wouldn't be hot with full
AC. Wonder what the body count was on those -- they were very popular and
the cabinet was extra. ;-)
Barry
----- Original Message -----
From: "Cecil Acuff" <chacuff at cableone.net>
To: <r-390 at mailman.qth.net>
Sent: Sunday, May 01, 2005 1:47 PM
Subject: Re: [R-390] Variacs and solas: additional thoughts.
>I agree with what you have said Barry...Good stuff!
>
> The only thing I will add and only because it has not been mentioned is
> the biggest benefit I have experienced when using a variac is in
> conjunction with some method of monitoring the current being drawn by the
> unit under test. The best variac I used years ago had a front panel
> voltmeter and amp meter. You would know right away if there was a problem
> with the load by watching the amp meter while bringing up the power.
> Another more inexpensive way of achieving this is to put a 60 watt 120V
> light bulb on a ceramic socket in series with the load. The lamp will
> light brightly on power up but should drop to just barely visible if all
> is well. If it stays bright chances are something is wrong and the nice
> thing...the bulb limited the current to limit the damage!
>
> That's a neat old trick that many of the newer techs probably hadn't heard
> of .....
>
> Cecil...
> ----- Original Message -----
> From: "Barry Hauser" <barry at hausernet.com>
> To: "W. Li" <wli98122 at yahoo.com>; <r-390 at mailman.qth.net>; "Roy Morgan"
> <roy.morgan at nist.gov>
> Sent: Sunday, May 01, 2005 12:27 PM
> Subject: Re: [R-390] Variacs and solas: additional thoughts.
>
>
>> Hi Roy --
>>
>> Many thanks for all the detail. Despite Ohms Law and various other well
>> established principles, there seems to be remaining variance in opinions
>> about the appropriate, safe and effective use of variacs.
>>
>> Very often, some list members tire easily of certain threads, even when
>> substantive. Oddly, it seems the complainants are those who rarely, if
>> ever, contribute anything. Even among the "regular crew" it has become
>> customary to trot out the dead horse lamentations.
>>
>> One of the reasons Seabiscut & friends get dug up and dragged to the
>> paddock again and again, is that, despite the length of the thread, there
>> is something short of a final resolution of a question sometime before it
>> peters out. Archives are difficult to search -- one of the reasons W. Li
>> created and periodically updates his "Pearls of Wisdom". Further, it's
>> difficult to come up with totally new material, and, if that were a
>> requirement (avoidus oderiferous moribund equinus), we might as well shut
>> down this list because the only new news would be the latest provocation
>> on the e-place.
>>
>> Another phenomenon: Even the most experienced vary in opinions on a
>> number of things. Often, this is due to differences in input or
>> assumptions. Given a complete and common baseline, there would be a good
>> deal more agreement, and less ambiguity facing the modern pilgrim.
>>
>> Anyway, persuant to the foregoing ....
>>
>> Here's some more <insert groan here> on variacs, AKA autotransformers ...
>> as followup to the questions Roy posed:
>>
>> Roy wrote:
>>> Here are some questions to investigate:
>>>
>>> Variacs:
>>> 1) What particular makes and models of "variacs" are set up for
>>> overvoltage as made?
>>
>> I have quite a few, both "plug 'n play" in enclosures, plus raw ones.
>> Most all of them I've found are set up with the "extra taps" -- at least
>> one, but usually two -- even if they aren't brought out to a switch
>> arrangement. At minimum the taps, at about 10 and 2 o'clock, usually at
>> least are brought out to the terminal board on the variac. I've got
>> Staco's, GenRad Variacs, Powerstats, etc. All pretty similar. If the
>> enclosed type has a "140" volt switch position, it's easy to defeat it
>> for safety purposes if you're sure you'll never want to step up.
>>
>>> 2) Do small variacs behave differently than bigger ones? (I doubt it.)
>>
>> Don't know for sure but also doubt it.
>>
>>> 3) Are fuses or circuit breakers normally in the input as made?
>>
>> Most I've noticed are on one side of the input. I have some variac's
>> that are part of multipurpose power supplies and may have additional
>> fusing.
>>
>> Possibly relevant: Autotransformers are not isolated. They are similar
>> to a wirewound pot, except the core is an iron doughnut which provides
>> the electrical characteristics of the creature. (Someone else can
>> explain the theory.) It is not the same as a true transformer with
>> isolated primary and secondary windings. Not sure, but I think the AC
>> and/or DC resistance is pretty low. So, there might not be much
>> additional protection from a fuse added to the load side. I haven't
>> thought it through, but there might be some hazard to a load side fuse
>> blowing while a supply side one remained intact under certain
>> circumstances.
>>
>>> 4) What errors are found in the dial readings due to line voltages being
>>> higher than the unit was made for?
>>
>> I, for one, never pay much attention to the dial readings as they amost
>> all read low. My AC runs about 126 volts most of the time, except during
>> Summer, exceeding the nominal 115-120 assumed by those dials. (Older
>> ones may have assumed 110 VAC). In addition, the rotor may not have been
>> calibrated right on if it's mounted with a set screw that allows for
>> rotating it relative to the shaft. The carbon wiper can wear in such a
>> way to cause changes. Always use an accurate AC voltmeter with them.
>>
>>> 5) Are the voltmeters found on variacs at all accurate?
>>
>> Some are, but should be checked against a known good voltmeter. If it
>> tracks accurately around the critical ranges, then you can rely on the
>> internal voltmeter. Assume nothing.
>>
>> I've posted on this before, but some more on safety: If you acquire a
>> used variac, or even an NOS one, open it up and examine it carefully. If
>> there is a buildup of carbon/graphite fallout from the wiper, clean it up
>> thoroughly. (NOS variacs may have been played with/twiddled). The
>> buildup may be confined to a small portion of the "race" due to tweaking
>> over a limited range. It doesn't take many bridged windings to start a
>> meltdown. Use a nylon detailing brush or old toothbrush - -no solvent and
>> no abrasives. The contacts are made up of the edges of the windings
>> which have been ground down, flattened and often thinly plated. If the
>> carbon fallout fills too many gaps it can cause dramatic failure -- with
>> plenty of smoke to go with it. The wiper normally straddles 2 or 3
>> windings - to avoid intermittents when you adjust it. That's OK.
>> However, if the wiper seems to be crumbling or leaving a heavy track
>> right after you cleaned it up, time for a new wiper.
>>
>> Check for prior damage -- prior overheating or burnout. There will
>> ususally be a blackened stripe around the doughnut. This may be
>> partially hidden by the terminal board. Not to be confused with the
>> black potting that most have partway up the doughnut, near the wiper
>> edge.
>>
>> As I mentioned above, despite all the dead horse thrashings, there is
>> remaining disagreement on the appropriate use of variacs. I suspect if
>> all the operating assumptions were pulled together in one place, there
>> wouldn't be much disagreement at all. Here are some factoids from
>> personal experience, but mainly gleaned from this list over the years:
>>
>> 1. "Bringing it up slowly on a variac" may often be false security. If
>> a tube rectifier is involved, B+ won't start up until about 90 volts or
>> whatever and suddenly. You can temporarily sub out tube rectifiers with
>> silicon rectifiers to do this. If so, ideally B+ should be monitored.
>>
>> 2. Same -- "bringing up slowly" means different things -- over what time
>> frame. Some will say very slowly, as in hours or even days. This may be
>> questionable. Even if you take care of the rectifier situation, the
>> operation of the filaments possibly heating up things at partial voltage
>> while the B+ is too low may be a bad thing -- much the same as using the
>> standby switch on the R-390's. I don't recall exactly, but there were
>> posts on this -- possible damage to some tubes - -I don't recall.
>>
>> 3. Not particularly effective as a way to reform electrolytics in place,
>> even if the rectifier aspect is dealt with. Not a good way to detect bad
>> electrolytics. Whether to reform or replace is arguable, however,
>> reforming is best done with a separate power supply or capacitor tester.
>> Disconnect the electrolytics and go about the test procedure with the cap
>> checker --one that tests caps at operating voltages. However, start the
>> leakage test at a lower voltage -- e.g. 50, switch to monitor leakage and
>> make sure it's going down, then step up, allow some time, re-check
>> leakage which should drop down to negligible/acceptable level if you're
>> going to try to use them. Some will say this has little merit and it's
>> best to replace them outright, and that makes sense, but, let's face it,
>> the realities are that many of us can't resist the impetus to go forward
>> and fire 'em up. Even if the caps are "acceptable", it's best to
>> reform/precharge them to avoid excessive stress on transformers after a
>> "long sleep". Further, if the leakage is bad enough, you will know not
>> to proceed.
>>
>> But using a variac to somehow avoid transformer failure and cap
>> explosions, etc. is false security. In that sense, I agree with Les. If
>> that's what you're going to do, don't get one.
>>
>> 4. You can use a variac to bring up a transformerless AC/DC unit (like a
>> 5-tube table radio or tube Transoceanic) if they have selenium
>> rectifiers, "as is", I would think -- but be aware that variacs provide
>> no isolation whatsoever. Use an isolation transformer in combination --
>> in front of the variac. There may be some value in a slow start to
>> long-unused/unknown solid state equipment. With these, the old cap
>> checkers don't have a voltage setting low enough.
>>
>> 5. Variacs are useful for checking regulation of B+ and short term for
>> dropping line voltage down to 110-115. Long term, permanent installation
>> would be better handled with a bucking transformer. (Unless your voltage
>> varies seasonally.) You can also check if the overvoltage condition you
>> have seems critical or not by comparing voltages at various test points
>> at full line voltage (eg. 127) vs dropped to 115/120 to determine if
>> there really is a need for a bucking transformer -- I suppose.
>>
>> 6. If you feel more comfortable with the idea, by all means, add a load
>> fuse. I usually plug a good surge suppressor or outlet strip into the
>> variac, plug the equipment under test into it along with a hookup (line
>> cord with banana plugs) into a DVM. Of course, the circuit breakers in
>> those things are typically set to trip at 15 Amps, so probably not that
>> much of a help, if any. Best to add a fuse at a lower rating if you feel
>> the need. Again, I'm not so sure it makes a difference with variacs which
>> may be why they typically have fuses or breakers on the input side only.
>>
>> 7. There is the notion that stepping the voltage up slowly may give one
>> the opportunity to pull the plug sooner if some crackling, arcing or
>> smoke occurs partway up the scale. This sounds like it makes some sense
>> and might help, depending on the exact circumstances and might limit
>> "collateral damage". However, it presumes that you are watching closely
>> full time and have the chassis situated so that you can hear and see any
>> developments immediately and you're quick on the trigger finger. To me,
>> that would require removal from any cabinet and up-ending the chassis so
>> you can see, smell, hear as quickly as possible. And then, there's
>> always the quintessential question ... "Do you feel lucky?".
>>
>> After reading Roy's info on Sola constant voltage transformers, I have
>> some questions and concerns about them. I've got a 20 amp unit supplying
>> a circuit with computer equipment on it. It runs hot and noisy even
>> heavily loaded. No apparentl problems in 8 years, but I seriously wonder
>> whether it was worth the bother. However, I'll pursue that off list with
>> Roy.
>>
>> Anyhow, that's my story and I'm stickin' to it -- until further notice.
>>
>> Barry
>>
>>
>>
>>
>>
>>>
>>> Constant Voltage Transformers:
>>> 1) Do small ones behave in a way similar to large ones? (I have
>>> examples from 60 volt-amps up to one Kilowatt.)
>>> 2) What peak voltages come out of the harmonic neutralized type, and
>>> what from the non-netutralized ones, as a function of loading.
>>> 3) Does harmonic content in the output lead to high voltages in
>>> rectifier power suppllies? Are choke input plate supplies affected in
>>> the same way?
>>> 4) What are the overload characteristics of these things? Are they the
>>> same for small and bigger units?
>>> 5) Where above rated output do these things collapse, and do all such
>>> transformers behave this way?
>>>
>>>
>>> Sooo many projects, sooo little time!
>>>
>>> Roy
>>>
>>>
>>>
>>>
>>>
>>> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
>>> 7130 Panorama Drive, Derwood MD 20855
>>> Home: 301-330-8828 Cell 301-928-7794
>>> Work: Voice: 301-975-3254, Fax: 301-948-6213
>>> roy.morgan at nist.gov --
>>>
>>> _____________________________________________________________
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>>
>>
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