[R-390] Variacs and solas: additional thoughts.

Cecil Acuff chacuff at cableone.net
Sun May 1 13:47:28 EDT 2005 

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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