FrankenVariac Capacitor Questions

[SgtRock]

Greetings EEVBees:

--I purchased this Variac setup (see pictures below) from an estate sale for $75. The heart of this device is a General Radio Co. 10A Variac. The bottom plate of the Variac was missing, so I had no choice but to try to fix what I have. I removed and tested the Variac. It made horrible, loud, ominous, humming and arcing noises. I took it apart, and the problem is that the previous jakeleg had left a screw inside the case which was shorting the coils. After I fixed that, the Variac was completely normal. I also realigned the brass shaft extension and re-shimmed the bottom to take the 1/2" wobble out of the dial, it was horrible. I have the feeling that the maker never got to finish his creation, and so the problems. The last work on this one had to have been 40 or 50 years ago.

Now, on to the electrical. There was a blown 32V 20A Fast Blow fuse in the 32mm fuse holder, eegad. The first thing I plan to use this on is a OCZ 600W MODSTREAMPRO PS, which draws a max of 10A. So I bought 20mm fuses (32mm are expensive and harder to find), at 10,5,3 and 1 Amps in fast and slow blow. I am assuming that slow blow is what I want to test the OCZ PS, since the caps will probably spike at over 10A. I will extend the 20mm fuses to 32mm using a piece of heavy gauge copper.

--The power comes out of the Variac and the two wires each go through one of the two coils pictured, and then on to the out receptacle and the front gauges. Each of these coils has a 400V 0.01 uF cap between the bottom of the coil and ground. There is a third 400V 0.01uF cap between the tops of the coils. Exactly what the purpose of the coils and caps is beyond my present state of knowledge. Please enlighten me. In any case each of the three 400V 0.01 uF caps are marked with a line (like a diode is marked) at one end. The bottom 2 caps both read 1.34uF but the top one is completely open. Would someone please tell me what to replace these with. 

"A three legged dog goes into a bar and says "I'm looking for the man that shot my paw"
Jim Varney  1949  -  2000
 
Best Regards
Clear Ether

[SeanB]

The chokes and capacitors are a line filter, to provide some noise reduction from noise from the load going back up the power line, and vice versa. The line on the cap is just to tell you which lead is connected to the outer foil. Preferably replace them with real Class Y caps to ground, and class X caps across the line terminals. Use 100n to 470n for the class X and 10n to 22n for the class Y. Old PC power supplies will have them inside, look at the power input side and grab those out for use. Look for an old circuit breaker of around 5A and use in place of the fuse, and add as well a switchable position to have a current limiting resistor ( 100W incandescent light bulb) in the line lead for testing unknown stuff.

[edpalmer42]

Replace the inductors and capacitors with a suitably-rated commercial powerline filter.  The performance will probably be better.

FYI, I have a variac rated for 1.6 KVA as compared to your 1.2 KVA unit.  I wondered what the no-load current draw was so I carefully hooked up my DMM in series with the variac and set it to the 2 amp scale.  When I plugged everything in, LIGHT came out of my DMM.  There was no place for light to come out, but it did anyway.  The wiring was correct.  I eventually realized that the initial current surge to establish the magnetic field was so great that it had vaporized the fuse in the DMM.  No damage to the DMM or the variac, but I think it took a year or two off my life! 

So, because of that current surge, I used a circuit breaker on the input side of the variac, rather than a fuse.  I used a fuse on the output side.

Ed

[SgtRock]

Dear Sean and Ed:

--Thanks a lot for the information. My understanding is much better now. I just ordered a 10A power line filter, see picture below. I hope it will suffice. The only circuit breaker I have is a 3M 15A GFI, see picture. Will it do the trick, or no? If not, could someone please point to one that will work, as I cannot find.

--As long as I am removing the existing chokes and capacitors, I am considering also removing the two receptacle boxes as well. I would then use a drill and nibbler to make cut outs to mount the female power line filter, and probably a single 120V outlet, to make a much neater appearance. But first I will just cut the junk loose and temporarily install the line filter so I can test the rebuilt OCZ 600W MODSTREAMPRO PS, which I need yesterday. I had to order the filter from W. H. Lo, so it will be a while before I return to this thread or OCZ thread. I had already planned to put a 100W bulb in series with the DUT. I assume that I do not need an Isolation Transformer ahead of the Variac, yes? I want to thank you all again for the good advice. I am trying to get some equipment repaired and a computer built, in order to make enough money to buy 100MHZ Rigol or equivalent scope.

"When people thought the Earth was flat, they were wrong. When people thought the Earth was spherical they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together."
Isaac Asimov 1920 - 1992

Best Regards
Clear Ether

[edpalmer42]

I wasn't thinking of that style of filter, but it should be fine.  I was thinking of one that had spade lugs on both input and output and mounted inside the box.  Something like this:  http://www.ebay.com/itm/111238372853.  If you're in a hurry, you don't need to wait for your filter to arrive.  Your variac will work just fine without it.

The GFCI isn't an appropriate replacement for a breaker.  A circuit breaker protects the variac, wiring, etc. from a fault which could cause excessive current flow, heat, fire, and other mayhem.  A GFCI protects you from a fault to ground that could electrocute you.  But now that I think of it, it might not be a bad idea to use one on the output.

The circuit breaker you want on the input looks more like this:  http://www.ebay.com/itm/231187078144.  Make sure that it's rated for 110 VAC.  Calculate the full load current of the variac (or read it off the label if you're lucky) and look for a breaker that has a slightly higher value.  Wire it in series with the input power.  Breakers like this are available on ebay (obviously!) or through places like Mouser, Digikey,  many of the surplus dealers or often from your favorite high-tech dumpster!

The isolation transformer is an optional thing.  If you're working on a piece of equipment that doesn't have some form of internal isolation or if you're working on a non-isolated power supply, you need it to work safely, otherwise no.  So typically, the transformer would be seperate from the variac.

I use a neat trick for the light bulb.  This is a North American trick - it might not be so convenient elsewhere, but the same concept applies.  Take an ordinary duplex outlet that has tabs on the side.  Normally you break off both tabs if you want to run each outlet to a seperate breaker.  But in this case, break off ONE tab.  It doesn't really matter which one.  Connect the power cord to the two terminals that you have just seperated.  You have now connected the two outlets in series.  Mount this in an electrical box and label it!!!  You don't want anyone to use this as an ordinary extension cord - which is what it looks like.  Now you plug the DUT into one outlet and plug any incandescent light into the other outlet.  You select the wattage of the light according to the expected load.  You can use anything from a 10W appliance bulb to a 500W halogen worklight.  Safe and convenient - no clip leads or exposed wires.

Ed

[calexanian]

A couple of things. Somebody built that box with the meters. Those variacs are sold pretty much stand alone. They have a pretty big inrush current when you plug them in depending on where in the AC cycle you make contact, as a result people like putting line filters on them to keep the noise out of things. In addition some people put them on the output too because the wipers on variacs are notoriously noisy. I find it handy to have a couple of old panel meters on them just for convenience sake. Like it was said before, get a regular line filter and replace those old paper caps and you should be good.

[SgtRock]

Dear Charles, Sean and Ed:

--I have bought one of these 5 Amp breakers http://www.ebay.com/itm/110923486725?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649, picture below. I plan on taking Ed's advice about 2 outlets in series. I will mount the double outlet in series with the 5A breaker into nibbled out holes in the top of the case, and the input filter into a hole in the side of the case. Then I will be able to use a 2 prong lamp adapter to hold the 100W or whatever bulb. When the 5A breaker is not needed I will just jump around it. To leave the bulb out I will just jump across the outlet. I am still mulling all the options, but that is where I am at, at the moment. In other words I plan to incorporate all the suggestions into the case of the FrankenVariac. I may even rename it the EdVariac. It is likely to be a while but I will report back when this project advances. If anyone has any more ideas for features, please hold forth.

"There are 10^11 stars in the galaxy. That used to be a huge number. But it's only a hundred billion.
It's less than the national deficit! We used to call them astronomical numbers. Now we should call them economical numbers."
Richard Feynman  1918  -  1988

Best Regards
Clear Ether

[richard.cs]

Variacs should have fuses/breakers on both input and output. Being a transformer at low output voltages they can supply many times their input current but because it's an autotransformer with only a single winding of the same wire that current can be enough to damage the windings. E.g. a 5A variac with 120V in, set for 20V out. A 5A fuse on the input would allow 5*(120/20) = 30A on the secondary which would burn out the bottom 1/6th of the winding. But you still need the input fuse to protect the variac when it's set for higher output voltages and against general faults like shorted turns.

Any fuse/breaker on the input must be a slow responding type. The one you linked looks like it's entirely thermal with no magnetic trip, that's good for protecting a variac which is thermally massive anyway.

[SgtRock]

Dear Richard:

--Thanks for holding forth. So, say for instance I am testing my rebuilt OCZ MODSTREAMPRO 600W PS, which draws 10A max, I have the 5A  breaker at the input, and for the output of the Variac I put a 10A slow blow fuse in the holder on the front panel, yes? Or should I start with a lower value fuse. I have ordered lots of fuses, so I do not mind destroying a few.

--Final thought, I guess if I encounter a situation where the 5A breaker is insufficient, I will just install in addition a 10A breaker and use a 15A toggle switch to select 5A or 10A.

"I can find in my undergraduate classes, bright students who do not know that the stars rise and set at night, or even that the Sun is a star."
Carl Sagan 1934 - 1996

Best Regards
Clear Ether

[edpalmer42]

I agree with Richard, a breaker is always needed on the input & output.  Typically, their purpose is to protect the variac, not the load, so they would be sized to allow for the maximum current that the variac is rated for.  I guess you could use different sizes of fuses or breakers on the output to protect the load, but it seems like a problem waiting to happen.  You'll have the wrong value in the circuit and either waste a fuse or blow up your device because the value was too high.

I don't like the idea of building the series outlets into the variac.  I really don't like the idea of having a shorting plug sitting around!  Remember that the purpose of a variac is to allow you to start the output voltage low and gradually increase it.  Usually, this will remove any current surge so the light bulb isn't needed.  This works best with a device that uses a linear power supply.  Now, if you're working on a device that has a switching supply, you can't do this.  You have to apply full voltage or the supply won't work right.  That's where you use the light bulb to limit the current in case something goes wrong.  Using the variac and the light bulb together would be an unusual, but not impossible, situation.  I put the series outlets in an electrical box with a power cord so that it looks like a rather goofy extension cord.  If it's in use, I have the DUT in one outlet and the light bulb in the other one.  No shorting plugs or switches.

Regarding the name change from FrankenVariac to EdVariac, are you implying that I'm a monster?       

Frank....uh....Ed