simple questions regardin isolation transformer and variac combo.

Just a small after-words, i noticed today when i plugged this on my lab, that it takes a LOT inrush current. My desk lamp dims breifly pretty much, and what i read about toroid transformers, this size transformer can take enought inrush current to trip fuse. Gonna need to get/design inrush current limiter. maybe some other way than switch in parallel to lamp...suggestions welcome, for now that is, project 'aint done yet .

Do you have slo-blow fuses/circuit breakers?

John

hmm, what i know, they are slow-blow. Didn't thought about that, so inrush current shouldn't be problem?

edit: yes they are indeed slow-blow

Some people use a negative-temperature coefficient (NTC) resistor in series. Here's one link: https://www.epcos.com/web/generator...ta__en.pdf;/PDF_InrushCurrentLimiting_AN2.pdf

John

ah, thanks i did find some information, but for now, this might not be issue, as no fuses are blown (yet...)

fezder said:

I do have schematic if you want to check it out, but hopefully you understand what i'm after with this ''bomb''....

Click to expand...

I would like to see it

Hey Fez:

There is one thing that you need to correct ASAP. When using a Variac, the output or wiper HAS to be fused. Lets' say its rated at 2400 W with 240 input and you fuse the primary at 15 A Slow blow.

Now you set the output to 10 V and try to draw 24 A from it. That's 240 W, In this case, the transformer wire will melt. 14 IA >> THAN 10 A,

I wish I saw this thread earlier. There was a nice Circuit Cellar article that had lots of bells and whistles such as a power meter, electronic fusing and the ability to select a tungsten light bulb (obsolete now) in series. It may have also had a GFCI. Pretty comprehensive and complete.

The one I built for myself when I was <12 YO, consisted of an external Isolation transfomer feeding a box with an Analog Meter fro Voltage and Current. My Variac was a 3 A unit and the transfomer was rated for at least 10A at 120 V and had dual primaries and secondaries. I did not use that option.

So, I had a front panel fuse, 3A variac, Binding posts for AC and Ground, a Grounded outlet. The switch I used was very different. It was a center off, SPDT switch. One position used the 3A variac and the other connected bypassed the variac and just used isolation.

What I miss is a light that tells be it's on independent of what it's set at. GFCI's didn't exist at the time.
Meters for power would have been useful as well as a bar graph and meters for voltage and current.

Adding circuits to help determine leakage would have been helpful too.

Dual 120/240 capability would have been cool too.

It was cheap to build and served me well, The isolation xfomer (step up/step down) was $10, the Variac free, The meters surplus. The case small and new.

In-rush isn't a problem because I always turn the variac to minimun.

So, there are just a few options that you can have. The light I miss the most. I would have needed a large 3PDT switch to accomplish that.

Another thing I would have done is to allow the use of an overhead cord reel which would be above the bench.

Fix the fuse location in the circuit! Datasheets for Variacs will show the wiper fuse.

hey keepitsimpletupid, i did install the fuse at wiper. i was aware about it before, but thanks anyway, you cant be too careful with ''mains'' voltage even isolated.

killivolt, i'll post schematic today, right when i draw it ''clean'' as it now is pretty, well fussy. it got little moisture when i was doing this thing outdoors.

ok, heres the schematic. sorry about the quality, i drawed it on paper, then scanned. I'm too oldish to use computer softwares....i just like more to draw these at paper .

OK, at this point, isn't the circuit working? The question you asked in post #21 was addressed in post #24. That is, if you want to stop the lamp from momentarily dimming, use an inrush current limiter.

Is there another question?

John

yes the circuit is working just fine, inrush current just got my attention, so i figured maybe there is something that should be done to it.

Inrush current is just one of those things that happens when you attach a big inductor. Motor starting, like a furnace motor, will do the same thing. I wouldn't worry about it, so long as nothing else is adversely affected.

Regards, John

ok, in that case, i'll just let it be

fezder said:

ok, heres the schematic. sorry about the quality, i drawed it on paper, then scanned. I'm too oldish to use computer softwares....i just like more to draw these at paper .

View attachment 80729

Click to expand...

Very good drawing for a hand sketch. I'm impressed

well i must confess, i first draw pretty much free-handly, then i draw with **broken link removed**

that helps a lot, makes drawing much easier . i recommend getting one of those, and good quality compass, which have adjustment wheel in them.
**broken link removed**
i got mine from local flea market (surpise....)

and what becomes to pencils, i always test them at shop. what i test, is their lead-mechanism. It must not be flexible, meaning the lead goes inside when you press it down surface, even without pressing the button. And the lead-mechanism must be metal.

I may sound too requiring, but i've used too much bad-quality pens ....

Nice I didn't think people even did that anymore. I like it.

Regards,
kv