how to protect autotransformer

[harp]

I am planing to build all in box variac, isolation transformer and current limiter, and just want to ask, if is current limitted like 1A, and on variac is set low voltage, is there something circuit or design that prevent variac, ammeter or fuse to burn? Say to test some device, starting from 0v, while ocasionaly get shorted...

[stj]

a lamp will limit current,
not knowing your target voltage range or current makes it hard to be more specific

[harp]

Autotransformer is range 0-300V. I am not familiar with autotransformer, will it do Vprim x Iprim = Vsec x Isec, like regular transformer? In other words, what the current show ammeter in case of short circuit from above circuit, say that output is set to 10V, 220V and 300v?

[stj]

i know the transformer range, i was wondering what range you intended to use.
the lamp should be on the output side to protect the device your testing.
if it's on the transformer input you may get a voltage to current issue where a low output voltage could draw a lot more current from the lamp.
which kind of defies the purpose of it.

[harp]

"what range you intended to use."................ it is variac, the whole range is intended to use! ................. "the lamp should be on the output side to protect the device your testing." ?................ but lamp has about 50 ohm, and for 240v, it will blow on 300v, and in low voltage output will interference with tested device? ?................. "you may get a voltage to current issue where a low output voltage could draw a lot more current from the lamp." ?............... are you sure about that, for me it looks like the same circuit... can on end of coil be pass higher current then the rest of it? ?................ I am sorry for the mess, from recently I cant quote properly... https://www.badcaps.net/forum/genera...28#post3474728

[stj]

a lot of variacs can output more than you put in, most people dont push them that far.
as for resistance, if you use a halogen lamp the cold reistance is very low.
i use a 40w incan, a 120w halogen and a 500w halogen depending on what i'm testing.
get those outdoor halogen floods while you can btw - they will be banned if they arent already!
luckilly there are good chinese ones on ebay.
i got 2x 500w flood with sealed heads AND a foldable tripod for them for the u.k. equiv of $45 inc post

[harp]

"a lot of variacs can output more than you put in, most people dont push them that far."............. We are still talking about current? From above schematic the input current provided to variac is about 1A, is there option there is on output be more amps?

[sam_sam_sam]

"a lot of variacs can output more than you put in, most people dont push them that far."............. We are still talking about current? From above schematic the input current provided to variac is about 1A, is there option there is on output be more amps?

What you have to worry about is the transformer tap rotor contacts that you do not over load them I not sure why you do not use a space heater coil
or
You can get a high current Variac but they are very expensive for brand name versions and much heavier than the low current ones

But do not try to reinvent the wheel it is not necessary because you can get a 20 amp version however I would highly recommend that you use some kind of current limiting device or very fast acting fuse do not use the time delay fuses in this situation because you will probably burn up the connection to transformer winding trust me I almost ruin one of mine doing stupid sh*t

Here is a 20 amp Chinese version of one which is very similar to one I have but the one I bought was a lot more expensive than this one so who knows what quality these are

https://www.ebay.com/itm/18644088438...BlBMUMa5xKfVZA

One note the volt meter are crap and you should not go by what it shows the upgrade I would make is a digital volt meter and you can find a size that will fit the opening or you might have use a dermal tool to make the opening a little bit bigger so you can mount the meter

One other note about the volt / current meter that I used on my 5 amp version you have to separate the voltage and power for the meter if it only has two wires here is the post where i did this project

https://www.badcaps.net/forum/troubl...e2#post1348156

[harp]

No, I already have all components, only lack for some answer here...

[sam_sam_sam]

"a lot of variacs can output more than you put in, most people dont push them that far."............. We are still talking about current? From above schematic the input current provided to variac is about 1A, is there option there is on output be more amps?

Would you please post a picture of Variac that you have so I can recommend something for you
I do not want to recommend something that your Variac can not possibly do safely

In the post that I referenced about the 5 amp version I have used several times to troubleshoot switching power supplies and battery backup especially for line conditioner function on the version that I have several of them to also repair line conditioners as well

I make sure that I do not over load because of the rotor on the transformer winding that is the biggest problem with these devices

[harp]

Thank you Sam I have found information whats I was looking for. Indeed, autotransformer act like regular one with two separate coil... it is not so clear to me yet, but... Here is video: https://m.youtube.com/watch?v=XgVxbjmdJ7s ............... And more info: https://pressbooks.bccampus.ca/singl...-transformers/

[redwire]

With a variac you have to use two fuses/circuit breakers - one to protect the primary side (which is a normal requirement) plus another to protect the secondary winding.
The secondary can get overloaded like at 10V 10A is only 100VA but will melt its winding wire.

Electrical safety standards generally require transformer fusing to be 167% of rated current. Bigger transformers (>9A) are fused 125% and smaller (<2A) are 300% here in Canada. You can check your local standards.
So for a 600VA autotransformer at 120VAC primary is 8.4A or 240VAC 4.2A fusing (167%).

If you used a 5x20mm fuse 232 rated 4A these will carry 4A forever (at least an hour). They need a lot more to blow.

For my variac I use W58 circuit breakers, a 4A one at 145% trips in one hour and at 200% trips in 10-45 seconds. Close enough.
The choice of fuse/circuit breaker is a bit of work. They are imprecise and different.

Aliexpress I had some DIN rail DZ47 EARU 1A that would not activate unless 5x rated current, got a refund. The 6A part was better meh a bit out.
Other circuit breakers there are cheap price but I say test every one.

[harp]

"The secondary can get overloaded like at 10V 10A is only 100VA but will melt its winding wire."................. According to this thread: https://www.badcaps.net/forum/troubl...orted-windings ... the only solution as I can see is to monitor direct temperature of copper where both windings ends, and cuttoff power when pass threshold, say 60 deg. C... anything else will interact with measurement or significaly downgrade usability range.

[stj]

it would be better if you could SAFELY bond a thermouple to the variac wiper contact

[sam_sam_sam]

it would be better if you could SAFELY bond a thermouple to the variac wiper contact

I had the same thought but the question is how to do this without it interfering with wiper assembly and causing issues

I would probably use my 5 amp one more often if could figure out how to monitor the temperature and keep it from causing issues with the wiper and the winding of the transformer

[redwire]

You want to protect the windings from overcurrent- which a fuse or circuit breaker can do.
That other thread with the roasted variac windings, I think the brushes or something failed and shorted and there was no proper fusing.
I think what might be a problem is thinking power in = power out for a variac... but when you dial down output voltage, the current is squared P=I²*R for the same power and so the winding easily gets overloaded.
Example 5A variac so 5*120VAC =600VA in but that with 10VAC output is uh 60A which will never happen without smoke. You're only good for 5A on the output side or 50VA at 10V out.

[sam_sam_sam]

You want to protect the windings from overcurrent- which a fuse or circuit breaker can do.
That other thread with the roasted variac windings, I think the brushes or something failed and shorted and there was no proper fusing.
I think what might be a problem is thinking power in = power out for a variac... but when you dial down output voltage, the current is squared P=I²*R for the same power and so the winding easily gets overloaded.
Example 5A variac so 5*120VAC =600VA in but that with 10VAC output is uh 60A which will never happen without smoke. You're only good for 5A on the output side or 50VA at 10V out.

So if I understand this correctly then you should have the fuse on the output side of the Variac for good measure so you can not overload the current output as well as the input fuse

But now the question is do you use a fast acting fuse or a slow blow type fuse which one would be better in this situation because the reason I asked this question is because if you are adjusting the voltage output your current might spike or use a slightly higher fast acting fuse that very close to the amperage of the slow blow but does not have the same time reaction the fast acting fuse would have a much faster reaction time

To add a output fuse is not impossible situation but not necessary easily done because of limited space in the front of the enclosure or just make a output power cord with a inline fuse and be done with it

[redwire]

Yeah that's what I am saying, fuse both input and output.
The primary fuse can be slow blow because there is inrush surge when you turn the variac on, and it's the entire winding that would need to heat up and roast which takes a bit of time. The fuse/circuit breaker response time is all about how fast the copper windings heat up and smoke. The fuse needs to be faster than that.

The secondary can be just a little bit of windings doing all the work if it's dialed down low, so a fuse acting is better.
I prefer resettable circuit breakers because they aren't super fast and just push the button if something went wrong.
I use W58 but Aliexpress has cheap ones, just test them first that they aren't fake. I bought some and tested them and the low value 1-3A ones are pretty terrible- they trip way above their rating.