Magnetic current limiter.

[Andrei917]

Hi, I have a 127V - 24V transformer, 500VA. I use it on a workbench for destructive tests, like to see if some component will "blow" or catch on fire. The problem is that I'm forcing that transformer and it gets really hot, because sometimes the circuit gets short circuited. I was wondering if it possible to make a current limiter to put between the wall socket and the transformer, it should allow a maximum current of 3.5A @ 127V.

I could use a big resistor, or a lamp, but then I will be throwing away almost 500w of useless heat. I don't wanna cut the wave with a dimmer or something like that, because it will destroy the sine wave.

Someone told me I could use a inductor or choke to limit that current, but I thought these things only made the current changes "slower".

If someone could give me an idea about how to make an inductor, I'm not sure if I should use it in series with my 24v transformer, or maybe a isolation transformer with a saturated core.

 

[tpetar]

There is one trick and in my opinion best way to limit and regulating transformer current and power.

Its old school trick, often is used in welding devices with iron core transformer. One part of iron core is movable between transformer primar/secundar windings, and on that way changing flux and power.



But in your case this is not applicable. I just post an idea for thinking.



You can use classic slow automatic fuse, or some circuit which monitoring current and when current touch limit then relay disconnect transformer. Transformers can have thermal fuse in windings as thermal protection.

 

[Andrei917]

Actually I think that is what I'm looking for, you said "But in your case this is not applicable", you mean in the 24v transformer itself? That is not possible, but I could make another 127v - 127v transformer. I think I will try to build something like that. I have some old transformer cores around, E - I plates, I could cut the middle "leg" from E plate and then move it as you said. Not sure if it will work, but I think I'm gonna try that.

Problem is that a fuse will cut the current, instead I need to keep the thing "working", even if it is short circuited, to see if there is any chance of things catching fire.

 

[tpetar]

Actually I think that is what I'm looking for, you said "But in your case this is not applicable", you mean in the 24v transformer itself? That is not possible, but I could make another 127v - 127v transformer. I think I will try to build something like that. I have some old transformer cores around, E - I plates, I could cut the middle "leg" from E plate and then move it as you said. Not sure if it will work, but I think I'm gonna try that.

Problem is that a fuse will cut the current, instead I need to keep the thing "working", even if it is short circuited, to see if there is any chance of things catching fire.

This is possible on any transformer, but transformer core must be designed for this movement mechanic. With moving part of core we changing transformer properties (flux). Its hard to modify existing transformer to this type.

You can make 100% load for your transformer if you want to test it, even test it on overload for some time, but I dont see point to overcurrent transformer to much to get burned.