Can a transformer alone blow fuses ?

[Hextejas]

I have this toroid connected via this wall receptacle and 9 times out of 10, I will blow a 3.15 or 4.0 slow blow fuse.
There is nothing connected to it and it is new. I had 1 good power-on this morning and I stopped after 8 blown fuses.
I thought that I would need to have some load connected in order to blow a fuse. Maybe not.

[Matje]

Larger toroids have a problem with high inrush current in the moment of powering on, your 500VA one definitely counts as a large one.

You will need to limit the inrush current, search for "inrush limiter" or "slow start" or similar.

[Benta]

Not unusual.
I had a 1 kVA toroid isolation transformer that would blow the fuses 10 out of 10 times.
A controlled start circuit can help.

[Kleinstein]

For such a size an type of transformer it is pretty normal to blow a fuse if turned on just be a hard switch to it's nominal voltage.

The problem with these torrid core transformers it that the core rather easy runs in to saturation. In this case there is very little inductive impedance to limit the current. So peak current could go well over 50 A than. This is enough to blow 16 A or similar circuit breaker.

Saturation is reached relatively easy in this type because the residual magnetization is often relative large and the core material (grain orientated FeSi) is used up to a relatively high field. Depending on the exact timing to turn on saturation will happen or not. So there is a chance not to blow the fuse, but only some 10%-25%.

So normally one needs a kind of soft start circuit with this type of transformer ( about from 100 VA up).

[Benta]

Yes, it's a major problem that most modern transformers are designed to operate very close to the core saturation point.
One solution is to choose a transformer with a higher primary voltage than needed (and corresponding secondary voltage as well, of course). This unfortunately makes it physically larger.
Another solution is switching on the transformer in a controlled way using SCRs or Triacs. In this case it should be switched on appr. at peak mains voltage.
Lastly, upgrading your fuse box and cabling is possible, but normally costly (electrician needed).


Edit: by a major coincidence, another thread is touching the same subject. Nice little circuit, should work:
https://www.eevblog.com/forum/projects/yet-another-mains-active-soft-start-is-this-design-decent/

[Globe Collector]

 A carefully chosen NTC Thermistor (or group of them in parallel) in series with the primary will solve the issue too. (All these types of circuits are usually referred to as "inrush limiters".)

It would also be worth checking to make sure your mains voltage is not exceeding the voltage rating of the transformer's primary.

You can do this with a Variac and Current Transformer, place the primary of the current transformer in series with the primary of your toroid. Load the secondary of the current transformer with an appropriate resistor and connect the Oscilloscope in parallel with the resistor.

 Power the whole lot off the Variac, wind the voltage up slowly from zero until the waveform on the oscilloscope starts to show steep peaks on the peaks of the sine wave (sort of looks like a "rounded off/low pass filtered" version of sine wave with cross-over distortion)...when these peaks start to appear, your transformer is starting to saturate. Most transformers are just saturating a little bit at their rated primary voltages, but any more than a little bit can be a problem.

[NiHaoMike]

I assume it's 240V mains since 4A at 120V would be much too little for that transformer.

As mentioned, a NTC is one possible solution, another is to try a thermal (not magnetic) circuit breaker instead of a fuse.

A carefully chosen NTC Thermistor (or group of them in parallel)

Do not connect NTCs in parallel since one will end up taking the majority of the current.

[Hextejas]

I assume it's 240V mains since 4A at 120V would be much too little for that transformer.

As mentioned, a NTC is one possible solution, another is to try a thermal (not magnetic) circuit breaker instead of a fuse.

A carefully chosen NTC Thermistor (or group of them in parallel)

Do not connect NTCs in parallel since one will end up taking the majority of the current.

NiHaoMike, actually it's 120v mains. Is that gonna be a problem ?

[NiHaoMike]

4A at 120V is just 480VA, while the transformer is designed for 500VA. Might be able to get away with it if you use a thermal circuit breaker (very slow response, even compared to a slow blow fuse), but most likely you'll have to go higher.

[Zero999]

Yes, it's a major problem that most modern transformers are designed to operate very close to the core saturation point.
One solution is to choose a transformer with a higher primary voltage than needed (and corresponding secondary voltage as well, of course). This unfortunately makes it physically larger.
Another solution is switching on the transformer in a controlled way using SCRs or Triacs. In this case it should be switched on appr. at peak mains voltage.
Lastly, upgrading your fuse box and cabling is possible, but normally costly (electrician needed).


Edit: by a major coincidence, another thread is touching the same subject. Nice little circuit, should work:
https://www.eevblog.com/forum/projects/yet-another-mains-active-soft-start-is-this-design-decent/

I assume it's 240V mains since 4A at 120V would be much too little for that transformer.

As mentioned, a NTC is one possible solution, another is to try a thermal (not magnetic) circuit breaker instead of a fuse.

A carefully chosen NTC Thermistor (or group of them in parallel)

Do not connect NTCs in parallel since one will end up taking the majority of the current.

NiHaoMike, actually it's 120v mains. Is that gonna be a problem ?

Use the circuit linked to by Benta but with a lower voltage TVS, MOV and relay for 120V operation.

[Hextejas]

Thank you Zero for your excellent idea.
Given my newbieness I have another idea and it is to buy another transformer closer to what was called out.
The original spec called for a 15-0-15 however the designer also said that bigger would be ok.
So i went bigger, apparently too much bigger.
I didn't know enough to worry about in-rush current.
Perhaps I will need to build this small limiter also but I think I will start with a smaller xformer.
Thanks again.

[Hextejas]

Ok, I added this thermistor to the input to the toroid transformer and it solved the fuse blowing issue of the transformer alone, however
Ametherm SL32 2R025 (Pack of 2) Aqua-Rite Thermistor, ICL 2 OHM 20% 25A 30MM

when I hooked it up to the power supply, it still blew the fuse, so something in the power supply is drawing a large current.
I tried hooking the power supply to a 24V, AC wall wart and it blew it also.

I will start a new topic re the power supply.

[IanB]

NiHaoMike, actually it's 120v mains. Is that gonna be a problem ?

It's inconvenient that the transformer is marked 115 V. In the USA the supply voltage is nominally 120 V, and depending on time and place might be up to 125 V. If the transformer is designed right up to the limit of saturation then using a higher than rated supply voltage will be pushing boundaries.