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Toroidal Transformer Inrush

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bokbaard

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I have a simplistic problem, but I am missing some basic inductive-load know-how to be sure. My toroidal is a 240V primary, I don't know the number of turns. I know you can't just measure the resistance across the primary, but doing that with an ohmmeter, gives me 5ohms. This is pretty damn low for a mains side 240V transformer, is it not? I don't want to power this thing up until I can get some assurance on how exactly such a low resistance is possible. I=240/5 = 48A (no way).

With this said, how would I determine which fuse to use on the primary mains side, slow blow yes (for the inrush current), but what kind of rating?
 
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the "inrush" current has nothing to do with the winding resistance. the transformer inductance prevents current from flowing when there is no load on the output.

the inrush current is actually a function of the load capacitor charging. there are a few ways to take care of it: severely overrating the fuse to the point of it being useless under some circumstances, adding an inrush current limiter, or ensuring that the power is turned on when the line voltage is at 0V.

Dan
 
Dan,
Thanks, yes I understand that, I think I just crossed my two interlinked questions there a bit.
More clearly, is it normal for a primary side windings to only be 5ohms?

As for the inrush current, I was hoping a slow blow would help, but OK, I will start rigging up a relay-delay load limiter fuse circuit of some kind.
 
Toroidal transformers have FAR higher in-rush surges than conventioal transformers - but it's normally only required to make slow-start circuits for large ones.

What rating is your transformer?.
 
Inrush current can be limited by using a ‘inrush limiting resistor’. These resistors have a high resistance when power is first applied (16 ohms). Then the resistance decreases to 0.27. (KC007L)

See DigiKey catalog page 1994.
**broken link removed**
Also search “Inrush Current limit Resistor”

Inrush is not a function of the transformer. It has nothing to do with the 5 ohms. The transformer may have 500 ohms at 60hz AC.
Inrush is a function of the filter capacitors.
 
Thanks everyone, appreciated.
I understand this now.
Just spent an hour reading transformer theory.
 
Sorry, I forgot.
Primary is 240
Secondary is 2 x 12V 12.5 Amp windings.

So that's 300VA - getting about the changeover point - below that you can get away with just an anti-surge fuse, much above it you probably need a cold-start circuit.

At this size I'd suggest trying an anti-surge fuse first.
 
Thanks Nigel, That was the kind of response I was looking for.
I still need to determine fuse ratings, but this is probably what I will go for.

regards
Marlon
 
Thanks Nigel, That was the kind of response I was looking for.
I still need to determine fuse ratings, but this is probably what I will go for.

Generally you need a higher fuse than you might think, probably a 5A A/S is what you're looking for. The fuse is there basically for if the rectifier goes S/C, and should blow before the transformer is ruined.
 
that is a lot of power... I am surprised it is 5 ohms, at that level I would suspect it would be lower still.

I am not familiar with the "antisurge" fuses that Nigel mentions, unless they go by a different name.

the hard core approach is often used at very high power: a current limiting resistor shorted by a power switch once the cap is charged. it is more efficient and protect better since inrush limiters "malfunction" if not allowed to cool down since they are simply negative tempco thermisters.

Dan
 
that is a lot of power... I am surprised it is 5 ohms, at that level I would suspect it would be lower still.

I am not familiar with the "antisurge" fuses that Nigel mentions, unless they go by a different name.

Sometimes called 'time-delay' fuses - essentially they don't blow with the sudden in-rush current - the vast majority of fuses are anti-surge, and generally prefixed with a 'T' - so T5A would be a 5A A/S fuse. F5A would be a fast-blow fuse, and M5A would be a special microwave fuse.
 
Sometimes called 'time-delay' fuses - essentially they don't blow with the sudden in-rush current - the vast majority of fuses are anti-surge, and generally prefixed with a 'T' - so T5A would be a 5A A/S fuse. F5A would be a fast-blow fuse, and M5A would be a special microwave fuse.
That is kinda what I suspected. I am used to the term slow-blow and occasionally time delay.
 
The impedance of an inductor is ZL = 2(pi)fL where:
f = frequency = 50Hz (in Australia).
L = inductance in Henries.
Being toroidal, the inductance of the transformer might be 500mH, so:
ZL = 2(pi)x50x500m ≈ 157Ω. A bit higher than 5Ω, don't you think?
 
Are you sure you measured the primary resistance rather than the secondary ?
 
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Are you sure you measured the primary resistance rather than the secondary ?

haha, yes. :rolleyes:

q5101997 kind of hit the nail on the head, I was hoping for a response in that direction, prompted me to read a few documents on impedance, which makes sense...
 
Every TV I've ever worked on that uses a Pulse Width Modulated power supply has had a 5W, 4.7Ω or so resistor connected in series with the mains full wave rectifier output and the posative terminal of the input filtering cap, to prevent excessive inrush current. It could be 100Ω to be safe, depending on the loading, since there is plenty of spare current on the mains side.
 
Would not a A.C. Capacitor (a.k.a motor start or ballast capacitor)work for the surge?
 
No inrush

There will be no inrush current if this is not a switching power supply, and the primary is connected directly to mains. A standard fast blow fuse will suffice. Whatever power is being drawn from the secondry, will equal the power drawn from the primary. If the output is 10V @ 1A, then P(secondry) = 10V x 1A = 10W. Then, P(primary) = 10W = 240V x I so I = 10W / 240V ≈ 42mA. Double this for about 100mA. No probs.
 
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