Hello again-
I am building a PSU using a nice, big, expensive toroid transformer. I've got this model here: the MT-2114

It is rated for 25-V-25 at 3.2A a side.

My quandry, ladies and gentlemen, is
a) should I use a fuse?
b) what value?
c) Where? (as in, the AC line or after the toroid)

I'm guessing a 7.5A (closest to 6.4 availiable) right at the start of the AC.

But, i wish to die no more, so please help me out.

Thank you all!


EDIT: if it's important, it's supplying power for two of these and about 1A worth of other things

DOUBLE EDIT: if the heat output is 20W max and sound is 20W max, and volts is 50, then amps is 0.8, correct? seems a bit low - thats all...

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-michael11298

 

Use slow blow fuses or they'll blow when the capacitors charge.

Also, I'd consider fusing the primary rather than the secondary.

Just a warning but 50VAC is quite dangerous so take precautions, even whilst playing with the secondary side voltage.

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edit: hero beat me to a better post.

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Well first off, it's not particularly big, nor is it particularly expensive

You need a fuse in the primary, and because it's a toroid, anti-surge or 'slow blow' are essential. But mains fuses, particularly for toroids, are difficult to size - a 160W transformer at 230/240V is a fair bit under 1A - so you might try an anti-surge 1A fuse?. If this tends to blow occasionally, then try 1.25A instead.

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You should consider fitting one of these
http://www.farnell.com/datasheets/76062.pdf
in series with the primary of the transformer.

They limit the surge current, and I have found that they work very well. Adding one of those will take the strain off the switches and rectifier diodes.

With a lower surge current, it means that you can have a smaller fuse, which will blow faster if there is a real overload.

Torroidal transformers tend to have very large turn on surges, so limiting is more useful on those than on standard transformers.

 

Diver300- they are just MOVs arn't they? like this?

Hey Nigel- what are "anti-surge" 1 amp fuses? Is that just a slow blow one, or do I want a slow blow 1A fuse then a MOV in series before the switches and everything, or should it be after the power switch?


While I have captivated you all, what about grounding? The transformer's mounting bolt may be out of the case. Should I ground it? it is insulated from the toroid, but to be safe, or will it create capacitance between the two potentials?

The case is plastic with no other pertruding metal. should I ground the plugs and whatnot that are used as output terminals or is this just asking for a ground loop hum?


Thanks all so much! You guys really are helping everyone so much.

__________________
-michael11298

 

An "anti-surge" fuse is the more common name (in the UK at least) for fuses that withstand a surge at switch-on.

You've no need for any other surge precautions with a toroid of that size, it's only large ones where you need to take more precautions.

Any exposed metal MUST be earthed - but be EXTREMELY careful not to cause a shorted turn around the toroid.

You need to design it to both make it safe, and to prevent ground loops, a central ground point is usually a good start.

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what is a shorted turn around a toroid?
The plate that keeps it all in is seperated from the toroid by rubber washers. The metal doesnt touch the toroid, but it shall be grounded nevertheless using a star ground.

Ok- slow blow fuses withstand surges so they are Ok- I'll try 1A first and I wont use a MOV.

The RCA jacks at the output should be earthed too? this brings the regulated negative supply to mains ground level. This is ok?

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A shorted turn on the toroid is when one of those wires that are wrapped around the core loose their insulation somehow, and come in contact with something in the same circuit, cause a large amount of current to flow, and alot of heat.

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Why would you be using RCA jacks on a PSU?.

Generally you would use nice large screw terminals, with a link (or switch) to earth either (or none) of the output terminals, depending on the particular use at the time.

A shorted turn on a toroid is a single turn through the core, a classic example is a toroid in a metal case - the bottom of the mounting bolt connects to the bottom case, so if the top of the bolt touches the top of the metal case, then it makes a single turn through the transformer. This overloads the transformer massively, hopefully blowing the fuse before the transformer dies - which it should!.

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Yes, those are the same things.

I also agree with others that the surge on a torroid of that size is not always needed, but I have had the turn on surge from a 330 VA torroid trip a 32 A breaker.

Also, on the bolt in the middle, it is vital that you earth one end only. If you take an AC voltmeter, pass one lead through the torroid, and then touch the leads together, you will see about 0.5V AC when the primary is powered. This is far too little to cause injury directly, so you don't need to insulate from fingers, but if you have a low resistance short, you will get lots and lots of current.

 

Did you know that there are different catagories of MCB's?, I didn't until the other day - the difference is their degree of 'anti-surge' behaviour. We went back to a wired fuse for feeding the TV's in the shop, it took the MCB out every morning when power was turned on.

Generally once you get above 500W toroids it's fairly common to include a 'soft-start' circuit, this feeds the main transformer via a large wirewound resistor, and shorts this out via a timing circuit and triac after a second or so.

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Yes, I did know that there are different types of MCBs. The particularly sensitive ones that tripped when we turned on the torroidal transformers are combined MCB/ELCB and I think that there isn't space for the delay elements, so they are a little touchy.

However, half a dozen NTC devices is a lot easier than a new distribution board.

I have also had to use the wirewound resistor and timer method on a conventional transformer. It is rated at 30 kW, and it took out every type of MCB until I added 2.2 hm: in series with each phase. I use a contactor to short out the resistors after a second or so. It now runs on a 16A MCB, but we only take about 5 kW from it.

 

I think that's a little beyond the scope of this thread!

(Probably a little beyond picking up as well?)

BTW, a common fault on such soft-start circuits is failure of the TRIAC that shorts out the resistor - sometimes they go S/C (resulting in intermittent blowing of the fuse), or O/C (resulting in a VERY hot resistor!).

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thanks nigel- the RCA jacks are for the amps' outputs, not power!
thanks all for your replies. I'm really busy with school right now, but I'll see how it gets on.

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-michael11298