replacing standard transformer with toroidal


2001-10-31 10:52 pm
It might result in less radiated magnetic field and reduce the hum in the output signal.
Keep in mind the inrush current of toroidal transformers is much higher than with conventional transformers. This might cause an unwanted fuse trip.

Advice: just try and see...


2001-10-31 10:52 pm
No doubt about it.
Perhaps you even need to add an thermistor (NTC) to limit the first peak inrush.
Under no circumstances the fuse should be replaced by one with a higher value!

And more: transformers in commercial equipment are often protected by a thermal fuse, located between the windings. This allows the designers to use a higher primary fuse value and thus prevent it from tripping because of the inrush current.
When you replace this commercial transformer by a toroidal transformer the original fuse doesn't protects the transformer anymore against overload!
In practice: lower the fuse value to about 1,25xInominal and add a thermistor when necessary.

Beware of the voltage rating,....

Most toroidals are spec'ed at full load, whereas the trad. way of spec'ing EI cores are for voltage at idle.......

In this the case, the idle voltage of a toroid with identical voltage rating as an EI can be up to 50% higher....

Toroids will generally give you lower drop between idle and full load. I think it is a sort of concensus that toroids gives a somewhat better transient response, due to lower voltage modulation, all other things being equal.
Cannot find the actual transformer in the Farnell catalogue, unless you can give me an order code...

A typical 50VA toroid of other make states a regulation factor of appx 15%. Thus a 25V full load toroid gives you appx. 28v at idle, multiplied by appx. 1,4 in a bridge rectifier. This gives appx. 40 V DC at idle........dropping to appx. 33-35 V on full load.

If you measure an idle voltage for an EI of 25 V AC, this thingie will most probably have a voltage of 20-21 V at full load.

Toroids usually has lower turns count, and thus lower internal resistance. The main point though is to check thorougly what the man spec's really are. A proper data sheet will also state output voltage at no load.
Changing an original EI with a toroid means making sure that one does not present the amp ( or whatever cicuit) with excessive voltage, as this can lead to breakdown, or i.e. increased heat dissipation, as an increase in idle voltage will also increase the idle current in the amp......

The main point is to make sure that an amp operating its PSU at idle, does not exceed the voltage limits of the amp board.

My first toroid PSU, many years ago, had 33 v transformers. At idle , the PSU gave 48 V DC. Testing with essentially no load, blew my marginally spec'ed 80V output transistors........ (well done old chap.. :) )


2001-10-31 10:52 pm
I meant the primary fuse. Transformer rated at 50VA, say 230V the primary current would be about 220mA, so a 250mA fuse would probably do.
Secondary fuses don't protect the transformer, only the circuit that's behind it.

If you read the specs, it say current rating is 1A. Is this nominal current? Based on your statements, I should get a 1.25A fuse (1.25 x Inominal).


2002-03-18 4:23 pm
Toroid RF Noise

I would re-state what Borisov57 said, a Toroidal is really good for injecting RF Noise in to whatever circuit, and I would imagine this would be worse for the digital side of things. Audio Amateur did an article on this subject, from my own experience, I well remember that a old Naim NAP120 (that was available with frame and toroidal transformers), the frame transformer type always sounded nicer. I'm not paranoid about this and I still use toroids, but I would not recommend changing from a frame type to a toroid. I've never had any hum problems with frame transformers, if there is hum its usually a symptom of incorrect grounding.