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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 18th Feb 2018, 6:48 pm   #1
TonyDuell
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Default Isolating transformer fusing

My over-stocked junk box contains a transformer with a primary tapped at 230V (let's say mains) and a pair of 55-0-55V secondaries. It says it is rated at 200VA. I think this would make a useful isolating transformer for working on vintage radios, etc.

My intention is to box it up with a switch to connect the secondaries in parallel to get 110V out (for US stuff) or series to get 220V (or so) out for European equipment. Also have the switch connect the output to one of 2 different connectors (to avoid the expensive mistake of plugging something rated for 110V input into a 220V supply).

My question, though is how should I fuse it. I would want a fuse in the primary circuit (say 1A or a little less), do I need secondary-side fuses as well? My initial thought is that the primary side fuse will be enough, an excessive load would blow it and transformers heat up slowly enough for that fuse to blow before any real damage is done. But any thoughts?

Also, if you use a lamp limiter with an isolating transformer, does it matter wether it goes between the transformer and the set-under-test or between the mains supply and the transformer? The magnetising current of such a transformer would be fairly small I think.
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Old 18th Feb 2018, 7:21 pm   #2
broadgage
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Default Re: Isolating transformer fusing

A fuse on the primary side is of relatively little use for protection against overload. It will probably have to be at least 5 amps to avoid blowing on the inrush current.
A 5 amp fuse will pass over 1KW forever, and 2KW for some time so not much protection for a 200 watt transformer.

I would be inclined to use a 2 amp fuse in series with each secondary winding, to protect against any gross overload, for moderate overloads, rely on your own vigilance.
With a fuse for each winding, a fault in 220 volt mode could blow either fuse as they will be in series, and a fault in 110 volt mode will blow both fuses.
Whilst 1 amp fuses would give better protection, I suspect that a radio set with a mains transformer might blow a 1 amp fuse in normal operation.
Or use a two pole 1 amp MCB with one pole in series with each winding.

A more sophisticated approach to protect against over load would be a thermal overload relay as used for motor protection.
A 1 amp relay, set to about 0.9amps in series with the primary should do the job. These units are designed to withstand the starting current of a motor so wont trip on the inrush of a transformer, but will trip in some minutes on a modest overload.
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Old 18th Feb 2018, 7:23 pm   #3
TonyDuell
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Default Re: Isolating transformer fusing

I was thinking of a 1A anti-surge fuse on the primary side, I think that would survive the inrush current.
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Old 18th Feb 2018, 7:31 pm   #4
usradcoll1
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Default Re: Isolating transformer fusing

It seems like a good plan and a 1A slo-blo fuse in the primary should be OK.
I always use my lamp limiter between the set and the isolation transformer secondary.
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Old 18th Feb 2018, 8:56 pm   #5
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Default Re: Isolating transformer fusing

Fuses that are likely to blow such as used in testing are IMHO a bad idea, you will replace it with a bit of silver paper sooner or later. MCBs are available for little cost a 1A one is about a fiver.
 
Old 19th Feb 2018, 9:21 am   #6
Boater Sam
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Default Re: Isolating transformer fusing

1 Amp type "C" MCB will not nuisance trip, type "B" might.
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Old 19th Feb 2018, 9:31 am   #7
G6Tanuki
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Default Re: Isolating transformer fusing

Personally, I'd use a circuit-breaker: available in a range of different time/current trip-profiles, and so much easier in practice to just press the 'reset' button rather than taking ten minutes out to remember just where you stored the 1.25A slow-blow fuses only to then remember you used the last one last month and haven't re-ordered!
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Old 19th Feb 2018, 10:10 am   #8
Radio1950
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Default Re: Isolating transformer fusing

I have a transformer functionally like this.
It is a HT transformer from an old valve colour TV.
Capable of about 150 watts before too much voltage sag.
Works well.
I deliberately did not fuse neither primary nor secondary.

Last edited by Radio1950; 19th Feb 2018 at 10:34 am.
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Old 19th Feb 2018, 12:38 pm   #9
mhennessy
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Default Re: Isolating transformer fusing

Sizing fuses for transformers is notorious difficult and very hard to get right. Ohm's law usually has relatively little to do with the final answer

If this is a frame type rather than a toroidal transformer, then the inrush will be rather more manageable and a 1 amp slow-blow fuse might well do the job, but I wouldn't be surprised if it occasionally fails if it is powered up with a load connected to the other side. You might already intend to be in the habit of powering up the transformer first, followed by the load - that's obviously good practice.

Really, the fuse is only there to protect against a catastrophic failure like shorted turns or a live-case short. To protect against abuse, a thermal switch mounted on the transformer itself is a good idea. A thermal switch buried in the windings at the time of manufacturer is best as the windings will warm up much more quickly than the surrounding metalwork, but the latter is usually the most viable approach for a finished transformer - naturally the thermal switch has to be set to a much lower temperature, and we have to hope that the overload isn't gross enough to cause the copper to warm up too rapidly (although a suitably sized over-current device should be able to mitigate against that).

I'd run the transformer at maximum load until the temperature of the frame has reached equilibrium, and choose a thermal cut-out based on that - perhaps only 50 or 60 degrees in practice. The bi-metallic thermal switches are cheap and reliable in my experience. The over-current device might end up being rated at more than an amp to avoid occasional nuisance-tripping, but the thermal switch provides useful reassurance.

I'd also add to the recommendation to use a breaker rather than a fuse as an over-current device - it also doubles as a power switch, which is useful. I used 2A magnetic/thermal circuit breakers with 500VA isolating transformers, and never had any issues, though at switch-on, the steel top/bottom panels make interesting noises as a result of the magnetism!

Of course, a cheap current meter (easily found in the usual places) might be useful to add, given that brief overloads are fine.
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Old 19th Feb 2018, 7:46 pm   #10
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Default Re: Isolating transformer fusing

If all fails I can assure everyone that the pyrotechnics associated with transformer failures are quite interesting be they low VA domestic items or distribution transformers. I had a Sony HMX music centre toroidal that went with quite a pop last year for some unknown reason. Sparks of different sizes and colours flew all over the place for about 1 - 2 secs.
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Old 20th Feb 2018, 1:51 am   #11
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Default Re: Isolating transformer fusing

I have had a good quality MK 13 amp socket switch welded to short circuit by a 1KW isolating transformer.
It will have to be fixed with a 45 amp switch.
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