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Author Topic: Wiring the Transformer  (Read 357 times)

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Offline sjgarth

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Wiring the Transformer
« on: June 21, 2019, 09:56:32 PM »
Hi,
I am going to use open transformers as the power supply for my Gaugemaster UQ 4 way panel mounted controller. I have purchased two Gaugemaster T1 transformers (16v x 2 at 1 amp per transformer) which I am going to mount in TO ONE ABS electrical box to create double insulation. Heat should not be a problem as each transformer will have a 2 cm clearance around it.
I was going to have the 230v mains input from one lead with jumpers from one transformer to the other. If I do this, should the mains fuse remain at 3 amps? I think it should.
Also, some people use thermal cutout on the transformers, but I was thinking about a fuse on the low voltage output, that is 4 fuses (1 for each transformer output), and i was going to use 1 amp glass fuses in inline holders.
Do the experts here think that this is adequate protection?
To be honest I cannot see the need of the cost of the thermal cutouts, and think that fusing is the way to go.
I attach a very rough diagram (fag packet type design) and hope you can see what I mean.



« Last Edit: June 21, 2019, 09:58:11 PM by sjgarth »
Yours

Steve

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Online ntpntpntp

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Re: Wiring the Transformer
« Reply #1 on: June 21, 2019, 10:33:36 PM »
The T1 is 1 amp per secondary output isn't it?

*very roughly*

You have a potential maximum 4 amps draw at 16V on the low voltage side, so applying the formula W=I x V that gives you a power consumption of 64 Watts. 

For the high voltage side, assuming 100% efficient transformers (which of course they're not) then to deliver that same wattage at 240V you an apply the formula I=W / V  giving 0.27 amps.   So even allowing for less than full efficiency of the transformers the mains side current draw is still going to be less than half an amp.

So you could argue a 1 amp fuse would be more than enough on the high voltage side.   If I remember correctly that's what I have in the case mounted fuse holder of my transformer box (which actually house a couple of other transformers as well as 2x Gaugemaster T1).

Should you have cutouts or fuses on the low voltage side?  I guess it depends what loads you're going to power. If it's controllers  then you'd expect most designs to have an overload cutout built in?   If it's for lighting and accessories then maybe include  a fuse, but on the other hand if you overload the circuit a bit you'll probably just get a reduction in voltage.  If you think you might get a full short circuit situation then yes a fuse is probably a good idea.  I've not bothered with fuses or cutouts on the outputs of mine. Not had a problem in over 20 years of use, but maybe I've just been fortunate :-)
« Last Edit: June 21, 2019, 10:46:34 PM by ntpntpntp »
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline sjgarth

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Re: Wiring the Transformer
« Reply #2 on: June 21, 2019, 11:05:43 PM »
The Gaugemaster UQ panel controller has 'All outputs are protected by a protection circuits and if overloaded some or all power may be lost' so maybe the output fuses are an overkill.
Thanks for the correction on the input HV side, the reason I said 3 amps is because the T1 instructions say 3 amps. I was querying if connecting the mains as shown would be okay. From your reply I would surmise that 3 amp would still be more than required even for 2 transformers primary windings to be in parallel.
Yours

Steve

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Online ntpntpntp

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Re: Wiring the Transformer
« Reply #3 on: June 21, 2019, 11:16:08 PM »
From your reply I would surmise that 3 amp would still be more than required even for 2 transformers primary windings to be in parallel.

That's how mine are wired. I guess the 3 amp fuse mentioned in the T1 instructions refers to the smallest typical household fuse rating most people have around the place in plugs etc.  It's often said the fuse in the plug only really protects the cable to the appliance. As I mentioned,  I fitted a fuse holder into my enclosure, wired to the incoming cable and with a 1 amp fuse as far as I can recall. I've never had to change it since I built the enclosure back in (I think) the late 80s or early 90s. It's powered all my layouts since that time.
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline stevewalker

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Re: Wiring the Transformer
« Reply #4 on: June 22, 2019, 12:44:16 AM »
The fuse on the mains side should be rated to protect the supply flex - it is NOT intended to protect the equipment itself, although it may have the side-effect of doing so. Obviously a lower rating is also suitable, as long as it is high enough to cover the load that the transformers present.

As for the outputs, if the controllers limit or cut the current in the event of a short, then no fuses should be required after the transformers - although you may prefer to install some. If the controllers don't provide protection, then a resettable device (thermal or combined thermal and magnetic) makes sense as it could be operated whenever a loco derails

Offline Bramshot

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Re: Wiring the Transformer
« Reply #5 on: June 22, 2019, 10:54:40 AM »
Noe that when switched on there is a current surge to transformers and a low current fuse on the input might get blown by this if it is a rapid action type. You can also get slow-blow fuses which are designed to accept short duration over current.

Online jthjth

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Re: Wiring the Transformer
« Reply #6 on: June 22, 2019, 04:52:20 PM »
How will you mount the transformers in the box? If you have metal screws protruding through the box to the outside you should fit solder tags on the inside and connect these to the earth conductor of your input flex, ie earth the transformer frame.

Offline sjgarth

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Re: Wiring the Transformer
« Reply #7 on: June 23, 2019, 01:09:52 AM »
I have an ABS box which is large enough to house the two transformers with about 2cm clear space around. I intend to mount them onto an ABS  sheet cut to size then insert the assembly into the ABS box and glue it in place. This will be secure and prevent the bolts being on the outside. This will protect the double insulation. I will probably earth the transformers anyway, it does no harm.
Yours

Steve

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Offline stevewalker

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Re: Wiring the Transformer
« Reply #8 on: June 23, 2019, 12:29:53 PM »
I intend to mount them onto an ABS  sheet cut to size then insert the assembly into the ABS box and glue it in place. This will be secure and prevent the bolts being on the outside. This will protect the double insulation. I will probably earth the transformers anyway, it does no harm.

Another way (that I have used as part of my central heating) is simply to fix the transformers to the inside of the box using plastic screws. Simple and can be undone in the future if required.

Offline themadhippy

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Re: Wiring the Transformer
« Reply #9 on: June 23, 2019, 01:24:50 PM »
Being pedantic i  makes it 64 VA,watts are a bit  different. Once you allow the usual fudge factors  for transformers your getting close to 0.5A,but the initial inrush current will be much more.I'd go for at least 0.75mm flex,this will let you use a 5A fuse if the 3A gets upset with the initial inrush current,and fit a  1A slow blow fuse on the input of each transformer.

 

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