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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 25th Jun 2022, 1:48 pm   #1
TonyDuell
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Default Reducing the mains voltage

This is an offshoot of my thread on European mains plugs, where it has been suggested that old European equipment is best run off 220V mains (which I agree with).

I will also add that the device I make, if I make one, is going to be used by me, in my workshop, and not left running unattended. So while I will obviously not do anything lethal (exposed metalwork, transformer cores, etc will be bonded to mains earth, no exposed live terminals, suitable fuse in the mains input, etc) it doesn't have to comply with any regulations to the letter.

Years ago, when most devices expected a rather narrow range of mains voltage it was quite easy to get autotransformers tapped at, say, 110V, 120V, 200V, 210V, 220V, 230V, 240V, 250V. Now of course such things are not common. And the old RS ones seem to have a flaw. The input and output cables are rubber-insulated, the 2 examples I've found are devoid of insulation. The transformer can itself is full of pitch, so replacing said cables is a messy project.

No matter, there is the well-known trick of a low voltage transformer with the primary powered from the mains and the secondary in series between the mainsa and the load, phased to reduce the voltage.

So, suppose I have a transformer with a 10:1 turns ratio, off load it will give 24V from a 240V mains input. Connect that in series with the main as above and I'll get 9/10ths of the mains voltage or 216V at the output. Now suppose that transformer, used as intended, was rated for 2A secondary current. Can I use it to power a 2A 216V load? My thought is 'yes'

But hang on. Suppose I put the primary and secondary windings in series, suitably phased, across the mains. Now the output is 10/11 of the input voltage or around 218V. Is it safe to connect like that, I can see no reason why not. And what is the maximum output current I can safely draw now?
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Old 25th Jun 2022, 2:27 pm   #2
emeritus
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Default Re: Reducing the mains voltage

Thinking back too many decades to my college days, and Thevinin's circulating currents theorem, I think the answer is 1.8A.
Consider the two windings in series with the load connected to the common point, and a lossless transformer. The 2A flowing in the secondary winding will be balanced by 200mA flowing in the primary winding. This will leave 1.8A to flow in the load.
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Old 25th Jun 2022, 2:43 pm   #3
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Default Re: Reducing the mains voltage

Try this explanation on Bucking, Auto transformer and Boosting transformers. https://sound-au.com/articles/buck-xfmr.htm
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Old 25th Jun 2022, 3:18 pm   #4
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Default Re: Reducing the mains voltage

If you want to operate your transformer to its maximum ratings, all you need to do is pull 2A from the secondary. The transformer doesn't care how it's connected - ampere-turns balance sorts things out.

So your first scenario, primary across the mains, secondary in series with the mains to your load - 2A is the load current you can draw. It doesn't matter whether the secondary is in series-aiding (216V on the load) or series-opposing (264V at the load), it's 2A you can go up to.

Now the second scenario, it's not quite so obvious. But as it's 10:1 ratio, then ignoring magnetising current, for 2A in the secondary there must be 0.2A in the primary.

You have them connected in series, voltages in-phase, so the currents will be, at any instant, flowing in opposite directions. So 2A flowing one way and 0.2A flowing the other way must result in 2.2A being drawn out of the junction point.

So you can pull 2.2A (at 218.2V) load current.
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Old 25th Jun 2022, 4:38 pm   #5
Lucien Nunes
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Default Re: Reducing the mains voltage

In the second (conventional auto) connection one can also simply equate input and output power, where the input power limit must be Vsupply * 2A, the rating of the low voltage part of the winding connected in series with the supply.

If one wants to be pedantic, repurposing windings between buck, boost and auto can change the I^2R losses associated with the magnetising current but this is normally trivial.
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Old 25th Jun 2022, 5:19 pm   #6
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Default Re: Reducing the mains voltage

I like it Lucien, it gets the same answer as my method but is more about obvious!
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Old 25th Jun 2022, 5:38 pm   #7
Lucien Nunes
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Default Re: Reducing the mains voltage

I remember the day when, being young and ignorant, I thought I had invented the concept of the bucking transformer. The epiphany was to visualise it with the current-driven series winding absorbing power from the load circuit, stepping it up to mains voltage and delivering it back to the supply terminals, reducing the supply current in proportion to the voltage ratio.

My initial experiment felt very cheeky and naughty, deliberately sending current 'the wrong way' through a winding. I imagined that if transformers had feelings, it would get frustrated and hate me for making it perform silly tricks.
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Old 25th Jun 2022, 8:11 pm   #8
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Default Re: Reducing the mains voltage

There is the subtlety that in the series connection giving 218.2V out, the original primary will see that voltage, not the 240V it is designed for. That being so, the magnetic flux density in the core will be less, so core losses will be less and it'll run cooler.

So for the same overall temperature rise, you could actually pull a bit more than 2.2A. How much more, we don't know, because we don't know the distribution of the total losses between core loss and winding loss. But at 2.2A, there will be a bit 'in hand.'
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Old 25th Jun 2022, 8:18 pm   #9
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Default Re: Reducing the mains voltage

My thought at this point for the problem I have is to get a transformer giving around 20-24V rated at 50VA. The machine I want to run from it has a 0.5A anti-surge fuse n the mains input circuit, yes there might be a brief overload but a transformer can cope with that. I'll fuse the mains input at 2A anti-surge I think.

No matter what the output voltage of the transformer circuit turns out to be, it's going to be near 220V and thus kinder on my German device than running it straight off the mains.
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Old 25th Jun 2022, 8:38 pm   #10
kalee20
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Default Re: Reducing the mains voltage

You've got a few options, but yes 50VA will be plenty, and your German gizmo will be much happier!
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Old 25th Jun 2022, 8:43 pm   #11
TonyDuell
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Default Re: Reducing the mains voltage

Yes, time to dig out/buy a couple of transformers, wire each one up in various ways and start measuring voltages.

'To measure is to know' as a Dutch friend told me
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Old 26th Jun 2022, 11:55 am   #12
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Default Re: Reducing the mains voltage

The latest on this...

I found a new transformer in the spares pile. It has 2 primary windings, one is 120V, the other 120V tapped at 100V. And a pair of 25V secondary windings, each rated at 2A. So 100VA total.

I'll call the windings 'primary' and 'secondary' as they were intended to be used, even though electrically that's wrong for my final configuration.

After doing a few tests, I've connected it with the primaries in series, using the 120V one and the 100V section to get a 220V winding, The secondaries are in parallel, these are then in series with the primaries. Mains across the whole lot. That gives me an autotransformer, nominally 245V input, 220V output. Well, I get 222V at the output -- my mains is always high, this morning it's almost 250V. I connected the Alibicord that I want to run, it works perfectly and the voltage doesn't change at all (OK, my meter has a resolution of 1V on that range, so it might be slightly dropping, but not enough to worry about).

And I am sure that 222V is a lot kinder to this old device than 250V.

'All' I need to do is find a suitable box, mount it up, add a fuse, etc.
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Old 26th Jun 2022, 3:54 pm   #13
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Default Re: Reducing the mains voltage

Couldn’t you just use the primaries? Both 120V windings in series for 240V input, then the 120V winding plus 100V on the other winding to give 220V out? Or doesn’t that give enough voltage reduction?

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Old 26th Jun 2022, 3:55 pm   #14
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Default Re: Reducing the mains voltage

I am not sure the primary would stand the extra current drawn by the load. I feel it's OK to do things like this to, say, run a 115V fan from UK mains, but not to run a complete device.
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Old 26th Jun 2022, 5:43 pm   #15
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Default Re: Reducing the mains voltage

Ah yes. The use of the low voltage secondary allows the transformer to be used where the load is greater than the transformer’s 100VA rating. I’d forgotten that.

Your configuration allows your “step-down box” to be used with much higher loads.

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Old 28th Jun 2022, 2:41 pm   #16
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Default Re: Reducing the mains voltage

Auto transformers are now sold as "energy saving devices". I recall being asked to look into it at work some years ago. The "25V" is the full load voltage so your "nominal input" is more like like 246 or 7V.
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Old 28th Jun 2022, 3:03 pm   #17
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Default Re: Reducing the mains voltage

My understsnding is that modern switch-mode power supplies use essentially the same amount of energy regardless of the mains voltage. Likewise, for something like an electric kettle that needs a certain amount of energy to raise a certain mass of water to boiling point, running it on a reduced voltage would simply mean that it would take longer to boil. It would actually result in using more energy than using it on its rated voltage, because it would spend longer losing heat to the surroundings by radiation and convection while getting up to temperature.
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Old 28th Jun 2022, 6:29 pm   #18
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Default Re: Reducing the mains voltage

I agree with emeritus on the smp thing, At work we had a 100hp hydraulic pump, it was pressure compensated so reducing the supply voltage would have caused it to draw more current. This might have resulted in an increase in efficiency but I don't know. I think I would agree with kalee20 that you can draw 2.2A from TonyDuell's autotransformer.
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