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Old 23rd Oct 2016, 7:33 pm   #21
emeritus
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Default Re: "Bucking" Transformer.

A voltage reducer is just what my cousin could have done with a few years ago for their lighting circuit: I didn't know that such things were commercially available. Their local substation is literally on the other side of the road, and their GLS lamps, even good quality ones, have always had short lives. Even today when mains voltage is supposed to be a nominal 230V, theirs is (just) within tolerance, but closer to 250V than 230V.
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Old 23rd Oct 2016, 9:16 pm   #22
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Default Re: "Bucking" Transformer.

Hi, it should be possible to make your own on something like a 1KVA site transformer core. The only really heavy wire is that from the input to the tap (say 10volts worth. The rest of the autotransformer is at low current.

THAT SAID: You should only undertake this sort of work for a device that will be in a high energy mains circuit if you are totally conversant with what you are doing and fully understand the relevant regulations.

Ed
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Old 27th Oct 2016, 4:25 pm   #23
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Default Re: "Bucking" Transformer.

OOOOOOPS!
I have just done a stupid thing.
I accidentally shorted out one of the secondaries on a new 12 -0 12 - 0V toroid thinking I'd wired the secondaries in series. .................................Stupid Boy.
I thought I'd caught it in time as the meter was showing the correct voltage as I switched off but it looks like the internal temperature was still rising and the primaries are now open circuit. These transformers are supposed to have their own internal thermal fuse.
So would they be accessible easily i.e would they be on the leadout wires or is the toroid basically junk? The insulation on the secondaries tests good at infinity on the 20meg range and they have the correct resistance when measured so I am confident no lasting damage has occurred. It would be a shame to have junked a £20 transformer.

Any ideas or have I to get a new transformer. Incidentally theres no phasing advice on a toroid so its guess work when series connecting secondaries and primaries. Its important when making a bucking unit.

A
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Old 27th Oct 2016, 5:11 pm   #24
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Default Re: "Bucking" Transformer.

I chucked it in the bin.
As the primaries are underneath the secondaries and the primaries are an overwind it's just not worth fiddling about. So its a lesson learned. I get why they put the thermal protection in but surely it's possible to wind the transformer so that any inline fuses are accessible on the outside?

Ho Hum. That's my pub night cancelled this week. Beer money spent on a new toroid.

Before I wire the new one in, given that I won't be so daft as to short a winding out, are there any dangers when series connecting the secondaries in a bucking configuration if I get the phases wrong? I am assuming that if I get mains minus 24V then I have got it right, and mains plus 24V I still have the series connection right but need to reverse the 2 free wires, but if i get them out of phase with each other will I just get a self cancelling effect and a reading of mains voltage with no difference?

A
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Old 27th Oct 2016, 5:39 pm   #25
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Default Re: "Bucking" Transformer.

Probably junk, unfortunately.

The primary will have been wound first and the thermal fuse buried with it. Then insulation, and secondaries last.

So to access the fuse, you'd need to remove quote a lot!

Edit: By the time I'd posted, you had already found it all out!

Last edited by kalee20; 27th Oct 2016 at 5:41 pm. Reason: As above
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Old 27th Oct 2016, 5:48 pm   #26
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Default Re: "Bucking" Transformer.

First connect the two secondaries in series, and confirm that they output about 24 volts, and not nearly zero as would be the case if one was the wrong way round.

Then, treating the series connected windings as a single winding, connect so as to reduce the mains voltage, and again confirm that the actual output is indeed reduced by about 24 volts and not increased.
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Old 27th Oct 2016, 5:50 pm   #27
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Default Re: "Bucking" Transformer.

Quote:
Originally Posted by kalee20 View Post
Probably junk, unfortunately.

The primary will have been wound first and the thermal fuse buried with it. Then insulation, and secondaries last.

So to access the fuse, you'd need to remove quote a lot!
I realised that pretty much straight away kale.
If it was a valuable toroid so to speak then it would be worth unwinding and rewinding as they look to be fairly agricultural things compared to a well made EI job.
Hey ho live and learn, and at least I lived without frying or electrocuting myself.
A new one is ordered and on its way. I have stuck with the same make on the basis that if it gets too hot all it will have cost is a bit of brass and won't cause a fire I hope. The rating at 160VA is twice what I actually need so I am confident that it will be reliable. (and silent).
Incidentally I have opted not to fit IEC sockets to the box, hardwiring the parts and feeding out a couple of flying leads with IEC connectors. If I need to supply kit that hasn't an IEC mains connector, I have some shrouded IEC plugs that I can make an adaptor lead with. I haven't fitted fuses I assumed the 3A one in the mains plug would suffice, but I am wondering about fitting a resettable breaker in the box rated below the 6.6A of the transformer.
A.
So any answers to my phasing question?

A.
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Old 27th Oct 2016, 5:53 pm   #28
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Default Re: "Bucking" Transformer.

Quote:
Originally Posted by broadgage View Post
First connect the two secondaries in series, and confirm that they output about 24 volts, and not nearly zero as would be the case if one was the wrong way round.

Then, treating the series connected windings as a single winding, connect so as to reduce the mains voltage, and again confirm that the actual output is indeed reduced by about 24 volts and not increased.
Brilliant! Thank you broadgage It is how I thought to do it so it's good to know my brains aren't totally addled.
I'm using "choc blocks" to set it up, then I'll solder all the joints and double insulate with tape and shrink wrap.
A.
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Old 28th Oct 2016, 12:45 am   #29
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Default Re: "Bucking" Transformer.

I dont know about commercial toroids in Britain, but when I have toroids made I specify what colour insulation or wire colour should be for all starts and finishes, for all the various windings.
I must admit though, for a single primary winding toroid ( 240 volts only), I just specify two orange wires.
For twin mains windings ( 2 X 120 volts) I use the standard from ILP transformers ( they are british I think) and that is: brown start, orange finish and black start, white finish, for the two primaries respectively.
I guess that is useless information now, Andy
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Old 28th Oct 2016, 11:16 am   #30
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Default Re: "Bucking" Transformer.

Thanks Joe,
The toroid in question did have it's colours clearly marked and I wired the primaries in series without any dramas. The cock up was when connecting the secondaries in series and entirely due to yours truly trusting to my own leaky memory instead of jotting down the colour code on a bit of paper. So I shorted one 12V secondary out and within seconds I had a trashed transformer. I don't think I did any actual lasting damage to the windings but the thermal fuses thought differently and failed safe.
Moral: Check then check again and check a third time before throwing any switches.
And modern transformers with built in thermal fuses are sensitive beasties so you don't have any leeway with them.
As I said I am now a chastened and hopefully wiser experimenter.

Andy.
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Old 28th Oct 2016, 2:03 pm   #31
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Default Re: "Bucking" Transformer.

Just a cautionary note from my personal experience.
I live in a country area. I helped out a long term friend who has a reasonably large cattle grazing property nearby. He has had a series of electric fence boxes fail because of mains power over voltage. His property is near the start of a long rural power line and to ensure the voltage for the consumer at the end of the line is sufficient, they bump up the voltage at my friend's end.
I could fix it easily by using a bucking transformer. No problems. Worked well for over a year then comes a little thunder storm.
A power line surge was just enough to open circuit the neutral end of the primary winding on the transformer. The electric fence box continued to draw current from the mains supply. This current the fencer was drawing was still passing through the secondary of the bucking transformer. The bucking transformer then acted as a current transformer with an open circuit secondary.
The voltage at the open end of the winding would have been very high as it did enough sparking to start a fire. Not a lot of damage done as it was seen quite quickly and extinguished.

I think anyone making use of a transformer as a voltage bucking device should be aware of this pitfall and make sure all connections to the primary are secure and well insulated.

In a previous thread it was stated that with the transformer that was used the primary circuit was open circuited by a thermal switch embedded within the winding. If the thermal switch was at the neutral end of the winding then the above thoughts would be relevant.

This was just relating my own experience.

Just another thought. When selecting a suitable transformer to use as a bucking transformer I would suggest checking the quality of the insulation around the low voltage winding.
The low voltage windings on a majority of modern transformers these days were never intended to cope with anything like 230 Volt RMS mains power along with the incumbent spikes and harmonics.

I use a bucking transformer for all of the power to my radio workshop so I do have some experience with them. My setup uses an old ex Navy transmitter filament transformer. 80 Ampere 15 Volt centre tapped secondary. It was used to supply the filaments of 2 mercury vapour rectifiers running at 3kV so the insulation around the windings is excellent for the job at hand.

Cheers,
Robert.
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Old 28th Oct 2016, 2:51 pm   #32
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Default Re: "Bucking" Transformer.

I wonder if there's any mileage in wiring a "bucking transformer" as an autotransformer? Put the series-connected HV and LV windings across the supply, with the 'load' taken from the tap and the bottom end?
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Old 28th Oct 2016, 4:49 pm   #33
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Default Re: "Bucking" Transformer.

I think if you do that you have to be sure that the primary can take the total current drawn, at which point you might as well find a genuine autotransformer rated to supply the desired current draw?
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Old 28th Oct 2016, 7:13 pm   #34
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Default Re: "Bucking" Transformer.

I am wondering why the primary winding would have to carry the full load current.

My thought-experiment goes like this :

Consider a transformer with a 220V primary and 2 separate 20V secondaries.

Connect one secondary in series with the primary (with the right phasing!) and connect across 240V mains. The combination acts as a 240V winding. After all, there is nothing special about 'primary' and 'secondary' windings. really. And connecting windings in series to make up the right voltage primary is common.

Now connect the other secondary as a bucking winding from the side of the mains that feeds the 20V part of the primary. Connect a 220V load. That is the sort of bucking transformer we have been considering.

Now, apart from losses (which had better be pretty small) and possible phase shifts due to the inductance of the windings (?) the 'bottom ends' of the 2 20V windings, one feeding the load, the other feeding the 220V transformer winding are the same voltage. Not just same RMS voltage, but same voltage at all times. So they can be connected, effectively paralleling the 2 20V windings. And there will be little change to the currents in the circuit if you do this (connecting 2 points at the same voltage will not change currents).

But now the device is electrically an autotransformer.

Are the possible phase shifts really that significant that the 220V winding (in my example) really supplies much current to the load?
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Old 28th Oct 2016, 7:37 pm   #35
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Default Re: "Bucking" Transformer.

It's a common enough circuit analysis problem: two points are at the same voltage, so you can pretend they are connected together electrically. (No current will flow in the connecting wire anyway, since there is no potential difference across it.) When the primary and secondary share a common connection, you can pretend that the end of the secondary is electrically connected to a tapping on the primary at the same voltage.

The bottom section of the winding is only carrying enough current to feed the top section of the winding, which is in series-opposing with the mains and therefore passing the full current drawn by the load. So, you could wind this as an autotransformer by switching gauges partway through the winding process. Only the "difference" portion of the winding will be carrying the full load current.
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Old 28th Oct 2016, 8:19 pm   #36
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Default Re: "Bucking" Transformer.

I think Julie M is agreeing with me. The 'bottom' (220V) part of the winding does not carry much current.

I wonder if (as Julie suggests is possible) autotransformers for small changes (say 240V to 220V) were/are wound with 2 gauges of wire. It would not be hard to do, given that the point where the gauge would change is a point brought out to a tag anyway.
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Old 28th Oct 2016, 8:38 pm   #37
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Default Re: "Bucking" Transformer.

Hi Gents, basically the bottom part of the winding carries the core mag current (usually Low) and the sec current out divided by the transformer ratio.
The heavier winding is thus only needed at the "top" end.
This allows for a fairly small and compact device.

Ed
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Old 28th Oct 2016, 9:17 pm   #38
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Default Re: "Bucking" Transformer.

Thanks, Ed Yes, I'm agreeing with Tony; he finished his post while I was still writing mine, so we sort of crossed over.

I also have "inside knowledge" that some autotransformers for powering non-frequency-sensitive 120 V US equipment from 240 V UK supplies were indeed wound with two different wire gauges. (I once had the task of reverse-engineering one, literally -- by counting the turns as I unwound them.) In this case, though, it was the bottom part (i.e., between the 120 V tapping and neutral) of the winding that was wound in the thicker wire, as this was the moiety (did I use that word right?) carrying the load current.
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Old 28th Oct 2016, 9:52 pm   #39
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Default Re: "Bucking" Transformer.

That was educational!
When I look at the way I will be connecting up the new toroid I think it will be more or less behaving as an auto transformer so I am reassured that I'm not stressing the primary.

Andy
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Old 29th Oct 2016, 11:51 pm   #40
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Default Re: "Bucking" Transformer.

A hint for julie m perhaps.
Having "unwound" hundreds if not thousands of dead transformers to reverse engineer them, there is a very easy way to do it!!
Simply cut the complete set of windings in half and simply count the ends of the wires.
takes one tenth of the time, is very accurate, and quite hard to get wrong as "usually" the different sections are well insulated, and in most cases different guages of wireare used. You can use the same method for squirrel cage motors!!

Joe
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