What Are Magnetically Coupled Inductors? - Technical Articles
This article discusses magnetically coupled inductors, more commonly known as transformers.
eepower.com
The reality of a transformer
- Thread starter bernardgreen
- Start date
eepower.com
Sponsored Links
- Joined
- 27 Jan 2008
- Messages
- 23,615
- Reaction score
- 2,659
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
First line
I do seem to remember in University doing calculations to work out how many turns are required, and working out the loses, but although as a radio ham designing my own radio I may need to calculate the size of an inductance to match the frequency in the main for 230/400 volt work at 50 Hz I will buy one off the shelf.
So what is an isolation transformer or current transformer?
I do seem to remember in University doing calculations to work out how many turns are required, and working out the loses, but although as a radio ham designing my own radio I may need to calculate the size of an inductance to match the frequency in the main for 230/400 volt work at 50 Hz I will buy one off the shelf.
... or, indeed, things like 'matching transformers'. Although any transformer which has a different number of turns on primary and secondary will result in a voltage change, that, per se, is not the primary purpose/intent of a matching transformer.
If I had to produce a comprehensive definition of a (wire-wound, 'passive') transformer, I think I would merely talk about inductive coupling between two 'coils', thereby encompassing most (hopefully all) possibilities.
Kind Regards, John
That all depends on what it is you are attempting to match.
Sponsored Links
You used the term 'matching transformer'... What do you mean by the term?
I regularly use a transformer to match the output voltage of a microphone to the sensitivity of an amplifiers microphone input, or the same function between a mixer and amplifier etc. Especially when mixing standards such as domestic HiFi and pro public address kit. The primary function of such a device being to match voltage levels.
However the transformer windins in question are frequently described with their ficticious primary and secondary impedances and require calculations to establish the voltage ratios which are frequently the more important factor.
A transformer which 'matches' the impedance of a source (e.g. RF or audio) to the impedance of the load it feeds (e.g. antenna or loudspeaker respectively), which optimises/maximises power transfer, probably most commonly seen at the output ends of RF and audio amplifiers (when they are often called 'output transformers') - or, for example, 'matching' the impedance of two RF transmission lines (e.g. 'matching' a 300Ω feeder to 75Ω coax).
In the case of RF, in addition to maximising power transfer, probably the most important purpose of impedance matchiing is to prevent reflections.
As I said, (impedance) matching will inevitable also change voltage, but I'm not sure that it will necessarily result in the voltage supplied to the load changing in the direction you might expect (from the turns ratio of the transformer).! ...
If you had a amplifier whose output was designed to feed a very high impedance load, it's output could well fall to near-zero (as well as possibly damaging the amplifier) if you connected its output to a low impedance load.
However, if you inserted a high turns ratio ('low turns' to load side) transformer between the two then, although the 'in-service' voltage would be much lower at the secondary of the transformer than at the primary, the voltage output of the amplifier (connected to the primary) would remain high and the secondary voltage (connected to load) could be much higher than it would have been without the transformer, despite what appears to be a (voltage) 'step-down' transformer.
Not that whilst, as you know, the (primary : secondary) voltage ratio is equal to the turns ratio of a transformer, the impdance ratio is equal to the turns ratio squared.
Kind Regards, John
Ah 3 posts to tell us you are referring to impedance.
Which is why I wrote:
Frequently impedance matching is irrelevant, especially in signal level audio where maximum power transfer is not required and sometimes undesireable, however voltage matching is more likely to be the requirement. As an example a low impedance dynamic microphone, typically 100-200Ω and 1mV is not likely to drive an amplifier with a 2mV (typically 1K-10KΩ) input designed for electret mic to full power. The impedance in this case is almost irrelevant and many 2.5:1-3:1V transformers will do the job adequately although one would usually be looking at 6:1-10:1 impedance transformers as that's how they're more often marked and even more often something like 1K-10KΩ (BBC LL81 for example). Chances are it's often used outside those sort of impedances.
If one is really lucky it may have both values marked, in this case with turns ratio and impedances:
From that it may be obvious the impedances are very wrong but it ran a pair of mono hospital radio channels for a number of years, in this case purely used for isolation/balancing.
(example of irrelevance of impedance)
I admit that I really didn't think it would be necessary to clarify, sinceI don't think I've come across the term "matching transformer" to be used in any context other than impedance matching.
If you type "matching transformer" (with the quotes) into Google, you'll get 100,0000+ hits and, looking at the first few pages, the great majority of those hits relate explicitly to impedance matching ...
... first few ....
I agree that power transfer is not relevant with (low) signal level audio although, as I implied, it is both relevant and important when it comes to the output of an audio amplifier which is feeding loudspeakers.
Yes, I agree that in such situations, it's voltage that matters, and that probably applies to most situations in which there is a low source output impedance and a high load input impedance, as well as to any situations in which power was not an issue.
However, as I illustrated, it's very different when things are the other way around. Hypothetically swapping your figures, if you had a source with an output impedance (hence intended to drive of load with an impedance of) 10 kΩ (probably at a pretty high voltage) and you connected it to a load with an input impedance of 200Ω, then you would get very little voltage (or power, if that mattered) going into the load and there may even be resultant damage to equipment.
Kind Regards, John
Well this is where it gets somewhat confusing.
I have loads of transformers designed to transform 100V to a power into 4/8/16Ω. yes they do transform the low impedance to a higher, yes they are voltage transformers, yes they are current transformers, but the transformation will simply be listed as a power and the dedicated test meter is quite likely to be calibrated in power (although mine show impedance). The man in the field installing such transformers will simply add up the selection of set powers of such devices to a total power. Yes it may be a 500:8Ω or 8:500Ω matching transformer but no one would dream of describing it as such, it will always be described as 20W. Many , many installers of such kit will not even have a clue it's 500Ω or that 10 of them will present a 50Ω load but they will know it's a total of 200W. Typically called a loudspeaker matching transformer or 100V line transformer by those who use them.
In one environment I've installed current to voltage "matching transformers" as listed by the manufacturer and as referred to by those who use them, to monitor AC current using a device with a 0-10V input.
Pretty much every transformer is an impedance matching transformer. But no it's not as obvious as you imply. In the real world the term may possibly not be used as frequently as may be assumed.
I've certainly never heard of anyone mentioning a 1060:2.9Ω 50W matching transformer for a downlighter but that is effectively what it is.
- Joined
- 27 Jan 2008
- Messages
- 23,615
- Reaction score
- 2,659
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
Some people would call them Baluns they are used in many areas to transition between balanced & unbalanced scenarios: one key area is for radio frequency, RF applications for antennas.
But they are a type of transformer, as it an auto transformer which could also be given different names. Is a ballast a transformer? Is an electronic transformer a transformer? Is a inverter a transformer? We call it a rotary transformer when no electronics, but an inverter when it has electronics, and we also have the synchronous vibrator to produce AC from DC.
I remember when the first vehicle activated traffic lights came out, the Mullard radar head needed a centre tap, and the generator did not have a centre tap so we had a centre tapped coil to give the centre tap, I would have called it an auto transformer, but seem to remember forest city gave it a different name. Reactance I think, it was a long time ago.
But they are a type of transformer, as it an auto transformer which could also be given different names. Is a ballast a transformer? Is an electronic transformer a transformer? Is a inverter a transformer? We call it a rotary transformer when no electronics, but an inverter when it has electronics, and we also have the synchronous vibrator to produce AC from DC.
I remember when the first vehicle activated traffic lights came out, the Mullard radar head needed a centre tap, and the generator did not have a centre tap so we had a centre tapped coil to give the centre tap, I would have called it an auto transformer, but seem to remember forest city gave it a different name. Reactance I think, it was a long time ago.
What do you mean by that?
I don't understand. A transformer cannot 'transform power'. With a perfect one, the power delivered to the load on the secondary will be identical to the power supplied byvthe source.
Again I don't understand. What on earth is meant by describing a transformer as "20 W", other than as an indication of the maximum power it can carry before becoming magnetically saturated? One has to have some indication of the 'transforming ratio', whether a turns ratio, a voltage ratio or an impedance ratio.
Are you perhaps just arguing about the word 'matching'? As for 'impedance transformation', I would saying that it's not simply 'obvious', but simply a fact that impedance transformation is an intrinsic characteristic of any (wire-wound) transformer.
If you connect a load of a particular impedance to the secondary of any perfect transformer, what you will 'see' (e.g. measure) at the primary is the impedance of the load transformed according to the impedance ratio of the transformer. For example, if you had a perfect 2:1 (i.e. 'voltage step-down') transformer fed with 200 V, and with a 100Ω impedance load connecting to the secondary, what you would see at the primary would be a load of 400Ω (since the impedance ratio would be (2:1)² = 4:1) - the secondary voltage would be 100 V (i.e. 1 A through the 100Ω load), whilst the primary current would be 0.5A which, with the primary voltage of 200V equates to an impedance (V/Z) of 200/0.5 = 400Ω.
Kind Regards, John
I think that may be a bit confusing/misleading because, although they often go hand-in-hand, Baluns and Matching Transformers are two totally different things/concepts, with different purposes.
As you say, a Balun 'deals with the interface between balanced and unbalanced feeders/whatever - which will usually (but, theoretically, not necessarily have different characteristic impedances, which also needs 'dealing with' ('matching').
Matching transformers are required to deal with the interface between feeders/whatever with different characteristic impedances, even if they are both balanced or both unbalanced - for example, the interface between 50Ω and 75Ω coax (both unbalanced).
Kind Regards, John
Somewhere along my editing I managed to delete the word 'nominated'
I'd have thought the action of a 100V loudspeaker line thansformer would have been easy to understand. I even confirmed they transform; impedance, voltage and current
No. the transformer I'm describing will typically have secondary terminals marked with the loudspeaker impedance, ie 4, 8, 16Ω and the primary will have power terminals, ie 5,10, 20W. in practical use the average installer will have no concept of turns ratio or primary impedance, they don't need to know any of that. All they require is the power put into the loudspeaker from the 'nominal' line voltage (typically in UK; 100V RMS at full power).
An example of a 10W max:
Very easy to understand (albeit there are several different ways of 'looking at it'). However, unlike your microphone transformer scenario, in this case one usually will want to use an (impedance) 'matching transformer', since one wants to optimise power transfer to the speaker.
Fair enough but you are only talking about how one marks the transformer, not what it is doing (which always is 'matching' the source impedance to the speaker impedance, to optimise power transfer) - the only important thing being that the 'power' figures specified only apply to a particular voltage (which I will assume is 100V RMS at full power) (and which I might have expected would be indicated on the transformer!).
Once one knows the source voltage at a particular maximum power, it is arithmetically trivial to determine what source impedance it equates to, and hence what impedance ratio (hence also turns/voltage ratio) is required to 'match' the source impedance to any given speaker impedance.
Assuming 100V RMS at full power (and also assuming I can do arithmetically correct in the middle of the night!), the following tabulation shows that for the example you illustrated. In other words, marking the transformer with powers (with an assumed voltage) and speaker impedances is just one way of doing it. The same information would still all be there if the transformer were specified in terms of turns, voltage or impedance ratios - but perhaps some of those who use them would not be capable of understanding that?
Kind Regards, John
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
