LED brightness question

[EFLImpudence]

When the ratio of turns primary/secondary is 1 / 1 then the current driven through each lamp is the same as the current in the ring

Ok.

Yes driven as the transformer and the primary current determines how much current flows through the lamp as apposed to the lamp determining how much current it will take from the supply.

Could you please explain how that works - OR what it means - when the document states:

 

[bernardgreen]

I cannot be 100% certain but logic would suggest that with with a ring current of for example 1 Amp then a ratio ( primary : secondary ) of 100 : 30 in the transformers would result in an LED current of 300 mA.

 

[JohnW2]

Yes driven as the transformer and the primary current determines how much current flows through the lamp as apposed to the lamp determining how much current it will take from the supply.

I cannot be 100% certain but logic would suggest that with with a ring current of for example 1 Amp then a ratio ( primary : secondary ) of 100 : 30 in the transformers would result in an LED current of 300 mA.

I must confess that I have always had difficulties getting my head around the concept of 'current transformers' and 'current transformation', so perhaps you can help to educate me.

I have always thought of wirewound transformers transforming voltage (according to the turns ratio) with the current flowing in the secondary (hence also primary) determined by the impedance of the load connected to the secondary (with its secondary voltage). Using some very simple numbers for convenience ...

If one has just one transformer, with a 100:30 turns ratio, with its primary connected to a 240V supply, the 'secondary voltage will be 72 volts. If one connects a 10Ω load to the secondary, then the secondary current will be 7.2A and the primary current will (ignoring losses etc.) be roughly 2.16A. If one changes the load to, say, 100Ω, then the currents will be 0.72A and roughly 0.216A respectively - i.e. the impedance of the load determines what current will flow in both secondary and primary.

On the face of it, I don't see why this concept changes if one has the primaries of several transformers in series. Say there are 10 such transformers in series across the 240V sup[ply, each with a 10Ω load connected to its secondary. The primary voltage of each transformer will then be 24V, hence the secondary voltage 7.2V ... hence secondary current at each transformer of 0.72A, with about 0.216A flowing through the primaries of all the transformers.

If one increases the impedance of the load on the secondary of one of the transformers, from 10Ω to 100Ω, then the maths then gets a little more complicated, since the 240V supply will presumably no longer be equally shared between all of the primaries (and I haven't yet tried to analyse the situation precisely), but there is surely no doubt that current through that one (much higher impedance) load will be much lower than it was when it was the same impedance as all the others, and that the current through (all of) the primaries will be a little lower..

To my simple mind, it therefore seems that, even with the transformers in series, the current flowing through each (secondary) load will be determined by the impedance of that load. If, as you are suggesting, the setup attempted to maintain the same current through the secondary when its impedance increased 10-fold, that would require a 10-fold increase in the secondary voltage - which, as well as being impractical/impossible, would violate my conception of the primary : secondary voltage ratio being equal to the turns ratio.

Can you help me?

Kind Regards, John

 

[bernardgreen]

It is more than 25 years since I had some involvement with a "runway" lighting system. It was used to power about 40 lamps along a private access road to an industrial compound.

The transformers were "odd" in that the impedance of the primary was not significantly affected by the load on the secondary.

The primary of a perfect transformer would have a very high impedance if the secondary was not loaded.

If "runway" transformers were perfect then a failed ( open circuit ) lamp would effectively open circuit the series connected ring of primaries causing other lamps to lose power.

 

[JohnW2]

The transformers were "odd" in that the impedance of the primary was not significantly affected by the load on the secondary.

Very odd. I find it hard to think how one could contrive a transformer to behave like that. In fact, it doesn't sound like a transformer, in the sense that we know such animals, at all. I can but suspect (guess!) that it has something to do with the transformer becoming 'saturated'.

The primary of a perfect transformer would have a very high impedance if the secondary was not loaded. If "runway" transformers were perfect then a failed ( open circuit ) lamp would effectively open circuit the series connected ring of primaries causing other lamps to lose power.

Exactly - that's one way of expressing the difficulty I had in understanding this. Maybe someone else can shed some light on this?

Kind Regards, John

 

[RandomGrinch]

If it helps at all (which it probably won't! ), The whole circuit is fed from a constant current regulator -

 

[JohnW2]

If it helps at all (which it probably won't! ), The whole circuit is fed from a constant current regulator -

Thanks. That helps quite a lot. I did consider that possibility, but wasn't too sure how one could engineer a constant-current AC regulator (it's child's play with DC!) - but what you quote explains how that's done, using 'phase angle control' with thyristors.

However, I don't think that alters the fact that there still must be something pretty 'odd' about the transformers. As bernard wrote, with a standard traditional transformer, the effective primary impedance (hence the impedance of all the primaries in series) would become very high if the secondary became open-circuit (e.g. due to a bulb dying) - so, unless the 'constant current regulator' was able to produce ludicrously high voltages (in an attempt to maintain the 'constant current') the primary voltage of all the other transformers would become very low - which clearly does not happen!

Kind Regards, John

 

[RandomGrinch]

Caveat - this information is from the "Remote Airport Lighting Manual" - ministry of transport - Ontario
(I can attach the .pdf if you would like - but it is 246 pages!)

I am 'usuming' at least some of this information is transferable

unless the 'constant current regulator' was able to produce ludicrously high voltages

Is a kV ludicrously high?

 

[JohnW2]

Caveat - this information is from the "Remote Airport Lighting Manual" - ministry of transport - Ontario
(I can attach the .pdf if you would like - but it is 246 pages!) ... I am 'usuming' at least some of this information is transferable

Thanks, but ....

... unless the 'constant current regulator' was able to produce ludicrously high voltages

Is a kV ludicrously high?

I presume that comment results from your having divided the "maximum regulator size" (7.5 kW) by the constant current (6.6 A) and getting an answer of 1,136 V. Even if you had used the "Power Output" (aka "minimum regulator size", = 4 kW) you would have got an answer of 606 V

However, the material you are posting seems as clear as mud (at least, to me), so I'm not at all sure of the validity of your calculation. For a start, I'm not at all sure what "minimum/maximum regulator sizes" mean. I can but assume that "voltage (rated)" (= "208V/240V") is the input voltage, and one does not expect a 'regulator' to be able to provide an output voltage greater than the input voltage - so I'm already lost!

Even if the 'regulator' is more than just a regulator, and first transforms the voltage up to 1+ kV before regulating the current, I still struggle to see how that could be 'enough' if we were talking about 'ordinary' transformers.

I therefore remain pretty 'lost'

Kind Regards, John