Led Halogen Drivers

[winston1]

The transformers used for aircraft ground lights have a set current all wired in series and these then have a volts output to work the light so every lamp has the same output.

Hm. What happens if one of the lamps blow?

 

[JohnW2]

The transformers used for aircraft ground lights have a set current all wired in series and these then have a volts output to work the light so every lamp has the same output.

Hm. What happens if one of the lamps blow?

Much less than would happen if the bulbs/lamps themselves (rather than their transformers) were wired in series - that is surely the point?

Kind Regards, John

 

[winston1]

The transformers used for aircraft ground lights have a set current all wired in series and these then have a volts output to work the light so every lamp has the same output.

Hm. What happens if one of the lamps blow?

Much less than would happen if the bulbs/lamps themselves (rather than their transformers) were wired in series - that is surely the point?

Kind Regards, John

But you have already said:

As you know, that's why leaving the secondary of CT open-circuit can be dangerous/damaging, since the voltage tries to rise to 'infinitely high' (the CT equivalent of putting a short-circuit across the secondary of a 'voltage transformer', when the current attempts to rise to 'infinitely high')).

 

[JohnW2]

But you have already said: "As you know, that's why leaving the secondary of CT open-circuit can be dangerous/damaging, since the voltage tries to rise to 'infinitely high' (the CT equivalent of putting a short-circuit across the secondary of a 'voltage transformer', when the current attempts to rise to 'infinitely high'))".

Indeed - so I can but presume that each transformer has a load connected across its secondary in addition to the lamp/bulb, so as to avoid that problem (in the event of an o/c lamp/bulb).

Kind Regards, John

 

[ericmark]

The transformers used for aircraft ground lights have a set current all wired in series and these then have a volts output to work the light so every lamp has the same output.

Hm. What happens if one of the lamps blow?

You loose one landing lamp. That is why transformers in series not bulbs. I will admit on the Falklands where this system was used, I was not directly involved with runway lighting, but saw the transformers and asked what they were for and the system was explained to me. Runways are very long and for the planes navigation equipment to work correctly and to assist the pilot they want all lights giving out the same amount of light. It seems this system does that.

 

[JohnW2]

Hm. What happens if one of the lamps blow?

You loose one landing lamp. That is why transformers in series not bulbs.

Yes, I've made that point. However, as I said, since they are being used as current transformers, the secondary voltage would be expected to get very, maybe catastrophically, high if a lamp/bulb blew o/c, unless there was also some other load across the secondary. Do you recall if there was such an additional load (or 'burden')?

Kind Regards, John

 

[bernardgreen]

The secondary voltage is set by the turns ration and the voltage across the primary. The voltage across the primary is set by the current flowing and the impedance of the primary. The current is set by the power supply system as means of controlling brightness of the lamps. The transformers are designed to have a constant primary impedance irrespective of the loading on the secondary. ( from memory )

 

[North_Wind]

As has been said you have to look at what a component does, not what it is called. Transformers (usually) simply transform (in the case of LEDs, usually 240v AC to 12/24v DC).

However "drivers" do all kinds of funky things. I am looking at some that can "communicate" with LED strip and change its colour (RGB) and dim it all via digital signals. This allows effect lighting in the kitchen (natural white during the day, warm white at night). Even red "party mode" haha.

 

[bernardgreen]

There needs to be a dictionary for translating the language engineers use and the language marketing and other lay people use.

LED Driver

( engineer ) a device for driving a set current through an LED element.

( marketeer ) a device that can do all sorts of things when 230 volts is put in one end and any number and any variety of light emitting devices are connected to the other end.

 

[winston1]

The transformers are designed to have a constant primary impedance irrespective of the loading on the secondary. ( from memory )

How is that done? The primary impedance is the square of the turns ratio times the secondary load impedance.

 

[bernardgreen]

The primary impedance is the square of the turns ratio times the secondary load impedance.

In a perfect transformer ( purely inductive with zero DC resistance in the windings ) that is true. By intentionally designing a less than "perfect" transformer any variation of the secondary load has less effect on the primary impedance.

 

[JohnW2]

The transformers are designed to have a constant primary impedance irrespective of the loading on the secondary. ( from memory )

Like winston, that's the bit I find difficult to understand. If (in some 'specially-designed' transformer) applying the load to a secondary didn't alter the primary impedance, I am struggling to see how it could supply power to that load.

Kind Regards, John

 

[ericmark]

As I said not my job, I just knew they existed and the basic way they worked. I remember with the old battery chargers the transformer was rather a low quality so a battery with a shorted cell did not draw silly amps. I do remember doing transformers in University and working out how many turns were required. But my son has all my books so can't refer back to them to brush up how it was calculated. As a young lad I thought it was simply a ratio, but that is not the case, frequency does matter. So there is a set number of turns which can be calculated. Lucky today I no longer need to know.

 

[JohnW2]

I do remember doing transformers in University and working out how many turns were required. ... As a young lad I thought it was simply a ratio ... So there is a set number of turns which can be calculated.

As I understand it ....

Electromagnetic theory (Maxwell etc.) is such that what is induced into the secondary of a transformer is current, not voltage.

When (as is the common situation) a transformer is used as a ‘voltage transformer’, it’s primary is supplied directly by a (constant-voltage - ‘negligible internal impedance’) power source, and the current through the primary is determined by the effective primary impedance. As has been said, in a perfect transformer that effective primary impedance will be directly related to the impedance of the load applied to the secondary. If that secondary impedance is ‘infinite’ (no load applied), the effective primary resistance will also be ‘infinite’ - so, with a perfect ‘unloaded’ transformer used as a voltage transformer, the primary current (as well as secondary current) would be zero. Because the primary impedance (hence current) is determined by the secondary impedance, the primary current will always adjust itself so as to achieve the same voltage across the secondary load. The voltage ratio of a perfect voltage transformer is thereby simply the turns ratio, and the current ratio the inverse of that .

When a transformer is used as a current transformer, it is not supplied directly from a constant-voltage source. Instead, the current flowing through the primary is determined by the rest of the circuit to which the primary is connected (in the landing lights example, all the other loaded transformers). Hence current is ‘forced through’ the primary of the transformer, even if no load is applied to the secondary. What is induced into the secondary is current, determined by the turns ratio, so that the secondary effectively behaves as a constant-current source. The voltage across the secondary will thus be the that current multiplied by the impedance of the secondary circuit. If, in a perfect transformer, that secondary impedance is ‘infinite’ (no load), the voltage will try to rise to ‘infinity’ as well. With a real-world (‘imperfect’) transformer, the voltage obviously will not become ‘infinite’, but it can get very high - potentially high enough to destroy the transformer’s insulation (and to be a hazard). This is the reason why CTs are always used with a load (‘burden’), which limits the possible secondary voltage.

As I said, that's how I understand it, but others may wish to correct me!

Kind Regards, John

 

[ericmark]

With no load the transformer should under normal conditions draw nearly zero power. To get to that point there is an optimum number of turns for a given voltage and frequency in the primary, that is the start point in any transformer design. The old turns ratio equalling the voltage ratio taught in school just touches the surface of transformer design. In simple terms the primary impedance is as important as the ratio. I wish I still had my books.

The miss matching of impedance can cause odd things to happen, and as we draw current from the secondary we do effect the impedance of the primary, there is often a trade off, the yellow site transformers are a good example, these are often rather poor, but as a result lighter than building a transformer which is better quality, we need to carry them so having them draw 1A when no load on secondary is acceptable in order to reduce weight.

At what point we start using high frequency transformers and inverters to reduce the weight of site transformers I don't know, but likely when we have another jump in copper prices that will come.

Cooling is also a problem, and using pipe rather than simple wires with cooling water inside the pipe is used, with generators we use hydrogen to cool, it is kept in place using an oil seal, that oil is called as one may expect seal oil, at Sizewell power station during the build, Greenpeace did a demonstration against the use of seal oil, at least until the type of seal was explained to them. Due to use of hydrogen the safety officer erected a sign "No smoking ban" he was rather embarrassed when it was pointed out the double negative in the statement means it actually says "There is no ban on smoking" which was clearly not the intended message.

I have hunted internet for info on transformer design, however each one I find starts with if we have 450 turns, rather than starting at the beginning with to work out how many turns we need on the primary. Which in turn is worked out by what the core is made from, frequency and voltage. They all seem to jump that stage of design.