Hi All,
A quick question because I'm feeling thick...
We have an IKEA light that takes G4 capsule bulbs, running at 12V. It has its own transformer driving a pair of wires forming a track. There are a minimum/maximum number of lamps (5/7) that can be connected across it - we've got 5.
I assume the minimum number is because it will simply drive too great a current through the system if it's underloaded - so I can't drop the power of the bulbs (currently 5 x 20W). Is that right?
Cheers - Adam
The message from "Adam-the-Kiwi" contains these words:
Yes. The output of the transformer isn't regulated - too few lights will allow the voltage to rise which will over-drive the remaining bulbs. If you want to run a lower total wattage you'll either need to ballast it with a dump-resistor, regulate it properly with a spot of electronics, or get a lower rated transformer.
From what you say, the transformer is expecting to work with a load of between 100W and 140W.
If the load is too small, the output voltage will rise above it's 12V nominal output and it will blow bulbs prematurely. If the load is too great, the transformer will either overheat, or blow it's fuse.
As you're already at the lower end of that range, reducing the wattage of the bulbs may make them blow. If you want less light, you could run the system via a dimmer, but it would be less efficient at lower settings.
Nope.
Reduce the number of the bulbs (or the wattage) and the total current will also fall. Each bulb will be connected in parallel.
Chances are the 12V transfermer is not actually a transformer at all, and is in fact a small switched mode power supply. These have the advantage of being very small and light, efficent, and are dimmable without placing a big inductive load on the dimmer etc. However they often also have a minimum current load. If you attempt to draw less that this, they will not work correctly.
So if you wish to reduce the number of bulbs, or reduce the size of them then you may need to change the transformer to something appropriate to the load you have in mind.
Lots to choose from:
-88-
That's approximately what I thought...
However (and I'm struggling to dredge up O-level physics here) I think this is a magnetic transformer, so surely the voltage step-down is determined entirely by the ratio of the windings? Won't the voltage be fixed by the transformer and the current drawn determined by the output power?
Cheers - Adam...
Correct, whatever you do you can't go below 100W. It's probably because the transformer is poorly regulated and the voltage will increase if you reduce the load.
Not quite because the windings have resistance. The (nominal) 12 V output from the secondary will have an internal resistance equal to the resistance of the secondary winding plus that of the primary divided by the square of the turns ratio.
A typical ~100 VA transformer might have a regulation of around 5-10%, so off-load the voltage will be 5-10% high.
The message from John Rumm contains these words:
But the individual current will usually rise because of poor regulation by the transformer.
That's the basic physics, yes. But the reality is a little more complicated.
Anything that tries to provide a voltage ( eg a battery, or transformer ) only has so much 'oomph'.
Imagine a small 12v battery, powering a 12v lamp. What is the voltage measured at the battery terminals? Around 12v. Now connect another lamp. The resistance of the load has halved. The current has doubled. What has the voltage done? Still around 12v, but slightly lower. Just how many lamps can we connect to this battery? Every time we connect another lamp, the total load resistance drops,and the total current increases. Can we expect the battery to provide unlimited amps? No. As the load increases, the load resistance drops. The current increases. As the load increases, the voltage droops. We are heading along the line that will ultimately result in the load being so low resistance that the battery is effectivly shorted out. At this point, the voltage will be zero.
If you connect a piece of wire between a battery's +and - terminals, would you expect it to be able to maintain 12v through a short circuit?
The way to think of it is this: All voltage sources are non-ideal. They cannot sustain infinite current without voltage drop. In otherwords, they can be modeled as an ideal voltage source in series with an internal resistance.
Real Battery = Ideal battery in series with resistor. ( same for transformer. )
Off load, I=0. Voltage drop across internal R =0, Terminal V = Ideal V. As load increases, Voltage drop across internal R increases. Terminal V = Ideal V- Vdrop.
So terminal voltage 'sags' as load current increases. A good voltage source has a low internal resistance, and so 'sags' less. This is called 'regulation'.
Your transformer has poor regulation ( high internal impedance ), and so the change between off-load and full-load voltage is significant.
Off-load it will rise above the nominal output V, On-load, it will sag to the nominal output V.
The message from "Adam-the-Kiwi" contains these words:
Off load - yes.
No because the secondary (and the primary of course) of the transformer has its own resistance.
Think of it this way. Off load the current is zero so the voltage across the output of the transformer is solely the result of the induced current from the primary.
When you start to draw current it's passing through the resistance of the secondary - and as we all know a current passing through a resistor develops a voltage across the resistance. That voltage will of course be opposite in polarity to the one the transformer's creating - so in effect you subtract it from the output.
Example. A transformer has an internal resistance of 1 ohm and an off load voltage of 12 volts.
At zero current you'll get 12 volts. At 1 amp you'll get an internal drop of one volt from V+IR so the output will have fallen to 11volts. It's that 11 volts that you use to work out the current that your lamps will draw. In order to make the lamps work properly therefore, the open circuit voltage of the transformer is rather higher than the nominal 12volts require so that when loaded with the correct number of the currect wattage lamp it works out just right. Fewer lamps won't pull the voltage down sufficiently. Of course, the lamps don't know this - all they see is too high a voltage and their life is accordingly shortened. Worse, once one's gone the others are more at risk - as a second bulb blows the voltage rises even more so more bulbs go....
You can get round this in two main ways. Firstly you could make the internal resistance of the transformer very low so that the current you're likely to draw from it doesn't pull the voltage down much. That way the off load voltage will be very similar to the full load voltage. That's great if you can afford socking great transformers with all that expensive copper and so on. Secondly you can electronically control the output. This is much cheaper as all it takes is something that can chop the incoming mains supply into bits and pass them into a capacitor, measure the voltage and when it drops to a threshold throw a few more lumps of mains in until it rises to a threshold then stop throwing bits in till it falls again. The two thresholds can be surprisingly close together.
Not sure I follow that... SMPS, usually have good regulation IME.
(if using a iron cored xformer then regulation does not come into it since you are running stepped down un rectifed and un regulated AC)
The message from John Rumm contains these words:
I was talking about just a transformer.
Regulation certainly does enter in to it - transformers are specified amongst other things by their ability to maintain their output voltage between limits under load. I didn't mean regulation in the active semiconductor sense, which I thought was clear enough from the text.
Yup, sorry talking at crossed purposes - I *was* thinking of active regulation in the SMPS sense.
In a halfway decent and efficient transformer of course you are correct.
But in an Ikea one, made to sell, and not to a decent spec? Who can say?
Let's say that winding RESISTANCE appears in series with the output..as a model..and if the iron saturates, the 1:1 correlation between input current, output current and turns ratio breaks down, usually followed by a 'nasty smell' :-)
Use of a decent larger transformer, would solve the problem.
No..they are usually unable to work AT ALL without SOME load on..
The regulation after that is a function of how complex and expensive the circuit is..
It does. Winding resistance and variations of the core reluctance with flux density cause it to sag under load.
Like what I said in the previous post you mean? ;-)
Well it's quite clear that simply reducing the power drawn by the bulbs won't be a good solution for you. As has already been made clear, the transformer is the limiting factor.
Would it be feasible to replace the transformer with one that's rated to work satisfactorilly at a lower power ? You'd need to ensure that it will still be within spec if somebody loads up the fittings with 20W bulbs though.
The new transformer could either replace the original or be sited externally from the fitting.
-88-
That is the longer term solution - someone posted a link to TLC's electronic transformers (at =A37 a pop, or something like that) - possibly in conjunction with much lower power bulbs. I'm also looking at some G4 replacement LED bulbs (obviously, with a new electronic transformer) - I know they'll produce a fraction of the light, but that might not be a bad thing...
I'd just like to say a big thanks to everyone who has contributed to this thread and helped bring my antiquated knowledge of transformers back up to scratch.
Cheers - Adam...
Hi,
When you pull a few bulbs, what happens to the brightess of the others, do they get much brighter or just a little brighter?
Maybe try some cheap low wattage G4s, they won't cost a lot or get any hotter than higher wattage ones.
If they run a little brighter then any shorter life might be made up by greater efficiency.
FWIW my the bulbs on my electronic transformer get dimmer the *less* it's loaded.
Also try a google for Osram 'IRC' ones.
cheers, Pete.
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