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Saturday, 31 May 2014

Electronic transformers and low voltage LEDs?

The electronic transformer is a favourite for 12 volt low voltage halogen lighting. Its small size and efficient operation make it a favourite. As the emphasis is changing to LED lighting there is a lot of confusion on exactly where you stand if you're using one of these devices. There is a lot of conflicting and confused information regarding their use. In this post I will simply describe the internal workings of a common electronic transformer and try and clear up some of those loose ends of information that are regularly ducked by many vendors, resellers and to a certain extent the manufacturers.







Contents.


Can I use an electronic transformer with LED lights?

Why do electronic transformers sometimes have a minimum load?

How does an electronic transformer work?

Can I use a power transformer with LED Lights?








Can I use an electronic transformer with LED lights?


Yes it is possible but you must check with the LED bulb manufacturer's requirements as well as the requirements of the electronic transformer. Regarding the LED, some vendors say "12V" but others say the LED needs to have a specific dedicated LED driver. In such a case you must follow the manufacturer's instructions and provide the driver as specified.


I have known resellers to state that some LEDs are 12V DC but I have yet to see a 12V DC LED in the GU5.3 format. Normally GU5.3 bulbs have no polarisation markings, indicating that the rectifying diodes must be incorporated within the LED bulb package casing. Certainly the 12V supplies do not have polarisation marking or keyed connectors that I've seen. I can only assume I've either missed something or the resellers are trying to cover their backs.
  • Note that an LED is actually a diode in its own right, as it's name suggests, and so would naturally half wave rectify in its own right anyway.  The separate rectifying diodes enable the LED to use the energy of the alternating current over the full cycle, as opposed to only half of it, reducing the possibility of any disturbing flicker.

Regarding the requirement of the electronic transformer: explained later in this post, insufficient load will cause intermittent operation of the electronic transformer, if it is below its "stated" minimum load. Sometimes this insufficient loading manifests itself as a pronounced flickering effect or more usually nothing.
  • Due to the way the voltage is generated internally within the electronic transformer, it is unlikely to damage the LED if it fails to meet the minimum load requirement.

A workaround, if your electronic transformer has a minimum load and it is feeding several light fittings but the sum of the LEDs alone does not reach this minimum value, then you can have one light fitting as a standard halogen bulb and all the other bulbs as LED lights. The combination of the Halogen and LED bulbs, ensures that the minimum load requirement is met and the electronic transformer operates properly. Otherwise you will need to get either a dedicated supply for LEDs or another electronic transformer that is capable of running from a 0 watt load (ie no load).
  • I have heard "Internet" talk that the high frequency output of the electronic transformer could damage the diodes in an LED.   I have neither seen this nor heard this from an LED 12V GU5.3 replacement manufacturer as yet.  But normal rules apply and you should use the manufacturer's instructions and guidance for their products.

Again, if you want a dimming feature it will be necessary to get an LED that is compatible with dimming, thus allowing it to be dimmed.
  • So dimming will only work if all components in the circuit have that feature, ie dimmer switch, dimmable electronic transformer and dimmable LED. You must confirm that all the components have the dimmable feature as it is not implied.

    • Be careful not to also get caught out by a minimum load requirement of the dimmer switch.




Why do electronic transformers sometimes have a minimum load?


As described next (and more in-depth in my previous article) the self oscillating part of the circuit is driven by the surge of energy caused by the load it's supplying. This energy goes through a tiny feedback toroid transformer and is cleverly used to drive the base terminal of the power transistors, directly driving the oscillation. If there is no load on the electronic transformer's output, there will be insufficient energy to drive the oscillator feedback toroid. Consequently, it will not oscillate and subsequently it cannot transform the volts.

This can be an advantage if the load is a standard low voltage 12v halogen that you fitted and the bulb breaks. In this scenario, the output of the electronic transformer naturally switches off, as the load is effectively removed by the bulb being broken. It is not so good if you're trying to retrofit LED lights though, especially if they do not reach the minimum load requirement of the electronic transformer. The load requirements are usually printed on the electronic transformer's case, showing both the upper and lower limits such as:

Electronic transformer 10-60 watt load requirement
Electronic transformer 10-60 watt load requirement

10-60w meaning minimum load of 10 watts and a maximum of 60 watts. Here is an example of such as transformer with the power requirement highlighted.


Electronic transformer 20-60 watt load requirement
Electronic transformer 20-60 watt load requirement

Similarly, 20-60w meaning a minimum load of 20 watts and a maximum of 60 watts. Here is another example of such a transformer with the power requirement highlighted.




Electronic transformer 0-50 watt load requirement
Electronic transformer 0-50 watt load requirement
It is possible to get an electronic transformer which has a load range which starts from 0 watts. In this case the circuitry that drives the oscillator is different to that described in my analysis of a typical electronic transformer as it cannot solely rely on the feedback of the load to self excite the oscillation.
Here is an example of such a device, which I personally use. It can drive both a 50w halogen or a 4w led, equally well. The transformer is shown above with its power requirement identifier highlighted. It is still an electronic transformer as it does not regulate the output, as would a switch mode power supply.



The affiliate links below are for the 0-50w electronic transformer that I personally use at home, the Varilight YT50L. I use this for powering my mixed system of both 12V Halogen and GU5.3 LEDs. It gives me scope and versatility to change from 4w LED to 50w halogen at will, plus everything in between. For the best lighting effects it is difficult to beat the coloured halogen lamps. For convenient economy, LED lights rule, but that particular discussion is for another post.



Varilight 50W Lighting Transformer for LV Circuits

This is a another affiliate link to the same electronic transformer YT50L but from the UK chain's B & Q website. This is where I actually purchased mine from which I personally use at home. 

If you are after slightly more potent versions then B & Q also have versions which are rated at 0-105 watt: YT105L and also the 0-150 watt: YT150 electronic transformer.

I have not had the chance to use either of the more powerful electronic transformers listed above, as I use a single transformer per light fitting rather than feeding several lights from a single transformer.  Although my setup is slightly more expensive, I believe it gives me the greatest versatility for my specific setup and design. One advantage is the 12V ELV leads are short as they only go to a single fitting.  
  • It is important to consider the length the and gauge of the ELV wires from the transformer. When driving high power halogen lights there are associated high currents. It is an important consideration to ensure the correct gauge of wire is used to minimise voltage drop over the length of wire. The longer the run the greater the gauge but also the greater the cost of the cable due to the increased material costs. In the case of retrofitting LED lights this is less of a consideration as the current is dramatically reduced. 
The more powerful electronic transformers YT105L and YT150 have the same property of being able to dive any load from virtually nothing to their full output capacity.  There is no need to meet any minimum load requirement as there is none.


If you are wondering about the cost implications of retrofitting a transformer for LEDs then you might like to look at my article retrofit lighting cost calculator.  If you're changing transformer you can enter its cost, plus the additional cost of any necessary remedial electrical work, into the calculation. This will give an unbiased view of the cost implications to help you with your decision.

If you are wondering about some of the other symbols and marks you can find on an electronic transformer especially those sold in Europe then please have a look at my article on them.




How does an electronic transformer work?


In this part of this post , I will skirt around the specifics and just go through the salient points of a typical electronic transformer. Previously, I created a post with a more detailed technical view of an electronic transformer's operation, which went into depth about the various components of an electronic transformer. You can link to that previous post here.

In yet another of my earlier posts titled, Transformers, Electronic transformers and switch mode power supplies, I discussed the technical differences between them and touched on each of their working operations. It is useful to understand what their differences are, if only to defend yourself from the pressure of a sales man, "giving you the full unbridled sales pitch."

  • A power transformer uses the low frequency provided by your mains power supplier to generate the flux energy, while an electronic transformer internally creates much less flux per cycle but much more frequently, resulting in the same power being delivered from a substantially smaller output transformer. This makes for a small form factor package that is also substantially lighter than a traditional power transformer.

There are many ways to electronically generate a higher frequency but the method most commonly used is an elegant circuit called a self-starting half bridge oscillator circuit. The core active components are two power transistors, which alternately switch the rectified mains through the output transformer.  The arrangement of the transistors is where it gets the name "half bridge" from. It's only one side of an H-bridge. On the other side of the bridge, the two transistors are replaced by two capacitors.



Here's a block diagram showing a black box schematic of the internal workings of the electronic transformer. If you click on the various elements you will be transported to the relevant part of my earlier post which describes that specific element.


Mains noise filter. Bridge rectifier Self exciting oscillator Half Bridge driver

Black box model of electronic transformer
Back box model of electronic transformer.
Click on item to be taken to relevant section in previous article.


Don't panic! The, "self-starting half bridge oscillator circuit," is an elegantly simple circuit and it turns up, in slightly modified forms, in many situations, such as electronic ballasts for compact fluorescent lights.  If you can follow the simple logic in the explanation below it may open a world you may not have even known existed.

  • I wrote an article on power factor and explained the two types of powerfactor.  The type of powerfactor the electronic transformer would suffer from is the rectified load type, where there is a power spike causing excessive harmonics.  As the capacitance of the electronic transformer is small and the charging and discharging of flux through the output transformer happens on each cycle at between 20 to 120 khz, this distortion is near undetectable.

As described in my previous post, the energy transfer of each cycle is tiny compared to a typical power transformer of the same power rating. Consequently, the inductive type of power factor is also very small and generally an electronic transformer can be safely used with a trailing edge dimmer. Which essentially means it can be used with all dimmers both trailing or leading edge. (Ensure you adhere to the manufacturer's instructions regarding choice of dimmers).

The transistors in an electronic transformer are driven by negative feedback, making for a low component count oscillator. This feedback arrangement is why it's called, "self exciting." The design uses the field energy generated in its output transformer to switch the base of the power transistors, thus changing the polarity of the current through the power transformer. Originally this switching circuitry would be powered via an additional secondary winding on the main output power transformer itself. For the fact that only a tiny current is required and simplicity, a second tiny toroid is utilised instead, which is in series with the primary winding of the main output transformer.
 
The above explanation assumes a DC source but we know that an electronic transformer is powered via the domestic mains supply. For simplicity the mains feeding the transistors is first rectified using a bridge rectifier, normally composed of four individual diodes (it doesn't get the "bridge" part of its name from this bit of the circuit). The raw rectified mains are put directly onto the power transistors without bothering to have any sort of smoothing capacitor to controle DC ripple.
  • Although at first sight no smoothing capacitor might appear primitive but this circuit configuration is essential to allow the electronic transformer to work correctly with dimmer switches.

As the switching transistors normally alternate in two states, either fully "on" or fully "off," they don't tend to consume power in themselves.
  • The consumed power is described by the equation I2R.  As "R", representing resistance, is either 0 or ∞ (infinity) and when it is ∞ the current flow, represented by "I", is 0. The result is the power consumed by the transistors is "near" zero.  

In reality nothing is perfect and the transistor must go between zero and infinite resistance. During that brief transitory period, it has a momentary resistance and consequently generates heat.  Fortunately the heat energy is negligible but there is normally a safety thermal protective circuit built into catch thermal runaway.

This is why when looking at the output waveform of the electronic transformer, with the aid of an oscillascope, it appears as a 100 Hz modulated 50 KHz output. This complex waveform is not a problem for a typical 12v halogen bulb. Because of the way the output voltage is delivered, it can be difficult to measure the voltage with some digital multimeters, thus giving the impression that the electronic transformer is not working or providing any output, even though you may see the bulb is unquestionably powered.



Can I use a "power transformer" with LED Lights?


Assuming that the manufacturer of the LED has specified it as a replacement for 12v halogen lighting, the answer is yes.
  • Note that a power transformer can also be known as a magnetic transformer. 
There are no special requirements for a power transformer if you only want normal operation but advanced features such as dimming need further considerations. There are issues with dimming in that:

  1. You would need a leading edge dimmer to be compatible with the power transformer.

    • It is vitally important to use a correct dimmer compatible with a transformer or inductor. This is usually the leading edge type, as it will not suffer from the flyback voltage spike caused by switching an inductor. A trailing edge dimmer is liable to be damaged by the inductive voltage spike. (check the manufacturer's instructions)

  2. You would also need to ensure the LED fitted is capable of being dimmed as well as having the correct dimmer. Not all LEDs are dimmable as some LEDs have a reservoir capacitor which will stop it from blanking out when the dimmer cuts the power part way through the mains cycle.

    • If you have the wrong type of LED it will either not dim at all or will dim very slightly. This is more of an issue with LEDs, as they have low power requirements. This low power requirement allows the manufacturer to fit a small reservoir capacitor within the body of the bulb's case. The reservoir capacitor stores sufficient energy to allows contiguous operation of the bulb through the natural zero volt crossing points of the mains' cycle. This is an advantage for normal LED bulb operation but a hindrance if you want to dim the lights.





For further reading and references: please see my resource page


7 comments:

  1. I have fitted a YT150 dimmable electronic transformer, purchased from B&Q. I found that with using a direct connection to mains and driving three 40W halogen bulbs the lights dimmed after half and hour. When I checked the output voltage with a digital voltmeter I found that it was 8.8V with 1 bulb connected. With 2 bulbs connected it was 7.4V. After half an hour this fell to 6.3V. With no load the output was 10.3V. It was supposed to be 11.5V. I got the same result with a second unit.
    Any suggestions as to what is going on here?

    ReplyDelete
  2. Hi Mike, there are a myriad of possibilities here but to address the first. The output of an electronic transformer is a high frequency modulated waveform, which usually plays havoc with readings from a DVM. Most DVM’s are designed to read simple sinusoid waveforms and get thrown by the messy electronic transformer output. The fact that you are getting any reading is something but you would need to confirm with a working installation, what you actually read is what you expect, using your multimeter. I am lucky in that I have access to an oscilloscope as none of my multimeters even give a reading.

    This is not really a repair forum but I understand that you would look for help wherever you can find it. I do not have a YT150 but Just as a gut reaction to your description, I would think that it could be heat related. Without knowing the previous history of the installation it is difficult to make any suggestions with certainty. If you switch the transformer off for say two hours, then switch it back on and it appears to work as before then dims after 30 minutes, it would suggest that it could be the built in thermal protection.

    1. Check if the transformer case situated in a place where it is getting too hot. Embedded in insulation or directly above a heat source such as pipes or a light is usually a bad idea.

    2. Bearing in mind that 120w at 12v is 10 ampers. This is a large current in anyone's book and will show up any poor connections. This poor connection could get extremely hot and cause thermal expansion as well as arcing. Confirm the integrity of the wiring and connections to all bulbs as this is a serious fire hazard. Poorly rated wiring could overheat and compromise the integrity of the insulation.

    3. A transformer is exactly that and the output voltage is a reflection of the input voltage. Confirm that the electronic transformer is supplied with the correct voltage and that the feed wire is not arching at a junction or at the switch, which is causing a voltage drop as it starts to get hot.

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  4. I've found your articles as the result of a google search on ELV transformers. They are very informative especially relating to why my moderately priced DMM doesn't read reliably on this circuits. I was trying to find out why most of the readily available electronic transformers available for halogen/LED lamps have a maximum wiring distance to the lamp (generally 2 meters) and sometimes a minimum distance as well. This presents a problem when replacing remotely mounted magnetic transformers with an ELV type. Thanks for sharing your time and expertise.

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