Dimmable transfomers

I read in sci.electronics.design that Ross Herbert wrote (in ) about 'Dimmable transfomers', on Fri, 4 Nov 2005:

Certainly, but asymmetric switching is very uncommon for dimmers, because of the flicker introduced. They may be using the term incorrectly.

They need to justify the higher cost of leading edge products. Some manufacturers claimed that they had lower harmonic emissions, until we stopped them with publicity. What they say above is correct, if you give full recognition to 'best suited'.

A transformer with higher than usual leakage inductance, intended to limit inrush current, MIGHT not work well with some trailing-edge dimmers. But this would be obvious, because the lamps would flicker or even flash.

A toroidal transformer, having very low leakage inductance, is most unlikely to be in trouble. For garden lighting, the cable resistances help a lot to limit inrush current.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Yes, not very clear was it?

Correct. Leading edge dimmers employ a triac.

Correct again.

Correct.

Yes, I noted that you had already posted this warning and again you are correct. John Woodgate seems to think toroidals are ok for garden lighting, but only if using sufficiently resistive low voltage cabling to limit cold start current. This goes against all the recommended practice which says that as heavy a cable as practicable should be used to minimise volt drop. John infers that one should use cable which tends to limit current but if this is true it means it is dropping voltage. He can't have it both ways....

That's a new line from you Phil, such masterful use of the English language. Where did you learn that I wonder??? Kindergarten near you is there???

"instate": to put into a certain state, position or condition: install. (Macquarie dictionary) I don't get your context.

If I did have expertise in designing such circuits I would know the reasons for the recommendations. However, since I don't know of any readily available technical description on the design aspects of the items in question I can only state the recommendations from those who should know. If you have further in-depth knowledge on the matter then why don't you enlighten me and others.

Completely irrelevant... If something did go wrong it would mean something had failed and I can fix that...

So I would be learning something when it did fail. The fact that it hasn't failed seems to indicate it is well designed and therefore should not fail for a very long time. Surely even you wouldn't be fretting that you wouldn't have the opportunity to learn something new just because all your electrical gear kept working year in year out without failing. You would probably deliberately bugger it up just so you could have an excuse to pull it apart I suppose.

look at the AN308 from ST called "HOWTO SELECT A SUITABLE CIRCUIT CONTROL BY A TRIAC FOR AN INDUCTIVE LOAD"

Anyway, I buildt this long long time ago (1996) and the circuit is still working fine as expexted on a 500VA "ordinary" EL-Core (or soething like that) transformer in the "power"range of 15 to 100% at the nomial resistive load of 500W@12V

hth

--

Michael Wieser

I read in sci.electronics.design that Ross Herbert wrote (in ) about 'Dimmable transfomers', on Fri, 4 Nov 2005:

For someone who claims to know nothing, you are very good at trashing people who do. I'm not even going to bother to explain why you are wrong. Arrogance has its own reward.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

They don't dim by much and then they are in standard incandescent mode- the rated halogen lifetime is significantly reduced and the attempt at removing the darkening by running at full power again is only partially effective. So he might as well go with the incandescents from the start if the dimming is going to be used often. " The standard operating temperature of the bulb wall to maintain a halogen cycle is 250C. When the temperature falls below this level the halogen cycle fails, and the lamp starts to act like a standard incandescent. It is estimated that the cycle fails when a lamp is operated below 70-80% of its rated power."

"John Woodgate"

** Ambiguous question - John.

Which remark are you enquiring/trolling about ?

The " any old triac dimmer" one surely does not need explaining to YOU yet again .

......... Phil

"John Woodgate"

** So you were trolling with a loaded question - as usual. ** Veni, vidi vici ???

** I doubt Ross knows a simple dimmer from a dim sim.

He is/was a telco tech.

........... Phil

"Ross Herbert"

** Masterful understatement.

Sir Humphry would be smiling.

** You have lost me a bit here.

Leading edge = triac dimmer - no ?

A triac is fired later and later in the half cycle to control power.

So, the "edge" leads the rest of the cycle.

** The key is the phrase "properly designed" - means the triac must be continuously triggered after switch on to preclude commutating off at the **wrong time** and producing half waving. ** Trailing edge = IGBT dimmers or maybe GTO triacs - no ?

The half cycle begins from zero and is cut short the vary the power.

Much kinder to diodes and filter electros.

** I agree - ELV lighting is one job toroidals are crap at.

The " high impedance " comment refers to the quite high leakage inductance typical of split bobbin transformers - hence a nice, soggy start up transient.

** All agreed.

As I informed the OP - standard toroidals are NOT safe or suitable for (esp outdoor) halogen lighting.

......... Phil

"John Woodgate"

** What nonsense.

Simple, pulse fired triac dimmers are normally somewhat asymmetric at mid and low settings.

It does not matter a hoot with incandescent lamp loads.

....... Phil

"Ross Herbert"

** Like some Soy on your "dim sims " - sir ??? ** Straw man fallacy.

The detailed and *instated* reasons behind such recommendations are the issue here.

** What does a bloke learn when nothing goes wrong ???

SFA.

More Soy ???

............ Phil

"Ross Herbert"

( snip agreed and good stuff)

** Oh, but I expect JW likes his *dim sims* to be very bright.

Easy on the Soy boy.

........... Phil

"Ross Herbert" "Phil Allison"

** Its a gift from God.

** If you had ever seen what goes wrong - you would be a wiser fellow.

( snip pedantic RH drivel)

** The opinion of a fool.

** Yes - you would be forced to analyse the system more fully.

Revelations and insights usually follow

Remember the old saying about " learning the hard way ".

Its how all engineering is really done.

.......... Phil

"John Woodgate"

** :-0

........... Phil

"Fred Bloggs"

** Huh ????

Halogen lamps often have SHORT lives compared to ordinary lamps - the 24 volt, 150 watt one in a 35 mm slide projector lasts 50 hours - at best !!

The life span of a tungsten lamp is an inverse function of filament temperature - it diminishes in proportion to the 12th or 13th power of the applied voltage ( depending who you ask) .

The " Quartz Halogen " sub species can operate at a higher filament temperature than the usual low pressure nitrogen bulbs without blackening the glass - however with reduced life.

When quartz halogen bulbs are operated at LOWER than specified voltage, life span increases - by large ratios.

Dimmed QI downlights last longer than non dimmed ones.

........ Phil

And where can you point to any of my previous posts "trashing" your "expert" opinion?

And where did I "admit to knowing nothing"? I said I have no expertise in designing transformers but that is a far cry from an admission to "knowing nothing" about "any" subject. I simply disagree with your statement that toroidal transformers make good candidates for elv garden lighting, particularly since you inferred that the cold switch on surge current in the lamps would need to be countered by using cables having a particular resistance in order to prolong lamp life. Doing it this way would imply that every cable run to any particular lamp would have to be specifically designed to achieve the required resistance to keep the surge current within the desired limit. Not very practical. It is far more sensible practical to use a transformer designed for the purpose which has the apprpriate characteristics rather than design around the undesirable characteristics of a non-recommended transformer.

You claim to be an "expert" in this area and I can't disagree with you since I don't know you or your achievements, however over my lifetime I have learned the wisdom of this observation about experts;

"An expert is a man who has made all the mistakes which can be made, in a narrow field."

Perhaps you are still making mistakes....

DISCLAIMER: I have been designing and overseeing garden and interior lighting systems for almost 15 years in the USA. I don't know a lot about the detailed electronics but I know what works.

1. The dimmer is of more concern than the transformer, but the two must be matched. That means the dimmer should be rated *by its manufacturer* for the type of transformer used (electronic and/or inductive). No exceptions - no smoke!
2. Dimming halogens works really well in most cases. HOWEVER, running them at low levels (

** The title of this NG indicates that detailed electronics is what it is all about.

That is just what a person HAS to know in order to predict if some arbitrary set up is going to be problem free.

Way out of the league of some installer dude - obviously.

.......... Phil

"Ross Herbert"

** You drew that inference - it is not in JWs post.

The "inrush surge " current of a toroidal transformer with a full halogen load attached is MASSIVE - enough to blow the triac in most dimmers.

Such a set up of 150VA size or more will need a "soft start" system fitted or some design feature in the triac drive that has that effect.

......... Phil