Downlight transformer

[ban-all-sheds]

I'm not sure about "horribly" - take a step back and consider it in context of the efficiency of incandescent lighting which we had for donkeys, and which 30% of the Brexit ******* want back.

On a whole-life basis, I'm not sure that trying to cram an unsophisticated SMPS into a small lamp envelope is such a good idea.

 

[aptsys]

I'm not sure about "horribly" - take a step back and consider it in context of the efficiency of incandescent lighting which we had for donkeys, and which 30% of the Brexit ******* want back.

On a whole-life basis, I'm not sure that trying to cram an unsophisticated SMPS into a small lamp envelope is such a good idea.

It is, you simply can't get rid of the heat otherwise. Most of these lamps will have two or three series strings of 3 LEDs, so you'd be dissipating 5 or 6W in any linear current limiting device at the higher end of their operating voltages.

 

[ban-all-sheds]

How does that compare to the efficiency of a small cheap'n'nasty SMPS?

And remember I said "whole life basis", so we need to compare the emissions and pollution involved from end-to-end with the production of the SMPS, and the disposal costs and issues.

 

[JohnD]

If I had a sudden irrational desire to festoon my home with small lightbulbs, I could buy these
http://www.wickes.co.uk/Wickes-LED-Mini-Globe-Light-Bulb---5-9W-B22/p/141664
or these
http://www.wickes.co.uk/Wickes-LED-Candle-Light-Bulb---3-4W-B22/p/141660
or even these
http://www.wickes.co.uk/Wickes-LED-Candle-Light-Bulb---3-4W-E14/p/141659

at a very modest price, with no need to fret about transformers and compatibility. I could find something to fit ordinary lampholders as found in 99.6% of the homes in the UK.

Why would I waste my time and money on added complexity for something that will soon be obsolete or go wrong?

 

[bernardgreen]

with small lightbulbs, I could buy these

each of those has an SMPS built into the lamp. If it is just one lamp on a switch then you may need to buy and fit some of these

https://www.maplin.co.uk/p/rc-contact-suppressor-rg22y

as snubbers.

 

[JohnD]

I have a feeling that I am most unlikely to buy any of those. I have a feeling that about 60 million other people in the UK won't, either. And I wouldn't want most of them to try adding such things.

 

[Taylortwocities]

each of those has an SMPS built into the lamp. If it is just one lamp on a switch then you may need to buy and fit some of these

https://www.maplin.co.uk/p/rc-contact-suppressor-rg22y

as snubbers.

Even if if were needed (it probably won’t be) then only one is needed per circuit.

 

[JohnW2]

Generally they do, anything else would be horribly inefficient.

A capacitor and bridge rectifier (as seen in some of the cheap LED lamps) is not particularly inefficient, is it?

Kind Regards, John

 

[aptsys]

A capacitor and bridge rectifier (as seen in some of the cheap LED lamps) is not particularly inefficient, is it?

Kind Regards, John

We were talking about 12V AC/DC lamps.

How does that compare to the efficiency of a small cheap'n'nasty SMPS?

And remember I said "whole life basis", so we need to compare the emissions and pollution involved from end-to-end with the production of the SMPS, and the disposal costs and issues.

Even the simplest buck converter should be able to drive LEDs at 80% or better and requires only around 5 components.

 

[JohnW2]

We were talking about 12V AC/DC lamps.

Ah! Although I realise that such was the 'on-topic' matter of this thread, I thought the more recent posts had been about the innards of LED lamps in general

However, and I guess this is what you are thinking of in what you go on to say, a DC-compatible LED does not need a SMPS in the normal sense - just a very simple 'chopper', which could probably be more efficient (no transformer) than even a conventional voltage-changing SMPS, would presumably be adequate (and could presumably be on a single tiny chip) - although the 'high current' pulses might have a detrimental effect on the life expectancy of the LED elements.

Kind Regards, John

 

[ban-all-sheds]

Even the simplest buck converter should be able to drive LEDs at 80% or better

How much do you need to drop from 12V for 3 LEDs in series?

 

[JohnW2]

How much do you need to drop from 12V for 3 LEDs in series?

Something probably approaching a quarter of the supply voltage - so if one uses a resistor or semiconductor, something probably approaching a quarter of the supplied power will be dissipated as heat in that component.

In contrast, I don't really see why a 'chopping' circuit should be as inefficient as the 80% suggested by aptsys - there could be well under 1V across the semiconductor element driving the LED.

Kind Regards, John

 

[aptsys]

Something probably approaching a quarter of the supply voltage - so if one uses a resistor or semiconductor, something probably approaching a quarter of the supplied power will be dissipated as heat in that component.

In contrast, I don't really see why a 'chopping' circuit should be as inefficient as the 80% suggested by aptsys - there could be well under 1V across the semiconductor element driving the LED.

Kind Regards, John

It depends on the topology, normally, it would be a standard buck regulator - so two switching elements (diode and transistor), inductor and capacitor and a sense resistor which must dissipate some power. All of these have losses plus the device driving these takes current too.

 

[JohnW2]

It depends on the topology, normally, it would be a standard buck regulator - so two switching elements (diode and transistor), inductor and capacitor and a sense resistor which must dissipate some power. All of these have losses plus the device driving these takes current too.

Yes, that's all true. I'm just a little surprised that one can't do at least a bit better than 80% efficiency. Let's face it, with just a resistor (and a bridge rectifier), if one assumes 3 x 3.3V LEDs, one ought to get about 82.5%, the other 17.5% being dissipated in the resistor and rectifier, so one would hope that something 'more clever' would do appreciably better than that.

Kind Regards, John

 

[aptsys]

Yes, that's all true. I'm just a little surprised that one can't do at least a bit better than 80% efficiency. Let's face it, with just a resistor (and a bridge rectifier), if one assumes 3 x 3.3V LEDs, one ought to get about 82.5%, the other 17.5% being dissipated in the resistor and rectifier, so one would hope that something 'more clever' would do appreciably better than that.

Kind Regards, John

There are times when DC-DC converters are not appropriate and linear regulators are more efficient - it depends on the circumstances.

In the case of normal 12V lamps, they have to deal with the nominal DC supply range in addition to the 12V AC rms which will have peaks at close to 17V. If you don't want any 100Hz flicker then the nominal voltage before any driver or regulator will probably be 15V or so.