20 mA LED SMPS buggy

"Ian Field" wrote in news:sqJBr.42046$ snipped-for-privacy@fx16.am:

dealextreme has LED "supplies" designed to work from 240VAC,perhaps he could tinker with one to change it's current output from amps to what he needs for his low power LEDs. or modify a Li-ion battery charger to supply the low mA he needs for his project;they also use a transformerless line-to-DUT supply,and are fairly inexpensive.

at minimum,they would give him the IC type and circuit needed to do what he wants.

Have you tried a capacitive voltage divider and connect the bridge, surge protection resistor and LED chain to the midpoint of the divider.

Of course, there should be quite a few LEDs in the chain (10-20) to keep the parallel capacitor at a reasonable size.

I chain of big LEDs (0.5-3 A) could be driven through an choke with reasonable inductances, e.g. through fluorescent ballasts (at least in the 230 V world).

There was recently a selection in the news & new products section of various LED drive modules, one in particular was a module with the form factor of the ballast for a linear florescent - it made big of being potted & having a high IP rating.

Potting seems to be a growing trend just lately.

"Ian Field" wrote in news:Uc0Cr.67235$ snipped-for-privacy@fx29.am:

My $7 dealextreme Li-ion 18650 2-cell charger is not potted. it's also very lightweight,so there's no XFMR in it(I haven't gone into it...) and the chip and circuit board can probably be accessed and modded. (unless it's chip-on-board!)

it runs off 100-240VAC.

There is almost without a doubt a HF transformer in there, just not a mains-frequency transformer.

If it's this one:

.. it's indicated to be Class II (double-insulated).

By the look of the adverts; planar transformers are starting to make an impact on the marketplace.

Back in the 80's Sony did a CTV PSU that switched a hefty electrolytic between the bridge rectifier & HT rails, IIRC it used GTO thyristors - very nasty things to faultfind.

"Ian Field" wrote in news:2%1Cr.89391$ snipped-for-privacy@fx24.am:

I opened it up,it definitely contains a small HF XFMR,about an inch square,0.5" high. 2 5-terminal SMD ICs labeled "5465",a TL431,and several xstrs. a pretty lousy soldering job,too.

Is it 10uS/division? If it is, it is slow:

That wont work because C2, E2 is not a negation of C1, E1 - sorry.

Glenn

...

...

The turn off seems to be 2uS?

If it is 10uS/div, you turn the transistor on for 60uS?

Are you sure that you have phased the transformer for flyback?:

Glenn

put a large low ESR capacitor in parallell with the LEDS

it's effectivly a capacitive divider, so if you're dropping 240V to 7V make the parallel cap 240/7 times the size of the series cap. then the series resistor only needs to be sized to protect the rectifier

An AC Xc potential divider might work - using a bridge rectifier and damping the rail with an electrolyitic might also help a little - but there is a serious warning! - If the LED chain is intermittent, even for a split second and the cap will develop enough voltage to punch out the chain of LEDs - intermittent for more than a split second could burst the cap.