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Gadget Freak Case #230: The Inexpensive Dimmable LED Desk Lamp

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The dimmable LED driver circuit inside the desk lamp.
The dimmable LED driver circuit inside the desk lamp.

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Re: An even simpler LED driver
cvandewater   12/8/2012 12:56:03 PM
Hi Andrew,

Yep - born & raised Dutch, but now Californian resident.

Clarification for others not familiar with the term - Elco means Electrolytic Capacitor. I was not aware that it is a typical Dutch expression.

Re: isolated, I referred to the housing, indeed the electronics are "hot" so it is required that it cannot be touched anywhere, including at openings for trimming pots to select sensitivity and timeout of the PIR detector. Unfortunately that are typical locations where (rain)water gets in, causing the demise of the detector. Usually it first starts acting weird, switching the light on and off randomly before finally failing completely.

"geisoleerd" is indeed the Dutch word for "isolated" and essentially means separated, so this can mean either the galvanic isolation from the grid or the protection of a "hot" circuit from touch by a non-conducting enclosure. So a further qualification is needed and I forgot to re-read my typing so I did not catch that my statement was ambiguous.

Most likely you found the same type of rope light that I had a section fail in.

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I want one
Cadman-LT   12/10/2012 3:05:12 AM
I'd like to have one of those!

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I found an error
armorris   3/31/2016 11:16:18 AM
If you use a zener diode as the voltage reference instead of an LED, put a forward-biased signal diode like a 1N4148 in series with the zener diode to block a sneak-path that slows down the startup of the circuit. The LED would be back-biased during start-up, blocking the sneak-path. Note that this turn-on delay might not be noticeable unless the electrolytic capacitor is discharged. It does not have a bleed resistor, so it can hold a charge for days, once the voltage has dropped below that required for the LEDs to conduct.

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Worn pot may destroy LEDs
armorris   9/9/2016 8:07:01 PM
When the dimmer pot starts to wear out or oxidize, the wiper might lose contact with the resistive element, causing the LED current to flow uncontrolled, damaging the LEDs. To prevent this, add a silicon signal diode, such as 1N4148 between the wiper (anode) and the high side of the pot (cathode). In normal operation, the diode will be reverse-biased and will have no effect on the circuit, but if the wiper loses contact with the resistive element (assuming the resistive element is intact), the diode will become forward biased, limiting the amount of current that can flow through the LEDs. They will glow brighter than normal, but probably will not be damaged in the short term.

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