Dropper resistor for sealed 12 to 24V LED to run it on 32V? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 10, 2022, 07:17:10 pm
content: Ebay sourced sealed LED panel light, advertised as 12 to 24V, Presumably it has an internal dropper resistor inside already. I want to use it as an indicator for a 32V supply, how can I calculate the value and Wattage for an additional series resistor please?https://www.ebay.co.uk/itm/393666454759Thanks.

author: langwadt
author_rank: Super Contributor
timestamp: July 10, 2022, 07:29:16 pm
content: measure the current at ~24V, add a resistor that drop the extra 8V at that current, 1K is probably in the ballpark

author: MikeK
author_rank: Super Contributor
timestamp: July 10, 2022, 07:36:00 pm
content: P = I2RP = power dissipatedI = current through resistorR = resistor valueUse a resistor rated 2X higher (or more).

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 11, 2022, 10:56:42 am
content: Thanks for the replies, just come to wire these up (I bought two, one for an indicator on a 12V DC equipment supply, the other for a 32v DC equipment supply).I noticed there were no polarity markings on them, and they work with either polarity. My multimeter doesn't show them as a diode on the diode test setting, just open. On resistance about 680k, slightly different according to polarity connection.So I assume they can't be incandescent, but how do you get an LED to connect to either polarity please? Bridge rectifier inside?

author: MikeK
author_rank: Super Contributor
timestamp: July 11, 2022, 11:52:00 am
content: Two back-to-back LED's?Woops, nevermind.

author: Per Hansson
author_rank: Unknown
timestamp: July 11, 2022, 11:57:47 am
content: Quote from: Chris Wilson on July 11, 2022, 10:56:42 amThere are neat antiparallel LED's available, I didn't actually find them with a quick Google search (maybe used the wrong terms) but here is a bigclive video that shows them:Of course there is the possibility that you have something different, only way to find out is to try to open it up and have a looksie inside...

author: langwadt
author_rank: Super Contributor
timestamp: July 11, 2022, 12:41:29 pm
content: Quote from: Per Hansson on July 11, 2022, 11:57:47 amthe one I have checked (not an ebay special) has four diodes, current limitng resistor and a cap across the LED

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 11, 2022, 02:27:58 pm
content: I use a lot of Schneider Electric illuminated push button and I can buy the LED for them that works on 24-120V both AC and DC. I don't know how they do it.

author: tooki
author_rank: Super Contributor
timestamp: July 11, 2022, 03:21:16 pm
content: If I had to guess (assuming it’s packaged in a standard epoxy LED housing) would be some kind of constant current driver, perhaps with a bridge rectifier. For the tiny amounts of current needed in modern LEDs, this is probably not a problem.

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2022, 05:07:41 pm
content: Quote from: BeBuLamar on July 11, 2022, 02:27:58 pmThe use an IC such as the AL5890-10?https://www.mouser.co.uk/datasheet/2/115/AL5890-1274651.pdf

author: langwadt
author_rank: Super Contributor
timestamp: July 11, 2022, 06:24:10 pm
content: Quote from: Zero999 on July 11, 2022, 05:07:41 pmor,https://en.wikipedia.org/wiki/Constant-current_diode

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2022, 06:36:13 pm
content: Quote from: langwadt on July 11, 2022, 06:24:10 pmAre they still made? I've not seen any rated to hundreds of volts.I could design on with a TL431 or similar with a cascode, but it's much easier to use an IC specifically designed for it.

author: langwadt
author_rank: Super Contributor
timestamp: July 11, 2022, 07:13:03 pm
content: Quote from: Zero999 on July 11, 2022, 06:36:13 pmLND150 is rated for 500VI've heard of some using them for series protection of inputs

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 11, 2022, 08:09:41 pm
content: Hmm, I seem to have started something here...I had no idea these would be anything other than an LED with a series resistor in a waterproof case.Does anything mentioned above preclude me from just adding an external series resistor to use one of them on 32V? From some replies it almost seems that they might work "as is" on more than 24V? If I had a spare I'd be tempted to try it, or cut it apart.But I haven't got a spare so the innards remain a mystery.Thanks for the replies, I had no idea these things could be so sophisticated, far too sophisticated for the likes of me

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 11, 2022, 08:16:48 pm
content: Measure current at 12 and 24V, if it draws considerably more at 24 it's probably just a resistor doing the current limiting. Given the narrow range I'm inclined to think resistor, probably sized for ~20mA@24V.That's part of how these Chinese parts are so cheap, they pass the cost of characterizing them on to you.

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2022, 08:22:30 pm
content: Quote from: langwadt on July 11, 2022, 07:13:03 pmThat's not a constant current diode, but a depletion mode MOSFET. It can be used as a constant current source, but it has a wide tolerance.

author: langwadt
author_rank: Super Contributor
timestamp: July 11, 2022, 08:25:58 pm
content: Quote from: Zero999 on July 11, 2022, 08:22:30 pmit is the same thing, if you need tighter tolerances they are sorted

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 11, 2022, 08:30:34 pm
content: OK, understood, thank you, I'll pop one on my metered bench supply and report back ASAP. Probably next couple of days, work is intruding

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2022, 09:44:48 pm
content: Quote from: langwadt on July 11, 2022, 08:25:58 pmHow do you get tighter tolerances with just a MOSFET?

author: langwadt
author_rank: Super Contributor
timestamp: July 11, 2022, 10:04:20 pm
content: Quote from: Zero999 on July 11, 2022, 09:44:48 pmthe manufacturer sorts them, just like resistors or that some transistor come in A/B/C variants with different hfe

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2022, 10:26:38 pm
content: Quote from: langwadt on July 11, 2022, 10:04:20 pmThe data sheet for the part you mentioned said nothing about that. It just says the drain current can be anything between 1mA and 3mA, when the gate voltage is zero.http://ww1.microchip.com/downloads/en/DeviceDoc/LND150-LND250-N-Channel-Depletion-Mode-DMOS-FETs-Data-Sheet-20005454A.pdf

author: langwadt
author_rank: Super Contributor
timestamp: July 11, 2022, 10:31:45 pm
content: Quote from: Zero999 on July 11, 2022, 10:26:38 pmlook at some of the datasheet listed on the wiki page, like this:https://linearsystems.com/lsdata/datasheets/J500_J501_J502_J503_J504_J505_J506_J507_J508_J509_J510_J511_Current_Regulator_Diode.pdf

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2022, 07:01:20 am
content: Quote from: langwadt on July 11, 2022, 10:31:45 pmThey're only rated to 50V though.A high voltage depletion mode MOSFET, such as the type you've listed could be cascoded with a lower current source rated to a much lower voltage though it's not as simple as just using a high voltage constant current LED driver IC.