Timeline for Easy way to figure out a LED's Vf in order to pick an appropriate resistor
Current License: CC BY-SA 4.0
27 events
| when toggle format | what | by | license | comment | |
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| May 12, 2022 at 16:13 | history | edited | JYelton | CC BY-SA 4.0 |
edited body
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| S Feb 26, 2019 at 16:38 | history | suggested | Electric_90 | CC BY-SA 4.0 |
improved formatting
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| Feb 26, 2019 at 15:50 | review | Suggested edits | |||
| S Feb 26, 2019 at 16:38 | |||||
| Jun 4, 2015 at 20:08 | comment | added | Ned64 | @akohlsmith Thanks again, I have: electronics.stackexchange.com/questions/173968/… | |
| Jun 4, 2015 at 19:45 | comment | added | akohlsmith | @Ned64 I think it would be worth posting a new question with your specific scenario. Include pics of the panel and tell us what you've tried and ask how to go about driving this thing. Include a link to this question since it's related but not quite the same. | |
| Jun 4, 2015 at 14:28 | comment | added | Ned64 | @akohlsmith (sorry, not allowed to chat, not enough points) Maybe I can try varying the current in a friend's lab (with his power supply), but where to stop? Is there perhaps a voltage limit over a single (white) LED that I could watch out for? | |
| Jun 4, 2015 at 14:17 | comment | added | akohlsmith | @Ned64 ahh see that's quite a bit different. :-) and yes, something like that would be a very good candidate for a constant current driver. You might want to take a look at backlight drivers for LCDs and whatnot; they usually have long strings of white leds and have constant current drivers which can go up to higher voltages. | |
| Jun 4, 2015 at 14:15 | comment | added | Ned64 | @akohlsmith It's a 600mmx600mm panel of 576pcs 3014 SMD LED in some unknown configuration. I thought the standard way to power it would be a constant current driver, like the one I use for my COB lights. I tried a 300mA driver, which resulted in 34V voltage drop. The panel clearly needs more, but I don't know how to find out without damaging the panel (e.g. by pairing it with a driver of too high voltage/current). | |
| Jun 4, 2015 at 13:33 | history | edited | akohlsmith | CC BY-SA 3.0 |
clarity edits, no technical changes
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| Jun 4, 2015 at 13:18 | comment | added | akohlsmith | @Ned64 Is it important to have a constant current driver? Is the supply voltage wildly varying (several volts) or is it absolutely required that the LED have a specific brightness that is unvarying? If not, guess at 20mA, calculate the resistor and slap it in. Tweak as needed. | |
| Jun 4, 2015 at 9:00 | comment | added | Ned64 | @akohlsmith Many thanks - too bad it's not so easy. I have an LED panel without driver and just don't know which constant current driver to buy. | |
| Jun 2, 2015 at 19:45 | comment | added | akohlsmith | @Ned64 without a datasheet you're reduced to experimenting. A light meter and variable resistor will give you a curve showing input current vs light output. It'll be a little more complex than this, but really there is no such thing as "perfect current" - all components will vary with PVT. Pick a value that works for the 80% case (which is really probably 99%) and run with it. | |
| Jun 2, 2015 at 13:07 | comment | added | Ned64 | Maybe it's not part of the original question, but how do I figure out the LEDs rated current? You assumed 10mA, but this may be suboptimal. Is there a way to assess the perfect current, too, without a data sheet? | |
| Jun 1, 2015 at 23:28 | comment | added | Peter Bennett | @diegoreymendez: Your meter apparently has a maximum reading of 1.99 volts in the diode test mode, so it can't indicate the Vf if it is higher than that. You'll have to use one of the methods mentioned in other answers for the higher-voltage LEDs. | |
| Jun 1, 2015 at 23:08 | vote | accept | clearscreen | ||
| Jun 1, 2015 at 23:03 | comment | added | clearscreen | My multi-meter is returning 1690 for red LEDs. I assume this is 1.69V? The weird part is it's returning a perfect "1" for green, blue and yellow LEDs even though they light up. | |
| Jun 1, 2015 at 22:23 | history | edited | akohlsmith | CC BY-SA 3.0 |
added 110 characters in body
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| Jun 1, 2015 at 22:19 | comment | added | akohlsmith | @diegoreymendez you're right, you do want to know what the Vf is in order to drive it correctly but what we're all saying here is that a very rough "rule of thumb" is appropriate for this kind of application. A lot of newbies get caught up in being very precise when there is no need for more than a bit of precision. When you must be precise and when you can be general comes with experience and usually with destroyed parts as well. :-) | |
| Jun 1, 2015 at 22:18 | comment | added | clearscreen | @akohlsmith - I'll use transistors but not just yet. I'll open up a new question for that. | |
| Jun 1, 2015 at 22:18 | comment | added | akohlsmith | @diegoreymendez Yes, my meter has them both on the same setting as well. Usually there will be another button to select between modes on the same selection switch position. Consult your meter manual for the details for your meter. | |
| Jun 1, 2015 at 22:17 | comment | added | akohlsmith | @Asmyldof I've edited my answer to eliminate the potentially misleading Vce spec. You're absolutely right, it depends a LOT on other factors. | |
| Jun 1, 2015 at 22:16 | history | edited | akohlsmith | CC BY-SA 3.0 |
elaborated answer, eliminated contentious 1.4V Vce comment as it needs more explanation
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| Jun 1, 2015 at 22:11 | comment | added | clearscreen | Thank you for the detailed reply. A few extra doubts I have: 1. My multi-meter has symbols 11 and 14 at the same position (diode on top of speaker-like symbol). Can I safely assume that's the correct diode position? 2. I have a feeling that since people tell me I'm trying too hard something I'm doing is inherently wrong - shouldn't I try to figure out the Vf first? Thanks! | |
| Jun 1, 2015 at 22:09 | comment | added | Asmyldof | I wanted to send you a message, but I can't seem to find a way, nor to start a chat, so I'll just do it here anyway (I've been away too long before a week ago). See figure 4: fairchildsemi.com/datasheets/BC/BC547.pdf at 20mA it's only ~0.06. And that's a crappy BC500 series. So, that is only very roughly 1.4. ;-) | |
| Jun 1, 2015 at 22:04 | comment | added | akohlsmith | Yep, hence the "very roughly" :-) Thank you for the feedback, I'll make some edits to clear things up | |
| Jun 1, 2015 at 22:02 | comment | added | Asmyldof | Sublime answer, except that C-E drop at saturation in a transistor doesn't need to be (and often ins't) 1.4V, otherwise everything perfectly explained and worked out. | |
| Jun 1, 2015 at 21:46 | history | answered | akohlsmith | CC BY-SA 3.0 |