Choosinga current liminting resistor when Vf = supply? - Page 1
Forum section: electronics / beginners

author: MarkS
author_rank: Unknown
timestamp: August 28, 2024, 10:02:58 pm
content: I'm making a digital clock with those 6" 7-segment displays Sparkfun sells. Each segment has a forward voltage of 12 volts and I plan on using a 12v Meanwell supply to power both the displays and the 5V logic. However, this poses an interesting problem that I have not encountered before. Vs - Vf / 20mA = 0 / 20mA. How do I make the math, math?

author: pqass
author_rank: Super Contributor
timestamp: August 28, 2024, 10:34:45 pm
content: Sparkfun has atutorialat the bottom of the page for the6" display.They have a SPI driver board using anopen-drain TI chip.Looking at the attached images tells you that the driver board overlays the display board and +12V goes directly to the display anodes.DANGER! DANGER! DANGER! DANGER!I just noticed there are resistors on the drains of theSPI driver board! I can't make out their value.Schematic of the driver boardsays they are 20R.

author: langwadt
author_rank: Super Contributor
timestamp: August 28, 2024, 11:05:44 pm
content: Quote from: pqass on August 28, 2024, 10:34:45 pmit has the calculation for 20R from the worst case Vf = 11.6V and then says 11-15R works well which makes sense since the driver is ~6-10R

author: Smokey
author_rank: Super Contributor
timestamp: August 28, 2024, 11:31:46 pm
content: From the datasheet, each "segment" is 6 LEDs in series. 12V/6leds = 2Vf/LED. That seems a little high for red, but reasonable.How about current sources/sinks instead of limiting resistors. Then it doesn't matter what the supply voltage does.https://www.digikey.com/en/products/detail/diodes-incorporated/BCR420UW6-7/5247739https://www.diodes.com/assets/Datasheets/BCR420UW6-BCR421UW6.pdfBCR420UW6-7 + one resistor per segment would get you 20mA (or whatever you wanted). Plus you can PWM dim it if you wanted.or...Toss these in there.https://www.digikey.com/en/products/detail/diodes-incorporated/AL5809-20S1-7/5030204https://www.diodes.com/assets/Datasheets/AL5809.pdfSingle device in SOD123 package. Limits current to 20mA. $0.22 each in 10s.

author: langwadt
author_rank: Super Contributor
timestamp: August 28, 2024, 11:43:57 pm
content: Quote from: Smokey on August 28, 2024, 11:31:46 pmexcept the dropout of that is ~1V so a 12V supply will no longer be enough

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 29, 2024, 12:30:34 am
content: Look into its datasheet, or characterize your display'sI-Vcurve if you want to be sure.When a LED display is "rated" at 12V, it's either:Having internal resistor or current regulator and is intended to be driven by 12V directly, no need to bother with detailsHavingI-Vcurve that slopes shallow enough thatIFis not changing dangerously fast with changingVFActually having 12V nominalVFthat will be way too marginal if you drive it with 12V directlyThe last case may work if you insist, but also may often completely break down under changing temperature.Anyway, looking at datasheets it appears to not be the 1st caseSo either use a small resistor and accept the slightly dimmer display, and hopeVFdoesn't wander down too far away. Or use higher supply voltage to get better margin, some supply module can be trimmed up a bit too depending on the model.

author: MarkS
author_rank: Unknown
timestamp: August 29, 2024, 02:02:03 am
content: After taking everything into consideration, I think I'll go with a 15v supply. It will make everything make sense.

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 29, 2024, 04:01:52 am
content: The datasheets specifies 11.6-12.4Vf for 20mA, 12V typ.Depending on manufacturing differences, temperature etc... it might work, or get too much / too low current, you're at the edge.Instead asking here, why don't you test it with a lab psu and see yourself what happens when varying the voltage around 11.6-12.4V?The current might be stable enough - or skyrocket!I wouldn't connect them directly to 12V, though I've never been in this situation.Those meanwell PSUs often have a pot to allow some voltage adjustment, if you get it to 13-14V it migh be enough to use a led driver.