Explain "Pull-up 300Ω to +5V" in regards to a PC Front Panel LED - Page 1
Forum section: electronics / beginners

author: BerMM
author_rank: Contributor
timestamp: February 06, 2024, 04:43:33 am
content: The specs on my mainboard say " Front panel LED (main color) Pull-up 300Ω to +5V". I'm putting it in a non PC case that has a 5v power LED. I could use some help figuring out what "Pull-up 300Ω to +5V" means.

author: golden_labels
author_rank: Super Contributor
timestamp: February 06, 2024, 04:50:35 am
content: It means that somewhere a 300 Ω pull-up resistor has to be placed to some +5V rail. WIthout the context it’s hard to tell where and how.300 Ω fits a resistor to be used for a typical indicator LED to be driven from 5 V.

author: BerMM
author_rank: Contributor
timestamp: February 06, 2024, 05:35:01 am
content: Thanks! I know what a resistor is, but not a pull-up resistor (what is the difference?). I want to connectthis Nintendo LED and switchtothis NUC(front panel header is page 36).Quote from: golden_labels on February 06, 2024, 04:50:35 am

author: Konkedout
author_rank: Frequent Contributor
timestamp: February 06, 2024, 05:54:50 am
content: "pullup resistor" is like "steering wheel". A resistor is a resistor and pullup is how it is connected and used.So you would connect one end of your 300 ohm resistor to +5 volts.The other end of the resistor would be connected to the LED Anode.The cathode of the LED would be connected to ground or a transistor drain or collector which switches it to ground.LEDs generally need to be connected in series with something (like a resistor) to limit current.Does that help?

author: BerMM
author_rank: Contributor
timestamp: February 06, 2024, 06:24:37 am
content: That makes sense, thanks! (I took an analog electronics class 25 years ago and I remember current limiting resistors when using LEDs, but I don't think we used the term pull-up)Quote from: Konkedout on February 06, 2024, 05:54:50 am

author: magic
author_rank: Super Contributor
timestamp: February 06, 2024, 08:31:46 am
content: Normal PC LED connectors look like your NUC's HDD LED connection: there is on-board pullup to the 5V rail on one pin, and an open drain output on the other pin. You connect a bare LED between the two pins in appropriate polarity and it "just works".There appears to be two power LEDs on the NUC and figure 10 suggests that they are wired in opposite polarities and the motherboard can select which one to light up by changing voltage polarity. This most likely means that there are selectable positive pullups and open drain drivers on both pins, under motherboard firmware's control.Do you have the original panel which connects to this header? What's in it and how is it wired?

author: golden_labels
author_rank: Super Contributor
timestamp: February 06, 2024, 11:08:51 pm
content: I guess this may be a weird use of word “pull-up”. On the same page they also drawn LEDs conducting from negative to positive (circled in the attachment).Normally I would say to just grab a multimeter and measure, but it seems that pins 2 and 4 are both actively driven. That is to support two-color LEDs. So I suppose measuring resistance from pin 2 to +5V is not going to help. This is probably a driver circuit with output resistance equivalent to about 300 Ω.If you are worried about destroying something, just put a 1 kΩ resistor between 2 and 4, turn it on and measure voltage across that resistor. It should read about 1000/1300 · 5V ≈ 770 mV when the thing is powered on and going up and down while in stand by. However, if it doesn’t it may still be because the driver continuously flips polarities to accomodate both LEDs. In such case I would go for a simple circuit with two LEDs at opposing polarities and something like 560 Ω resistor. They will be pretty dim, but enough to tell if the driver works as expected.

author: Stray Electron
author_rank: Super Contributor
timestamp: February 06, 2024, 11:22:31 pm
content: Quote from: BerMM on February 06, 2024, 06:24:37 am"pull-up" simply means that the other end of the resister is connected the +V rail. Pull-down would mean that it's connected to negative side of the power source (often referred to as "ground").