Help needed with LED's from someone with a very simplistic knowledge of electronics.

[1stFalloutBoy]

Help needed ppwith LED's from somone with a very simplistic knowledge of electronics.

I am a tech support person - not someone with a lot of knowledge when it comes to electronics, I have been asked to hook up LED indicators that monitor the Raid Controller on a companies LSI-9361-8i and I don't want to blow it up as that one card is worth $1600, I have found some information regarding the headers on the card and asked for info from broadcom who do the support for the cards, there response was as follows.

The header current is limited to 25 mA at 3.3V.
Keep in mind, however, that it was designed to drive an LED with a forward voltage drop which is typically around 2V, therefore pulling approximately 10 mA.

Now there are four headers in question 3 two pin headers and one 16 pin connector with 2 pins per led indicating drive missing/unconfigured/bad.

I don't know if this response is telling me that the header current is 25mA at 3.3v for all the headers combined i.e 25ma/11 LEDs = 2.27ma per LED which seems awfully low, or if it is 25mA per led. and as for the statement.

"Keep in mind, however, that it was designed to drive an LED with a forward voltage drop which is typically around 2V, therefore pulling approximately 10 mA" - I have no idea what this means.

Can someone help me out please?

 

[KlausST]

Re: Help needed ppwith LED's from somone with a very simplistic knowledge of electron

Hi,

The header current is limited to 25 mA at 3.3V.
Keep in mind, however, that it was designed to drive an LED with a forward voltage drop which is typically around 2V, therefore pulling approximately 10 mA.

This sounds as if there is an internal 130 Ohms resistor:
* if you short the output, then there is the whole 3.3V across the resustor: I = 3.3V / 130 Ohms = about 25mA
* if you connct the LED with it's 2.0V, then there remains 3.3V - 2.0V = 1.3V across the resistor. 1.3V / 130 Ohms = 10mA.
--> you don't need an extra current limiting resistor, just the bare LED.
There is about no risk to fry the LED output. But use ESD protection like an ESD wrist band.

I'm sure this is true for each single LED, not the common LED current.

Use LEDs with 2.0V: red, green, yellow, amber. (Vf: 1.6...2.3V, usually they have around 10..30mA typical forward current)
(but not: blue, white)
In doubt use standard LEDs, like 3mm or 5mm diameter, they may be in a case for easier mounting.
Mind that you need to take care about LED direction.
Usually the longer LED connection is anode, connect it to "+"
The shorter wire is cathode, "-".

Klaus

 

[Audioguru]

Re: Help needed ppwith LED's from somone with a very simplistic knowledge of electron

Modern bright green LEDs use the same chemistry and have the same higher voltage as blue and white LEDs. Old dim green LEDs had a forward voltage of about 2.2V.