Putting capacitor on HDD LED help

[da na]

Adding a capacitor to a hard drive activity light so it's more of a slow fade than a rapid blink on/off. Using a 1500uf capacitor. I'm getting the power straight from the motherboard HDD LED pins.

Should I add the capacitor in series or parallel to the LED? (Across the LED's pins or before it in the circuit?) It's an electrolytic so I can't really afford to get it wrong.

[Kilrah]

Parallel, otherwise it would do the opposite and the LED would only briefly flash once the first time it gets power and then stay off.

[AbydosOne]

I wish that motherboard status LED output electrical characteristics were published somewhere, because I have several open questions about how they're supposed to work...

 

I'm going to preface this with "I'm pretty sure they're configured for constant current and not constant voltage so you might run the risk of burning out the driving stage or never lighting your LED."

 

23 minutes ago, Mel0n. said:

Should I add the capacitor in series or parallel to the LED?

Parallel. Preferably with a resistor in series with the cap to make it an RC circuit (since the LED isn't technically a linear device). You'll need to do some math to figure out the values for R and C compared to the LED voltage/current curves.

 

23 minutes ago, Mel0n. said:

Using a 1500uf capacitor.

That might be (way) too big.

[mariushm]

An 1500 uF capacitor is way too high of a value... and it will not work like you think it will.  You won't get fade effect. 

 

The chipset will probably limit the current to around 5mA, maybe even less, so it would take a lot of time for the capacitor to even charge up (maybe up to a second or two worth of continuous led on)... and then unless you limit the current between the capacitor and led, the led will simply stop lightning once the voltage on the capacitor drops below the led's forward voltage (for red leds, that's around 1.7v..1.9v)

So basically you'd have no blinking led while the capacitor charges, then once the capacitor charges enough to get the voltage above the led's forward voltage threshold, the led will light up and pull a lot of energy from capacitor unless it's current limited through a resistor. So without a resistor to limit current, it will basically be as if there's no capacitor in circuit once capacitor fills up.