SATA Activity LED help~?

Thread Starter

hekokimushi

Joined May 3, 2012
6
hi, had left electronics for more than 10 years. now doing some personal projects with the computer. looked up SATA's definition, where the 15Pin Power connector, pin 11 does provide an active low output that can be used for activity indication.

as i am a bit out of designing circuit, would like to ask some assistance. please refer to the attached for SATA's specfication of Pin 11's electrical characteristic. i am not exactly sure what that is telling me, IHAct, is that the Max 300uA that the harddrive can sink, or capabale of sinking?

i intend to use a PNP transistor, so whenever Pin11 is active (driven low), the LED will come on with just a base resistor, (do i need to pull it up to 5V?) a Collector resistor on the LED.

thanks for your help. cheers
h
 

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crutschow

Joined Mar 14, 2008
23,159
Yes, it would appear that the maximum current you should draw is 300μA. That means for a 3.3V supply, the smallest base resistor to the PNP should be about 8.6K.

Note: You can't use 5V for your supply, the PNP emitter must be connected to no more than 3.3V. You don't need a pull-up resistor but you do need a resistor in series with the LED to limit its current.
 

Thread Starter

hekokimushi

Joined May 3, 2012
6
thank you crutchow.

Would you recommend any quad SOIP PNP transistor that would be fully switched on at this little current? and it only states Max 300uA, so it could be less? would a FET be better suited?

thanks a bunch
 

crutschow

Joined Mar 14, 2008
23,159
You typically use a forced beta of 10 for a transistor switch so that would give a maximum collector current of 3mA. To get the 20mA for a typical LED you would need to add an NPN stage driven by the PNP.

A PMOSFET might be better, since you need only one transistor, but it would need to be a logic-level type that fully turns on at 3.3V such as this one.
 

Thread Starter

hekokimushi

Joined May 3, 2012
6
please correct me if i am mistaken,


p11 (active low), pull up to 3V3, would this be good enough for that output to drive the PMOSFET?
 

Thread Starter

hekokimushi

Joined May 3, 2012
6
3V3 - 20k ohm --+
p11 --------------+---- gate

5V - source

drain - Rlimit - LED - GND

seem reasonable? thanks a lot. :D
 
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