Hi All, I was hoping for a review of this circuit before I build it. I have a sata controller (3Ware 9500-12) with led connactors for each drive on it but the manufacture says that the board will be damaged if the leds share a common ground. I designed this circuit to isolate the board from the common ground for the leds. The headers on the board have 3.3V and are ment to drive a led so I don't think I need a resistor on the input. I chose the LTV-846 instead of a 4N2* because this is a 12 drive controller and I want to make the board as small as possible. This actually only 1 of 3 circuits I will have to build. Any help would be appreciated. Thanks Elliyanna
I would connect the LED + Resistor into the output transistor's collector, not emitter, and connect the emitter (the pin with the arrow) straight to the ground.
Doesn't seem to make any difference with opto-isolators, the base voltage isn't referred to anything. I'd lower the led current, maybe 1k resistor on any colour, unless you're trying to light up the whole room.
Thanks all for taking the time to look and reply. cpemma, I will up the resistance. All I am trying to drive is activity leds in drive cages (Supermicro 5in3 M35ST1). Actually I wouldn't have to do this at all except 3Ware in their wonderful wisdom decided to design the led header on their card without a common cathode (ground). I suppose it is so you have to buy their drive cages as all of the other cages I have seen have a common cathode on the drive leds. Again thanks for the review
That is because the - is what is switched, the same way as in motherboard LEDs and if only one is on and you have 4 leds hooked up, it will try to sink 80 mA and it can probably only handle 20-30. It is a lot easier for things to sink current than source it, so that is why they use that. The leds in most things are set up like that.
Conkbot, Actually this is not the case. The drive cages I have now are supermico 5 drives in 3 bays. It has 6 connections for activity leds. 1 for a common ground and 5 other connectors 1 for each drive. This means that for each path to ground you are sinking 20 mA on the anode side. The leds in a cage are wired so you can monitor activity of each drive not the total activity like you see in a case/motherboard activity led. Only the leds for the drives accessing data should be on. This is the standard wiring method for hard drive cages. Now what 3Ware did is give an indepent Anode/Cathode connection for each activity led and state that a common ground/cathode can damage the controller. Their own brand of cages have 2 connections for each led. So what they have done is if you want to monitor individual activity you have to buy their cages (which are more expensive then other brands). This is marketing you want this feature you buy our product nothing more. In my case I am building an adapter and using the cheaper cages.
