PC 3.5 FoC Voltage Question: 3.3V or 7.4V?

[Bark]

I'm spec'ing out my first blade, and I am confused on the PC 3.5 manual pages 27-28: Simple Flash on Clash / FoC effect

I'll have a 7.4V 3400mAh battery. I want to get a Tri-Rebel with Blue-Blue-White. Blue-Blue will be wired in series on the main PC LED pads. I want the White to only light for FoC. Looking into the manual, I understand the following (and please correct me if needed)

-No experience with 2512 2W SMD resistors? Then bridge this on the board and use an external resistor
-Connect Board Power Supply + (flip side of board) to FoC Die+
-Connect FoC Die- to resistor
-Connect resistor to SMD footprint pad (flip side of board)

It seems to me that the voltage will be 7.4 due to the battery, but the text says something about, "The FoC pad is only a 3.3V remote control pad..." Then, it gives an example of 7.4 when doing the resistance calculation. I'm just not understanding the wording.

Question: When calculating the external resistor needed for the simple FoC LED die, do I use 7.4 or 3.3 for the source voltage?

[cvsickle]

The FOC pad that the manual is referring to is the FoC1 or FoC2 remote pad. These are just responsible for sending the FoC signal, which is a 3.3v signal. In your case, this signal is being sent to the onboard PEx, which will be used to apply the battery voltage to the LED. You are correct in thinking the voltage from the bridged pad will be 7.4v because that is your battery pack's voltage.

[Bark]

Thanks!

[jamesfuss]

When your talking about the 2512 2w smd resistor, are you meaning you put that on the pc along with an external resistor or do you mean if your not going to use the 2512 2w smd resistor you just bridge that part of the board and instead use and external resistor? Sorry new to this just trying to get my head around it

[Forgetful Jedi Knight]

When your talking about the 2512 2w smd resistor, are you meaning you put that on the pc along with an external resistor or do you mean if your not going to use the 2512 2w smd resistor you just bridge that part of the board and instead use and external resistor? Sorry new to this just trying to get my head around it

Typically you just bridge the pads, and use an external resistor, UNLESS the resistor you need is 2W or less. Excess heat is bad for the board.

[jamesfuss]

Oh right got it, if that's the case. If you are useing White for your FoC would the 2512 be fine or would you need an external resistor? (In the process of learning ohms law)

[snilam]

I always bridge then use an external resistor just because I don't want to deal with some SMD. those things are tiny!

[jamesfuss]

Ah fair enough, do you get the same result for using either?

[Forgetful Jedi Knight]

Ah fair enough, do you get the same result for using either?

If if they are the same value, then the result would be the same. I also use external resistors to keep heat away from the board.

[Forgetful Jedi Knight]

Oh right got it, if that's the case. If you are useing White for your FoC would the 2512 be fine or would you need an external resistor? (In the process of learning ohms law)

it depends, if you are using 7.4v, you would likely need a huge external resistor.

Also, there is another recent thread that discusses resistors. You might want to read through it.