# How to not-fry a component

1. Dec 4, 2013

### _Bd_

So I am looking at my spec sheets, I am running most of my boards on 5V (Arduino)

There are however 5 LEDs that say can have a maximum FV of 1.5V, so Im pretty sure they will fry if I run the 5V through them, there are also 5 Photodiodes that have a 1.3 maximum FV, so they will fry as well.

What I was thinking is connecting them all to my VCC, but to add a resistor right before the actual component, which would create a voltage drop. (I am not sure if this would work though)

So I am thinking of doing this:
http://img12.imageshack.us/img12/1894/una0.png [Broken]

Now I am not sure if this will work, I tried setting that up in MultiSim, but it didnt show any voltage drop across the resistor, it kept saying the Voltage going to the PD and LED were still 5V which I really want to avoid.

I calculated this using the following:

Knowing the current and voltage for the PD

10 mA, and 5V, I want a drop of 3.7V, so V=iR
so that gives a resistor of 370 Ohms for the Photodiode, and using the same logic a Resistor of 175 Ohms for the LED.

Will this work? or will my components still fry?

Thank you for all your help

Last edited by a moderator: May 6, 2017
2. Dec 4, 2013

### AlephZero

That's a sensible way to do it, and you seem to have calculated the resistor values correctly.

I don't know why it didn't "work" in Multisim. Did you have the diodes the wrong way round in your input? Or are you misinterpreting the output somehow?

Cut the problem down to the minimum, and simulate just a resistor, a diode, and a 5V voltage source.

3. Dec 4, 2013

### _Bd_

yeah well, In multisim I just did the resistor, the diode and the source, and where I think the problem might be is that I connected the other lead to ground? since I didnt know what to put there (I didnt want to completely re-draw my circuit, which I just showed in that picture as a "black box")

I'll try it again and upload some screenshots, but it was pretty much that same picture up there but with ground instead of the black boxes.

Or maybe it was that I listed it as VCC? (I am not sure, but it says vcc are TTL sources, i dont know if maybe a CMOS source? or what type of source should I use? or does it even matter, just for the multisim schematic?)

Anyways, Thanks, ill try it like so and hopefully i wont fry my components

4. Dec 4, 2013

### davenn

it would depend on what the connection in the "black box" is that the LED is being connected to
ie. is it a current sink -- that point is grounded when an operation occurs therefore allowing current to flow from 5V through the resistor and LED to ground
or is it a current source ?, which will require the LED to be reversed and going to an external ground via the resistor

Only you can tell us what is occurring in your "black box"

Dave