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# Do integrated resistors in a LED make you work with an specific power source? Answered

Hello, everyone! Totally new here.Well, I'm going to buy a pair of 12 volt "eagle eye" LED for a medicine experiment, and I planned to connect them to either a rectangular 9 volt battery or 3 cylindrical 3.7 volt batteries. But now, these LEDs are meant to be connected to a 12 volt car battery. Does the difference of the amperes between each power source (car battery, rectangular and cylindrical) conflict with the LED's functionality, or does the integrated resistor allow me to use any power source without trouble?

Here's a picture of said PCB with the integrated resistor, for example (found on the internet), it just reads "300" and I don't know what that means. Sorry if all these questions and doubts are really silly, but again I'm a medicine student trying to make this work and electricity is a really complex world. Thanks in advance, wise people!!!

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Does the resistor placed in series with an LED make you work with a specific power source?

I think the short answer to this question is, "yes".

However it will take a longer answer for me to explain why I believe this to be the case.

Moreover, if this light source is required for some kind of repeatable experiment, you might want to consider the fact that the actual brightness, the amount of light energy emitted, per unit time, depends on other parts of the circuit apart from the LED, including the battery voltage, and the size of that current-limiting resistor.

Or at least that is true for simple LED circuits, like the one in the picture you have attached. More complicated circuits can keep LED brightness constant, erm, "regulated" is the technical word for it, over a wide range of battery voltage.

The brightness of an LED, light energy emitted per unit time, is roughly proportional to the amount of current (measured in amperes, or milliamperes) flowing through it. Also it is typically assumed that constant current through an LED will give constant light output, and this is the goal of a, "constant current", type regulator, when that is used for driving LEDs.

Also the amount of current that flows through the LED is basically a design choice. The designer of the circuit powering the LED, he or she can choose the amount of current, I, that flows through the LED, as long as I is somewhere in the range:

0 < I < Imax

Imax is an upper limit. More current than that makes the LED burn out, and destroys it.

Also, the person who designed the circuit in the picture you attached, used some very simple math for this, specifically the formula,

I = (Vs -Vd)/R

where I is the current through the LED, Vs is the voltage of the voltage source (e.g. 12 volts), Vd is the (approximate) characteristic forward voltage drop across the LED, and R is the size of the resistor, in ohms.

This is the same formula used by the well known, online "LED calculator", found here:

http://led.linear1.org/1led.wiz/

I claim that web site is well-known, and dang near as old as the hills, or maybe the internet itself. Although, if you've never seen that site before, and none of your friends have seen it... then I guess there's no way for me to prove that is an old and venerable web site. I suppose archive.org has seen it, but again, what does that prove?

By the way, the "300" printed on the resistor, in the picture you attached, means 30*10^0 = 30 ohms, if I am reading it correctly, according to the usual resistor color code,
https://en.wikipedia.org/wiki/Electronic_color_cod...

I am guessing that is a 30 ohm resistor. If you have a multimeter in your possession, and if you know how to use it,

https://www.sciencebuddies.org/science-fair-projec...

then you can measure the resistance of that resistor directly, or indirectly, by measuring the voltage across it, the current through it, etc.

Getting back to what I was saying about current and brightness, I want you to consider the simple formula, presented earlier,

I = (Vs -Vd)/R

and think about how I depends on Vs, particularly in the circumstance where Vs is supplied by a battery, and Vs is not truly constant. Rather Vs decreases with time, as the battery loses energy. Thus current I, and thus brightness of the LED, are decreasing with time as Vs decreases.

This is something to think about, if your circuit has to supply constant light intensity, over an amount of time long enough for the battery voltage to change.

By the way, I think the solution to this problem of declining battery voltage is to use a regulator of some kind. Either use a, "constant voltage", regulator to keep Vs constant, e.g. exactly, constantly, 12.0 volts. Or use a, "constant current", regulator, to watch the current going to the LED, and regulate that directly.

I have some vague ideas for how to do this. I mean, there are inexpensive, swiching-type, power converters, that can supply constant voltage, or constant current. You can find these things on eBay, for a few dollars per unit, at the time of this writing. I actually have some of these little gizmos, and I think in the past I have uploaded, unto this forum, pictures and explanation of how to use them for driving LEDs.

Although, I don't know if your experiment actually, like, requires, constant and repeatable, levels of LED brightness.

Anyway, if you want to know more about these switching regulator gizmos, please reply, and I will try to find that thread where I was writing about them previously.