Why don't I need to use resistors with this project?

Other Hardware General Electronics
1

I'm making this device and the tutorial says resistors aren't needed because they're using a 3v battery. But they don't specify what led's to use. Don't different led's have different voltages and currents? Don't those factors change whether you need to use resistors or not with 3v?

All the white led's I found on digikey have 3.2V forwards voltage and 20ma current test. When I put that in a resistor calculator, it says 3v isn't even enough to power that. I don't yet know how to calculate these things, but it makes sense to me that you can't power something that needs 3.2V with a 3v battery. I found some that weren't white but were under 3V forward voltage, but even with those the calculator said I need to use resistors.

Why does the tutorial say you don't need to use resistors without specifying the led's? Are there some "standard" leds they are referring to? What kind of led specs should I look for that don't need resistors with a 3V battery?

2

3V coin cell like that have a lot of internal resistance (hundreds of ohms or more), which is enough to limit current through the LED nicely.

3

but it makes sense to me that you can't power something that needs 3.2V with a 3v battery

It is not as simple as that. The 3.2V is only the forward voltage drop at the specified current, normally 20mA. There is a small range of voltages under the specified one where current will flow but not 20mA so you will still be able to see something.

Using the internal resistance of a coin cell is an old trick but in my opinion should never be used in a beginners tutorial because you need to know so much more about how it works and how to apply the technique than you are capable at that stage. Hence your confusion.

The bottom line is you always need something to limit the current in an LED, resistors are simple, but using a hidden resistor is a bad thing to do especially if you are too stupid to explain why it works. There are a lot of rubbish tutorials out there.

Good tutorials should give you transferable knowledge not one off hidden tricks.

4

Ok, after reading the comments on here, I decided to learn a bit more of the basics, ohm's law etc. There are a few things that I still need clarification on.

Internal Resistance of Batteries
Am I correct in saying that the internal resistance of batteries change depending on your circuit? Originally I thought you meant that coin cells have a consistent internal resistance, for example something like 10ohms that I could just find on the package and plug into ohms law. But the tutorials I'm looking at such as this one show that you should find the internal resistance when your circuit is put together. And this battery data sheet shows that load resistance (which I believe is the same as internal resistance) changes with different voltages and capacities.

So just to clarify, internal resistance isn't consistent? And it's best to figure it out as I make my circuit? I'd like to know what resistors to order before I make this, but if you need to figure out the internal resistance as you make the circuit, then it seems I can't know beforehand what resistors to order. Is that correct? Or is there a way to figure it out from those graphs on the data sheet?

Forward Voltage
In the data sheet for the led on page 13, it shows that the forward voltage at about 5mA current is about 2.9V. So if I did ohms law which shows that it would work with a 3V battery and a 20ohm resistor. The graph doesn't show the forward voltage lower than that. Is there a way to figure out the forward voltage/current lower than that?

Pulling it together
In the end, am I supposed to use a resistor that, combined with the internal resistance, gives me the sum of the resistance needed? Let's say I'm using a 3v battery and going with the forward voltage and current of 2.9V and 5mA, which requires a 20ohms. Then should I find the internal resistance and subtract that from 20 to get the resistor I should use? But this confuses me again, since wouldn't changing the resistor then, in turn, change the internal resistance since the circuit is changed again, and the load resistance changes depending on the circuit?

Am I making this more complicated than it has to be?

5

dangerousdashie:
Ok, after reading the comments on here, I decided to learn a bit more of the basics, ohm's law etc. There are a few things that I still need clarification on.

Internal Resistance of Batteries
Am I correct in saying that the internal resistance of batteries change depending on your circuit? Originally I thought you meant that coin cells have a consistent internal resistance

No, internal resistance depends on temperature, charging state, history, many things.

So just to clarify, internal resistance isn't consistent? And it's best to figure it out as I make my circuit? I'd like to know what resistors to order before I make this, but if you need to figure out the internal resistance as you make the circuit, then it seems I can't know beforehand what resistors to order. Is that correct? Or is there a way to figure it out from those graphs on the data sheet?

All you need to do is not drain more current from the battery than the datasheet says, and expect the voltage will fall a bit under max rated load

Forward Voltage
In the data sheet for the led on page 13, it shows that the forward voltage at about 5mA current is about 2.9V. So if I did ohms law which shows that it would work with a 3V battery and a 20ohm resistor. The graph doesn't show the forward voltage lower than that. Is there a way to figure out the forward voltage/current lower than that?

The diode equation applies fairly well to LEDs, ie exponential dependence of current on forward voltage untill the series resistance of the device starts to have effect. Roughly the current will double for each 26mV increase in
voltage, per diode (if a series string). Note the "roughly", device temperature has an effect, and that depends on power dissipated, ie on current.

Pulling it together
In the end, am I supposed to use a resistor that, combined with the internal resistance, gives me the sum of the resistance needed?

You are overthinking this

Am I making this more complicated than it has to be?

As I said, over thinking - you are probably overloading the cell anyway, its a hack to start with.

6

The graph doesn't show the forward voltage lower than that. Is there a way to figure out the forward voltage/current lower than that?

No.
Data sheets only show what the manufacturer will guarantee, for points off the graph different devices could behaved very differently.

But this confuses me again, since wouldn't changing the resistor then, in turn, change the internal resistance since the circuit is changed again, and the load resistance changes depending on the circuit?

The internal resistance is not dependant on your circuit.