MrChips
I don't have so many Resistors values, I should buy some potentiometers to try, but yes I have a larger value R to turn it on.

If you have some 680Ω resistors, just keep adding another in series.

 

I think it's starting to make sense although consfusing, and let's asume I didn't have a circuit, led datasheet, and a meter, just a exam paper with a schematic, a pencil and a calculator, there we will need an ideal led to solve it right?

Just stating your assumptions should be sufficient. When I was in school, we used 1.4V for LEDs; now I use 2V if color isn't specified.

 

Okay guys I did again the circuit with different resistors values, now R1 = 680omhs and R2 = 2.2Komhs, led remains the same warm white, without the Led multimeter measured 2.76V in the resistor divider, when plugged the Led into bread board the voltage sag to 2.53V, this is confuse tbh but yeah that's how the Vfw Led works.

When you say an ideal Led you mean a perfect Led on paper with a exact Vfw at what it will turn on?

I was reading the Thevenin's theorem, but haven't made a calculation yet.

WBanh
I understand the calculations you did there now, so according to the measures my meter did when led is connected, 3.6V-2.53V = 1.07V, so 1.07V across R1, this will allow a current throught R1 of 1.07V/680ohms = 1.573mA, since V across Led is 2.53v for R2 is the same 2.53V/2.2Kohms = 1.15mA, 1.573mA-1.15mA = 0.423mA left and that goes to the led.

Correct.

I think it's starting to make sense although consfusing, and let's asume I didn't have a circuit, led datasheet, and a meter, just a exam paper with a schematic, a pencil and a calculator, there we will need an ideal led to solve it right?

Then you need to make assumptions. The basic assumptions back in the day were the following:

These are not engraved in stone and many people used different values. These are the ones I typically used.

IR: 1.5 V
Green: 1.8 V
Red: 2.0 V
Yellow/Orange 2.2 V

There were no blue, UV, or white LEDs back then, but when they came in (blue first), the rule was 3.6 V.

In practice, it didn't matter much because you designed a circuit specifically so that it didn't matter much.

It was also generally assumed that an LED was supposed to have somewhere between 10 mA and 20 mA of current (this is for indicator LEDs, which was the dominant use back then).

Today, the range of currents go from sub-milliamp to tens of amps, with the voltages spanning a much broader range accordingly. Plus, there are a lot more materials used to make the same color, so those add to the spread. More than ever, it's important to refer to the data sheet for the LED you are using. But, when you are just wanting to make an indicator LED in a circuit, the above rule of thumb should still work well because it is pretty easy to find LEDs that are a close match to them.

 

Man this is so cool, I had not idea that in the beging there weren't white leds, are you talking about 80's, 90's? I just read 3 years ago to use 2.2V for red and orange and 3V for blue and white.

So guys when solving pure schematics the results are just an aproximation, compared to when you build the circuit as there could be some variations due components like resistors, capacitors and inductors I see they are sold with tolerances normally from 10%-30%, and 1%-5% are generally more expensive?

So okay it is better to buy components from trustable sites as digikey, arrow, mouser and lcsc, because they provide the part number, plus they save you time because they also provide schematics in the product page.

I was watching that transistors and mosfets do something similar, but transistors are controlled with current at the Base and mosfets with voltage at the Gate, can you give me some examples when is better to use each one?

 

So guys when solving pure schematics the results are just an aproximation, compared to when you build the circuit as there could be some variations due components like resistors, capacitors and inductors

When we design circuits, we take tolerances into account. Normally I'd use 5% tolerance resistors, but will use 1% when necessary. I've also cherry picked parts, but that's not a good design philosophy.

In your LED example, we'd design for a nominal current (say 10mA) and not care if it was 9.5-10.5mA.

If you take your first example where a 9V supply, and an LED being turned on by a transistor with a 1k current limiting resistor.

If we assume a forward voltage of 2V and a saturation voltage of 0, that would give 7mA through the LED. We don't really care if the 9V (perhaps from a battery) was slightly higher and the saturation voltage was 0.1V. The difference in LED current wouldn't make a noticeable difference in brightness. The resistor could be 1.05k or 0.95k and not make a difference in LED brightness.

So okay it is better to buy components from trustable sites as digikey, arrow, mouser and lcsc, because they provide the part number, plus they save you time because they also provide schematics in the product page.

For people not able to troubleshoot problems caused by counterfeit parts, they should always buy from reputable sources (i.e. not Amazon, eBay, AliExpress, and the like). The same goes for people who don't want to waste their time on defective/counterfeit parts.

If you have nothing better to do with your time and want to save money, go ahead and buy parts from untrusted sources.

I have thousands of dollars worth of components I bought from eBay before counterfeiting became such a huge problem. Now there aren't many components that I'd buy on eBay and a large list of things that I won't buy if they ship from China.

I was watching that transistors and mosfets do something similar, but transistors are controlled with current at the Base and mosfets with voltage at the Gate, can you give me some examples when is better to use each one?

MOSFETs have replaced BJTs in most designs. They're now fast enough, cheap enough, and can handle sufficient power. That wasn't always the case. Try looking for a reasonably priced P channel MOSFET in through hole. For N channel, there's 2N7000 for about a dime each in quantity 100. Try to find a P channel for that price.

Going back to your LED example. When using a BJT, you need to provide sufficient base current to saturate the device. That requires some calculations. If LED current is 7mA, a general purpose transistor like 2N3904 would require a base current of 0.7mA Now you need to know what voltage will be applied to the base resistor to calculate its value. If you were using 9V, then the base resistor would be:
\( R_B = \frac{V_B}{I_B} = \frac{9V-0.7V}{0.7ma} = 12.4k\Omega \)
The nearest 5% value is 12k.

If you used a 2N7000, you don't need a gate resistor. But you might want to use one from the gate to ground to make sure the MOSFET was off when no voltage was applied to the gate.

BJTs aren't very susceptible to ESD, but most MOSFETs are. Some discrete MOSFETs have ESD protection, but the vast majority don't. If you're lucky and you zap a MOSFET, it'll die outright. If you're unlucky, it may exhibit out-of-spec behavior or die prematurely.

I know how to handle MOSFETs, but I've still damaged several 2N7000 in the past 5 years or so. None died outright, but all exhibited high leakage current.

 

Man this is so cool, I had not idea that in the beging there weren't white leds, are you talking about 80's, 90's? I just read 3 years ago to use 2.2V for red and orange and 3V for blue and white.

Blue LEDs were invented in the early 1990s, but took quite a while to commercialize and get them to price points that were economically viable for many types of applications. The inventor got the 2014 Nobel Prize for their work.

So guys when solving pure schematics the results are just an aproximation, compared to when you build the circuit as there could be some variations due components like resistors, capacitors and inductors I see they are sold with tolerances normally from 10%-30%, and 1%-5% are generally more expensive?

Almost all circuit analysis, whether by hand or by a simulator, involves approximations. Real devices are very complex. Even a simple resistor not only has a tolerance to its resistance, but its resistance changes with temperature and other factors. It is also usually modeled as a device that has a constant resistance as a function of voltage and current, but this is not the case. First off, it will heat up, which changes the temperature, which changes the resistance. But, even if it is held at constant temperature, the resistance of virtually all materials is not such that current is strictly proportional to voltage -- they are non-ohmic to some degree.

Then there are things like the noise voltage produced by the component, which can be very complicated.

In most situations, we can ignore these factors. If we can't, then we need to model them and take them into account in sufficient detail for our purposes. Knowing what to take into account, how to model it, and how to determine that the model is good enough is the full-time job of some engineers. But for most, it's good enough to just gain an appreciation for the kinds of things that aren't being taken into account and being able to identify when some of them need to be considered.

As an example, the voltage drop across the base-emitter junction of a BJT transistor is often taken to be fixed at some voltage near 650 mV (usually either 0.6 V or 0.7 V). That's good enough for many designs, because the circuit was designed in such a way that any voltage between, say, 500 mV and 800 mV would result in deviations in behavior that are well within acceptable limits. But other circuits are not only sensitive to these differences, their very purpose relies on them being there. For instance, the Vbe of a transistor at a given current depends on temperature, so we can use that relationship to construct a thermometer based on that reliance. In that design, we don't dare ignore these effects.

So okay it is better to buy components from trustable sites as digikey, arrow, mouser and lcsc, because they provide the part number, plus they save you time because they also provide schematics in the product page.

You definitely want to buy from reputable sights if you need to have parts that you can have confidence in. For a lot of hobby work, this isn't the case. So if you can get components at 1/10 the cost from Bob's Electronic Parts and All-night Video Rental, then if half of them are bad, you are money ahead. You've wasted time, effort, and frustration in figuring out which parts are good and which aren't, but if that is worth the cost-savings to you, it's a smart decision. For a lot of us, this was the mode we were in at the beginning. We were poor students and so scrounging parts, including resistors, off of circuit boards from equipment found in a dumpster was worth it because we couldn't afford to buy real parts. At some point, many of us reached a point where our time was more valuable to us than the savings, and so we paid DigiKey's and Mouser's outrageous prices without any hesitation because we knew they were virtually guaranteed to have what we wanted in stock, to be willing to sell it in unit quantities, and be willing to ship it the same day. But I still know people that scrounge from scrap circuit boards, even after 50 or 60 years in electronics.

I was watching that transistors and mosfets do something similar, but transistors are controlled with current at the Base and mosfets with voltage at the Gate, can you give me some examples when is better to use each one?

BJTs are most properly considered voltage controlled devices. But for a huge fraction of designs, it is reasonable to treat them as current-controlled devices. While you can always perform the analysis using a voltage-controlled model, there are lots of circuits that simply can't be analyzed properly without treating it as voltage-controlled.

As for BJTs versus MOSFETs, lots of circuits can use either, while some applications strongly favor one of the other. In many cases, the factors that determine this are second-order parameters, such as input capacitance, frequency response, noise, etc..