I still don't understand one thing. Does b actually depend on the rest of the elements? (Resistors, capacitors, etc?) I'm still confused with that Hef.

In theory exercises, the exercise would always give me b and all the characteristics of the circuit (resistances, capacities, sources, etc) and I just had to find everything else (voltages and currents).

But in this case, I'm still not sure how to do this.

 

hi,
For a saturated collector its usual to assume Beta of 10 or 20.
E

 

Or even a voltage divider w/ 2 resistors to give the base 0.8V with little Ib.

 

For simple switching circuits β is not considered. You simply want the base to turn on the CE.

 

I still don't understand one thing. Does b actually depend on the rest of the elements? (Resistors, capacitors, etc?) I'm still confused with that Hef.

Beta depends on a number of things: temperature, collector current, collector-emitter voltage.

When you're using a transistor in saturation mode, a beta of 20 is used for BC547. For 2N small signal transistors, e.g. 2N3904, 10 is used. For power transistors, it can be lower; unless they're darlingtons. For darlingtons, e.g. TIP120, a beta of 250 is used.

 

At least I did not burn it. I used 1k for the base and 100 for the led. I grounded the emitter.

 

hi,
What type of LED is that.? do you have a datasheet.?
E

 

hi,
What type of LED is that.? do you have a datasheet.?
E

No, but the site where I bought it, it said that it operates from 2 - 3.6 volts and 20mA.

 

Run it at 15mA. for long life,

 

No, but the site where I bought it, it said that it operates from 2 - 3.6 volts and 20mA.

20mA is likely the maximum DC current. That doesn't mean you need to operate it at that current. For an indicator that isn't exposed to bright light, a milliamp or two is usually sufficient.

It's also possible to drive most LEDs at a much higher peak current, for example when multiplexing.

These Kingbright LEDs, have a peak current of over 100mA:

In the notes, it states the conditions for using the peak current.

EDIT: Specs for an old HP LED that had a maximum peak current of 1A:

 

Typical for 5V is 220Ω but I use 560Ω for up to 9V. Different colors have different parameters also if you want to split hairs. For the "standard" 5mm LEDs.
http://www.gizmology.net/LEDs.htm

 

When using a transistor as a switch, a Beta value of 10-20 is typically used to insure it is fully on (saturated).

 

Re - #1 : Shows (somewhat) also the determinedness of the hFE


+ the ß varies item to item and also depends on many e.g. the junction temperature

• http://pdf.datasheetcatalog.com/datasheet2/d/0kq9ua20u27ka13c66efh960khpy.pdf
• http://pdf.datasheetcatalog.com/datasheet/philips/BC546_547_3.pdf
• http://pdf.datasheetcatalog.com/datasheet2/4/091ajoxgqj7kllx0jixohlhq11wy.pdf
• http://pdf.datasheetcatalog.com/datasheet/motorola/BC546B.pdf
• http://pdf.datasheetcatalog.com/datasheet_pdf/general-semiconductor/BC546A_to_BC549C.pdf
• https://www.mccsemi.com/pdf/Products/BC546-BC548C(TO-92).pdf
• https://www.taiwansemi.com/products/datasheet/BC546A series_C1703.pdf

 

So in other words, you are saying I don't need to know b and the transistor currents?
...

Not exactly. In most designs you can reduce the effect of β on the final circuit by choosing a voltage divider biasing circuit. The stable DC bias current means the device will perform as an AC amplifier over a wide dynamic range and the actual value is less relevant because you are looking at the small signal gain (hfe) rather than the large signal gain (hFE or β).

 

Hi,

There are a lot of transistor circuits used with LEDs.
The two most common are the:
1. Switch
2. Constant current

When used as a switch, the transistor just has to turn on and off. For this circuit you ensure that the base current is around one-tenth of the collector current. So if you have an LED that takes 20ma in the collector (with current limiting resistor) then you need at least 2ma base current.
If the LED takes 200ma in the collector, then you need 20ma base current.
The Switch circuit is used mostly when there is a constant supply voltage like 5v or 10v.

When the supply voltage can vary widely, like from 5v to 15v, then a constant current circuit is used, and this can also be a Switch too. There are a number of ways that the constant current circuit can be built so it is a good idea to look some up or ask here.

In any of these circuits the circuit is designed such that it does not depend highly on the Beta of the transistor. That is because the Beta can vary widely from device to device and with temperature and with supply voltage.
So it is not that the Beta does not matter, it is that the circuit is designed so that the Beta does not matter as much, but it must be designed properly in order to achieve this design goal.

 

Thank you guys I will search more about this.

 

Would you be able to use a Zener diode in that circuit?

 

Would you be able to use a Zener diode in that circuit?

You can always add a component, but for what purpose?