Transistor questions

[Richardcavell]

Hi, everyone. Please look at my schematic here.

1. How fast can that BC558 transistor switch on and off?

2. With the BC558 transistor, is the current that goes through E-C always a fixed multiple of the E-B current?

3. If 'yes' to question 2, does that mean that if I removed the resistors from the circuit, the LED would immediately burn out and my batteries would be drained, by the E-C current rapidly increasing?

4. (I know I've asked this before but I'm not understanding it) What does that 100k ohm resistor do? The circuit appears to work perfectly with that part of the circuit removed entirely. Does the base somehow 'store' electrical potential that needs to be drained? Is there a buzzword or a wikipedia article that I should know about?

 

[MikeMl]

When you have a running thread with lots of replies, don't start a new one on the same topic!

 

[audioguru]

You didn't ask this question about switching speed in your other post. Why not? Why make this new thread?

To find out the spec's of a transistor, Mosfet or IC you simply look at its datasheet from a manufacturer.
I show the datasheet from Fairchild where the Gain Bandwidth Product (the frequency where the AC gain has dropped to only 1) is typically 150MHz.

 

[EternityForest]

Without the resistors the LED and the transistor too would most likely fry very quickly.
The E-C is not quite a fixed multiple of the E-B current but it is close. in real life transistors are not quite perfect and in some applications like hi fi audio this is important.

the "multiplier" is called the current gain or the hfe. Datasheets specify this within a pretty large range and HFE can vary quite a bit between the same type of transistor.

I don't have a flaming clue what that 100k is for, and I've been doing this for 11 years. The first thing i thought when i looked at it is "what is that 100k doing"

I assume its a continuity probe? i think it would be more useful with a non flashing light or a buzzer, but that is just me.

And yes, there is an article you should read that will be very helpful to you :Bipolar junction transistor - Wikipedia, the free encyclopedia

Edit:
Forgot to mention,not enough sleep
transistors go into "saturation" past a certain base current, after that, the output current does not change much.

 

[KeepItSimpleStupid]

(4) It helps drain the charge out of the base when the probes are disconnected. There is always a small amount of capacitance associated with a transistor. That charge, applied to the base, needs a place to go and it goes through the 100K resistor.

 

[EternityForest]

I would think that because bipolar transistors amplify the current not voltage. that the bias resistor would not be needed in this application.
I have never seen a pull up resistor used on a bipolar in such a simple application before.

Imagine that we literally put a capacitor between the base of the transistor and circuit ground. the charge would just drain through the transistor to the collector, maybe slowing the response to changing base current a little, not enough to notice unless it was a big giant electrolytic cap or something.

If we were dealing with a MOSFET, which operates on voltage, not current, then yes, we most definately would need a pull up or else the charge would just stay until it slowly discharged. but this is just a bipolar PNP.

 

[Richardcavell]

Yes, it's a continuity probe, designed to simply sort between 'conductor' and 'insulator'. Obviously it's intended mainly as a teaching exercise. Thanks to all the replies.

 

[Jaguarjoe]

Jaycar sells you a book of simple circuits and they don't explain how they work?

 

[Richardcavell]

Each circuit has a two-page writeup. This circuit's explanation explains in basic terms what the transistor does. It doesn't explain the need for the 100k resistor. Thanks to all who replied.

 

[audioguru]

Without the 100k resistor, nearby radio stations or mains hum will be picked up by the input probe which will keep the transistor turned on.
The 100k resistor will drain away some of the radio signals and keep the transistor turned off.