Saturation voltage of transistor

Thread Starter

Ryan$

Joined Dec 14, 2018
178
Hi guys, may anyone explain to me when I've a transistor works as switch and it's connected on a general circuit .... so sometimes when switch is off ("there's no current in its branch") the voltage at transistor between Vce is 0.2 and it's stuck on that voltage and not going to zero although the switch(transistor) is "disconnect"... but how is it stuck on that voltage? and how would be voltage at all in sides of transistor(switch) without existing current in its branch? is there in electronic a stuck voltage that if we change the cases of transistor..it will stuck on saturation?!


thanks.
 

Thread Starter

Ryan$

Joined Dec 14, 2018
178
Post a schematic of the transistor circuit you are referring to.
Can't understand what your question is. o_O
Uhh .... I will explain more , I'm not accepting the idea how my switch is "off" and on its sides there's any voltage value like "0.2v or anything else", why it's not zero?! I've no current then It would be zero voltage.

if it's "off" then there's no current, and if no current, then it wouldn't be voltage(voltage=zero) ..
 

crutschow

Joined Mar 14, 2008
34,280
When a transistor is "off" it does not conduct current and has the full supply voltage across it (the same as a mechanical switch would).
When a transistor is fully "on" it conducts current with a small voltage drop across it due to its on resistance.
This voltage will vary depending upon the on resistance of the particular transistor you are using and the value of the on current.

Make sense?
 

Thread Starter

Ryan$

Joined Dec 14, 2018
178
When a transistor is "off" it does not conduct current and has the full supply voltage across it (the same as a mechanical switch would).
When a transistor is fully "on" it conducts current with a small voltage drop across it due to its on resistance.
This voltage will vary depending upon the on resistance of the particular transistor you are using and the value of the on current.

Make sense?
Started making a sense for me, so when it's off it's like "capacitor" but not exactly capacitor ..
 

Bordodynov

Joined May 20, 2015
3,177
What is the maximum voltage on this key transistor. If it is less than 5 volts and less than the breakdown voltage of the base-emitter junction, then you can swap the emitter and collector (reverse mode) and then without current you will receive a voltage that will be significantly less than 0.2 volt.
 

WBahn

Joined Mar 31, 2012
29,976
Hi guys, may anyone explain to me when I've a transistor works as switch and it's connected on a general circuit .... so sometimes when switch is off ("there's no current in its branch") the voltage at transistor between Vce is 0.2 and it's stuck on that voltage and not going to zero although the switch(transistor) is "disconnect"... but how is it stuck on that voltage? and how would be voltage at all in sides of transistor(switch) without existing current in its branch? is there in electronic a stuck voltage that if we change the cases of transistor..it will stuck on saturation?!


thanks.
You are confusing two very different situations.

Saturation voltage comes into play when the transistor is being driven hard ON, not when it is off.

When driven hard on, ideally a transistor would have zero volts from collector to emitter -- it would look like a switch that is closed. But real transistors aren't ideal and there is a low voltage between the collector and emitter even when it is being driven hard on. We call this the saturation voltage. We often treat it as a constant voltage, but in fact it varies with both base current and collector current as well as temperature.

When the transistor is OFF, saturation voltage is not a factor because it simply doesn't apply -- the transistor is cutoff, not saturated.
 

bertus

Joined Apr 5, 2008
22,270
Hello,

From the wiki :

Saturation
With both junctions forward-biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector (or the other direction in the case of NPN, with negatively charged carriers flowing from emitter to collector). This mode corresponds to a logical "on", or a closed switch.

More info can be found on this page:
https://en.wikipedia.org/wiki/Bipolar_junction_transistor

Bertus
 

Thread Starter

Ryan$

Joined Dec 14, 2018
178
Hi guys ; I'm still a lil confused about saturation.
what's I'm thinking about the case of saturation like we are feeding someone and at one time we will arrive to a case that's totally fully, so if we feed him more, then he will get "more" food so the same analogous to voltage and transistor, if we feed transistor by voltage more and more at one time the transistor will be totally fully, but if we feed more, then he will get more, i'e the voltage will not stuck onto the "saturation voltage"..but that's wrong and I don't know why.

May please anyone explain more about saturation voltage?! I've read resources from wiki but they didn't clear up my situation above
 

crutschow

Joined Mar 14, 2008
34,280
if we feed transistor by voltage more and more at one time the transistor will be totally fully, but if we feed more, then he will get more, i'e the voltage will not stuck onto the "saturation voltage".
I don't understand this statement. :confused:
What "voltage" are you talking about?
What do you mean "stuck"?

A BJT acts like a current operated device when used as a switch, so the base is fed a current, with the base-emitter voltage looking like a forward-biased diode.
When the base current is sufficient to fully turn on the transistor (i.e. the collector-emitter voltage is less than the base-emitter voltage for a given collector current) then additional base current will have only a small effect on further lowering the collector-emitter voltage.
 

Thread Starter

Ryan$

Joined Dec 14, 2018
178
I don't understand this statement. :confused:
What "voltage" are you talking about?
What do you mean "stuck"?

A BJT acts like a current operated device when used as a switch, so the base is fed a current, with the base-emitter voltage looking like a forward-biased diode.
When the base current is sufficient to fully turn on the transistor (i.e. the collector-emitter voltage is less than the base-emitter voltage for a given collector current) then additional base current will have only a small effect on further lowering the collector-emitter voltage.
Voltage is any value of voltage, I mean with "stuck" : when we arrive to critical voltage value then the input voltage after that value in spite of increasing that value, we will get the same output and doesn't change from the "critical voltage value", i.e if (Vi >Vsat) => we get specific output and doesn't change, why?!
 

crutschow

Joined Mar 14, 2008
34,280
we get specific output and doesn't change, why?
Because once the transistor is fully on (saturated) it can't turn on any more.
For example, when you close a relay's contacts with its rated coil voltage, then applying move voltage will have no effect.

Your question is puzzling.
What would you expect the transistor to do?
 
Top