Transistor base voltage

[dark]

I need help regarding transistor base voltage. I attach a pic in which the transistor is wired to monitor over current .
My question is about the high voltage application to the base (Q1(B)) , most datasheet says maximum -5~-7V as maximum base voltage. Can we apply high voltage in this manner.

Thanks

 

[N.Carsane]

I don't know much about the PNP type transistors but after reading the specs from the following link, www.alltransistors.com/transistor.php?transistor=49339 , i think the voltage may be just underthe max (200V) as the i think you may agree if you read the specs but i'm not entirely sure...

 

[dark]

I don't know much about the PNP type transistors but after reading the specs from the following link, www.alltransistors.com/transistor.php?transistor=49339 , i think the voltage may be extremely high as the i think you may agree if you read the specs but i'm not entirely sure...

Hi, N.Carsane, Welcome to the forum .

 

[atferrari]

deleted

 

[MrAl]

Hi,

When they talk about the base emitter voltage like that they mean 'reverse' voltage. That means when the voltage switches the transistor 'off' if the base emitter voltage goes opposite to what turns it on then it must be about 5v or less. If the reverse voltage goes up too high it blows out the base emitter diode.

Normally a transistor connected as a current sense (small resistor base to emitter) does not experience too high of a reverse voltage because it would take far too much current to drive that small resistor to have a large voltage across it, so in this application this isnt usually a problem.

What could be a problem when a transistor is used like this is the base current could becomes too large for a short time due to a high surge current in the load which then drives the base too hard. To remedy this usually a small resistance like 100 ohms is placed in series with the base, which does not change the normal operation too much.

 

[Nigel Goodwin]

I need help regarding transistor base voltage. I attach a pic in which the transistor is wired to monitor over current .
My question is about the high voltage application to the base (Q1(B)) , most datasheet says maximum -5~-7V as maximum base voltage. Can we apply high voltage in this manner.

You're not applying high voltage to the base - voltages are relative - and while it's high relative to where it's been measured from, it's VERY low in relation to the base/emitter (which are joined by a 0.1 ohm resistor).

As others have mentioned, the limitation in the datasheet is for reverse zener breakdown of the base/emitter junction, which would require in excess of 70 amps flowing the wrong way through the 0.1 ohm (pretty unlikely, and almost certainly totally impossible).

 

[Mosaic]

The use of small resistances to switch a transistor 'ON' is often found when a 'PASS' power transistor is used to increase Voltage regulator currents.

Here is a useful explanation and diagram.
**broken link removed**

FYI, Reverse zener breakdown IS used as a feature for fast pulse devices. Some transistors tolerate this, it's a voltage phenomenon. Usually you have to characterize them and bin them for these types of circuits.

 

[Nigel Goodwin]

The use of small resistances to switch a transistor 'ON' is often found when a 'PASS' power transistor is used to increase Voltage regulator currents.

Here is a useful explanation and diagram.
**broken link removed**

FYI, Reverse zener breakdown IS used as a feature for fast pulse devices. Some transistors tolerate this, it's a voltage phenomenon. Usually you have to characterize them and bin them for these types of circuits.

What has this got to do with this post?, absolutely nothing? - just adding confusion by posting about something entirely different.

 

[Mosaic]

I guess I misread the OP's requirement.