max base collector current npn transistor

Thread Starter

sdan

Joined Jan 10, 2019
3
Hello

I have the case where the collector base current on a npn transistor could be negative. I know, the path between base and collector feels like a diode. So there will be a voltage of about 0.5V, but I don't find any data which current is allowed thrugh this diode path. Does anyone has an idea (e.g. for BC846)?
 

KeithWalker

Joined Jul 10, 2017
1,216
Hello

I have the case where the collector base current on a npn transistor could be negative. I know, the path between base and collector feels like a diode. So there will be a voltage of about 0.5V, but I don't find any data which current is allowed thrugh this diode path. Does anyone has an idea (e.g. for BC846)?
The data sheet shows that the maximum peak base current is 200 mA so I would think that the maximum continuous base current will be about half that. This will be the limiting factor in your circuit.
 

LvW

Joined Jun 13, 2013
1,084
Hello

I have the case where the collector base current on a npn transistor could be negative. I know, the path between base and collector feels like a diode. So there will be a voltage of about 0.5V, but I don't find any data which current is allowed thrugh this diode path. Does anyone has an idea (e.g. for BC846)?
Under normal operation conditions (amplification mode) the path between base and collector acts as a pn junction which is reverse biased.
Hence, there is a very small current only which, normally, is neglected.
 

Thread Starter

sdan

Joined Jan 10, 2019
3
The data sheet shows that the maximum peak base current is 200 mA so I would think that the maximum continuous base current will be about half that. This will be the limiting factor in your circuit.
Thanks for your answer. Normally the base current goes from base to emitter. In my special case the current goes from base to collecter. In this special case the maximum allowable base current is the same?
 

dl324

Joined Mar 30, 2015
11,506
In my special case the current goes from base to collecter. In this special case the maximum allowable base current is the same?
Please post a schematic showing the source of this current.

The CB junction is usually reverse biased unless the transistor is in saturation.
 

Thread Starter

sdan

Joined Jan 10, 2019
3
Please post a schematic showing the source of this current.

The CB junction is usually reverse biased unless the transistor is in saturation.
It's the circuit in the attachement. Normally V1 is positive, but in the discussed special case V1 is negative. The circuit doesn't has to do anything, but it should not be destroyed.
The function of the circuit is to compare a voltage with a reference as long the voltage is below a specified voltage level. When the voltage is higher than the specified level, Q1 is switched on and the compare_voltage is lower than the reference voltage.
In other words, with the value of compare_voltage, a load should be switched on or off (not drawn in the attachement), when the voltage V1 is in a certain window. Unforunately V1 can be reversed.
 

Attachments

Alec_t

Joined Sep 17, 2013
11,640
In your circuit, if V1 is -30V and we take a diode drop as 0.6V, then the collector-base junction diode current will be about (2 x 0.6 -30)/30k ≅ -0.9mA, as LTspice tells you. This reverse current is not harmful to the transistor. It's possible to use a transistor with collector and emitter connections inter-changed; though the current gain would probably be less in the reversed state.
 
Last edited:

KeithWalker

Joined Jul 10, 2017
1,216
Thanks for your answer. Normally the base current goes from base to emitter. In my special case the current goes from base to collecter. In this special case the maximum allowable base current is the same?
The junctions between the base and emitter and the base and collector are the same junction. The emitter and collector can handle identical currents. Why would it not be the same? In your circuit, the maximum reverse base/collector current will be about 2.5 mA. That is not enough to cause any damage to the transistor.
 
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