Hi pp,
You already know that Q1 and Q2 are a nominal ~20mA current circuit, also you also have a 48Vdc drive signal via a 10k, [4.8mA] so why are you unable to calculate the voltage drop across the 1k resistor.????

Do you know Ohms law equations.???

E

Hi Eric,

Off-course, I know ohm's law. The voltage drop across 1k is 4.8V. Then, how 25V at the Q1_Emitter?.

 

Hi all,

While I'm searching for the answer for my question, I found the post thread by title named as "What is use of R41 (base to emitter) resistor in Transistor Relay Circuit ?".

But, one of the post in the thread stated that, first need to be calculated the leakage current of the transistor. Again, I don't know how to calculate the leakage current. However, MisterBill2 posted the text by saying what are the thinks need to be calculated to find out the leakage current.

The text from MisterBill2:
https://forum.allaboutcircuits.com/...-resistor-in-transistor-relay-circuit.149416/
[OK, the process for determining that resistor starts elsewhere. As already mentioned, first the required collector current to operate the relay with a worst case transistor saturation voltage must be known. That will require reading the specification sheet.
Next, the base current to bring the switching transistor into saturation, for a minimum gain transistor must be calculated.
Once the required base current is known, the value of R3, the base current limiting resistor can be selected, allowing for both the base-to emitter voltage to vary with temperature, and allowing some current through R41 (R4??). The value of R4 is then selected to pull the base of the transistor below the cutoff voltage even at the maximum leakage current of the opto-isolator plus the off-state base leakage of the transistor.
Of course an adequate margin must be used to allow for tolerance variations of the resistor values. Then an appropriate standard value for the resistor is selected. This is why most engineers just pick values based on experience and common practice.]


In my case, I don't have any relay in my circuit. Just I'm learning how to calculate the Rbe resistor?. I'm not understand that much from the text, but, I have tried my way please could you also help me.

1. Max. case Collector-Emitter Saturation Voltage (Vce) - 305mV (according to ZXTN25060BZ datasheet)
2. Max. Collector current (Ic) - 5A (according to ZXTN25060BZ datasheet)
3. Min. DC current gain (hFE) - 20 (according to ZXTN25060BZ datasheet)

I have just assumed and consider that 1A current (Ic) is the required and 5V (Vbe), such that I have calculated the Rb.

According to datasheet (from page 5), 50mA (Ib) current is required to turn on the transistor to pass 1A current from collector to emitter.

Rb = (Vbe - Vbe(on)) / Ib => (5-1.050) / 50m => 79 ohms ( according to datasheet Vbe(on) = 1.050V)

From here on-wards not understand what I have to do to calculate the leakage current of the transistor. Please could you help me?.

Thank you, all.

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

 

Trace the path from the emitter to ground, and see if that helps.

Bob

 

Trace the path from the emitter to ground, and see if that helps.

Bob

 

Hi all,

While I'm searching for the answer for my question, I found the post thread by title named as "What is use of R41 (base to emitter) resistor in Transistor Relay Circuit ?".

But, one of the post in the thread stated that, first need to be calculated the leakage current of the transistor. Again, I don't know how to calculate the leakage current. However, MisterBill2 posted the text by saying what are the thinks need to be calculated to find out the leakage current.

The text from MisterBill2:
https://forum.allaboutcircuits.com/...-resistor-in-transistor-relay-circuit.149416/
[OK, the process for determining that resistor starts elsewhere. As already mentioned, first the required collector current to operate the relay with a worst case transistor saturation voltage must be known. That will require reading the specification sheet.
Next, the base current to bring the switching transistor into saturation, for a minimum gain transistor must be calculated.
Once the required base current is known, the value of R3, the base current limiting resistor can be selected, allowing for both the base-to emitter voltage to vary with temperature, and allowing some current through R41 (R4??). The value of R4 is then selected to pull the base of the transistor below the cutoff voltage even at the maximum leakage current of the opto-isolator plus the off-state base leakage of the transistor.
Of course an adequate margin must be used to allow for tolerance variations of the resistor values. Then an appropriate standard value for the resistor is selected. This is why most engineers just pick values based on experience and common practice.]


In my case, I don't have any relay in my circuit. Just I'm learning how to calculate the Rbe resistor?. I'm not understand that much from the text, but, I have tried my way please could you also help me.

1. Max. case Collector-Emitter Saturation Voltage (Vce) - 305mV (according to ZXTN25060BZ datasheet)
2. Max. Collector current (Ic) - 5A (according to ZXTN25060BZ datasheet)
3. Min. DC current gain (hFE) - 20 (according to ZXTN25060BZ datasheet)

I have just assumed and consider that 1A current (Ic) is the required and 5V (Vbe), such that I have calculated the Rb.

According to datasheet (from page 5), 50mA (Ib) current is required to turn on the transistor to pass 1A current from collector to emitter.

Rb = (Vbe - Vbe(on)) / Ib => (5-1.050) / 50m => 79 ohms ( according to datasheet Vbe(on) = 1.050V)

From here on-wards not understand what I have to do to calculate the leakage current of the transistor. Please could you help me?.

Thank you, all.

Just saw this thread so not sure if anyone mentioned this yet but...

In a switching circuit the base emitter resistor is used to sink the collector base leakage current so that the transistor can turn all the way off.
Without some current sink (from resistor or other) the collector to base junction can forward bias the base emitter junction and therefore turn the transistor on or keep it on or keep it one a little bit which is also not good in most cases.
There are other ways to do this but if using a base to emitter resistor then it's the leakage current if it's a switching circuit.
If an amplifier, it is usually to bias the transistor into linear operation and set the input impedance at the same time.
If you have another transistor driving this transistor then the base to emitter resistor will help to keep the transistor off by sinking any leakage current and also sinking any left over current from the transistor that drives it if it happens to stay on a little bit.
This is used a lot in switching circuits but in an amplifier it could also help to bias it and set the gain and improve the linearity of the stage.


Good luck to you.

 

hi pp,
In that type of current limit circuit, a series Emitter resistor is always required.

E
View attachment 266089

Because I'm bored, here is a solution that uses the 12V instead of a 5V/10k

 

hi pp,
Read this AAC Article, ask if you have a problem.
E
https://www.allaboutcircuits.com/technical-articles/all-about-bjt-beta-understanding-terminology/

Hi Eric,

Yes, I have gone though the above article, even though my question is not clear which I have asked in the post #5 and marked in bold letters.

Thank you.

 

200 to 250 might be a good guess


A good design can handle hfe drift - You should never assume it to be an exact value.