High side switch with bjt

[gmarocco]

I have built a high side switch using bjt: the circuit is powered with 18V and the base of the NPN transistor is driven by a pulse between 0 and 5V I would like to use a 50mA load. The circuit is in the image attached to the post.
The load is powered only if I remove resistor R1





Here are the calculation I have done:

Ib(Q3) = 5mA as Ic/Ib=10 in saturation
Hypotesys: Ic(Q4)=4mA

R3= Vbesat(q3)/9mA = 0.9V/0.009 = 100 Ohm

R2 = (18- Vbesat(q3)-Vcesat(q4))/4mA = 4.2kOhm

R4 = (5-Vbesat(q4))/600uA = 7.3kOhm

R3 = 0.6V/200uA = 3kOhm

What's wrong with these calculations?

Many thanks

 

[erikl]

Ib(Q3) = 5mA as Ic/Ib=10 in saturation
Hypotesys: Ic(Q4)=9mA

R1= Vbesat(q3)/4mA = 0.8V/0.004 = 200 Ohm

R2 = (18- Vbesat(q3)-Vcesat(q4))/9mA = 1.8kOhm

R4 = (5-(Vbesat=0.7V(q4)))/600uA = 7.2kOhm

R3 = 0.7V/200uA ≈ 3.6kOhm

Are you sure the input time constant (≈25ms) isn't too long?

 

[Audioguru]

The value of R1 is so low that Q3 will NEVER turn on.
R2 provides a current of about 4.1mA but R1 needs about 8mA when Q3 barely begins to turn on.

Use 3.3k for R1 then it needs about 0.24mA. Use 3.3k for R1 then the base current in Q3 will be about 5mA and it will turn on properly.

The circuit does not need R3 unless the input never goes low.

Why do you have C1 smoothing the input pulse?

 

[gmarocco]

The value of R1 is so low that Q3 will NEVER turn on.
R2 provides a current of about 4.1mA but R1 needs about 8mA when Q3 barely begins to turn on.

Use 3.3k for R1 then it needs about 0.24mA. Use 3.3k for R1 then the base current in Q3 will be about 5mA and it will turn on properly.

The circuit does not need R3 unless the input never goes low.

Why do you have C1 smoothing the input pulse?

Thanks for the reply, I have put C1 in order to avoid spurious activation of the NPN transistor

 

[Audioguru]

I have put C1 in order to avoid spurious activation of the NPN transistor

Spurious signals will activate the NPN transistor if the circuit is built on a breadboard because all the long connecting wires and many long strips of intermittent contacts are antennas.

 

[ALERTLINKS]

R1 value is very low. Chage it between 1K-10K.