Darlington pair from two transistors

[darshelectro]

I am confusing myself on calculating values for making a Darlington pair from two transistors.

Having trouble calculating the base resistance.

I want to connect the second transistor to a transformer,
If my load resistance is 3.2 ohm (transformer) and my supply voltage is 9 Volts, then my load current is 2.81 amps.

I have attached a picture. It is a bit crude but i hope it will make it clear. The 555 timer output Feeds into NPN transistor Q1 and another transistor Q2. Q2 feeds the transformer T1.

So Q2 has to have an Ic greater than 2.81. maybe 4A transistor

Question

1) Does Q1 need to be a high power NPN. If it is connected on the same wire as Q2, then it will also be receiving 2.81 A.

However, i have read that Q1 in a Darlington pair is a low power NPN. therefore, can someone correct my understanding please.

2) How do i calculate the base resistor. R1. Do i use R1=0.2*load resistance*Hfe . Formula from (http://www.kpsec.freeuk.com/trancirc.htm)

The Hfe would be: (Hfe of Q1) * (Hfe of Q2).

Therefore, if (Hfe of Q1) is 40, (Hfe Q2 is 40) for example. Hfe = 1600.

R1 would therefore be 0.2*3.2*1600=1K resistor.

3) If there is a better method or I’m doing something wrong please advise.

Thanks

 

[chemelec]

I am confusing myself on calculating values for making a Darlington pair from two transistors.

Having trouble calculating the base resistance.

I want to connect the second transistor to a transformer,
If my load resistance is 3.2 ohm (transformer) and my supply voltage is 9 Volts, then my load current is 2.81 amps.

I have attached a picture. It is a bit crude but i hope it will make it clear. The 555 timer output Feeds into NPN transistor Q1 and another transistor Q2. Q2 feeds the transformer T1.

So Q2 has to have an Ic greater than 2.81. maybe 4A transistor

Question

1) Does Q1 need to be a high power NPN. If it is connected on the same wire as Q2, then it will also be receiving 2.81 A.

However, i have read that Q1 in a Darlington pair is a low power NPN. therefore, can someone correct my understanding please.

2) How do i calculate the base resistor. R1. Do i use R1=0.2*load resistance*Hfe . Formula from (http://www.kpsec.freeuk.com/trancirc.htm)

The Hfe would be: (Hfe of Q1) * (Hfe of Q2).

Therefore, if (Hfe of Q1) is 40, (Hfe Q2 is 40) for example. Hfe = 1600.

R1 would therefore be 0.2*3.2*1600=1K resistor.

3) If there is a better method or I’m doing something wrong please advise.

Thanks

If I am reading your Post Correctly:

Considering the 555 will Sink or Source 200 mA, there should be no reason to need a Darlington configuration.

Just use an NPN Power Transistor.

 

[audioguru]

The output high voltage of the 555 is about +7.8V. The voltage drop of the darlington is about 1.5V so the load gets only 6.3V if the supply is exactly 9V.

If the 555 drives a power transistor and the load is at its collector, then the load gets about 8.8V with a 9V supply.