PNP darlington pair calculation understanding

Thread Starter

KranthiKumarR

Joined Aug 27, 2017
13
Dear all,

I have a 20ohms discharge resistor connected to my PNP darlington pair. I have simulated the circuit in my mobile (every circuit app).please find the attachment for circuit. But here I'm unable to understand how the PNP transistors are turned on. Please help me understand this circuit
 

Attachments

ericgibbs

Joined Jan 29, 2010
8,882
hi,
As I said in post #3, The +4V of the supply is connected to Ground of the circuit and the minus side of the supply is connected to the transistor Collector path.

Is this homework or a college assignment.?
E
 

ericgibbs

Joined Jan 29, 2010
8,882
hi K,
This is your circuit in LTS.
The Emitter currents are a close match, the Base current in the upper NPN is going to be higher as its got the extra 4v from the lower battery.
E
I have marked the diagram with labels, ask if you need explanation.
 

Attachments

ericgibbs

Joined Jan 29, 2010
8,882
hi K,
This sim shows the effect on the circuit as the battery voltages fall from 4V to 2V.
Note: it is still discharging the batteries at 2V at approx 60mA, compared to the 160mA at 4v.
Its IR1 and IR2 plots
It looks OK.
E
 

Attachments

Thread Starter

KranthiKumarR

Joined Aug 27, 2017
13
hi K,
This is your circuit in LTS.
The Emitter currents are a close match, the Base current in the upper NPN is going to be higher as its got the extra 4v from the lower battery.
E
I have marked the diagram with labels, ask if you need explanation.
Hi Eric,

My simulation and values on mobile are also exactly the same. I need some explanation. Please help. Consider the lower PNP darlington U1.
Here how is Vb1 calculated. according to the voltage divider rule. Vb1 should be 4V*3k/(200k + 3k) = 0.059V. Please correct me if I'm wrong.
 

ericgibbs

Joined Jan 29, 2010
8,882
hi,
We assume that for a Darlington that to be ON, conducting the Vbe is 1.4V, in forward bias.
The 200k is large compared to the 3K and I would ignore it.
For the Lower circuit, thats 4Vbty - 1.4Vbe = 2.6V across the 3K resistor. That is 2.6v/3k = ~0.87mA
For the upper circuit add the 4Vbty lower battery, so thats (4Vbty+2.6V)/3k = 2.2mA

Compare these values with the simulation, they are close enough.

The difference is due to the actual Vbe value of the Darlington's compared to the assumed value of 1.4V

E

Update:
Checked the actual Vbe's on the sim , they are approx 1.25V thru 1.3V.
I always assume 1.4Vbe for a Darlington and 0.7Vbe for a regular transistor.
 
Last edited:

Thread Starter

KranthiKumarR

Joined Aug 27, 2017
13
Thank you eric for the explanation. While selecting a PNP darlington pair. I've selected a component whose Maximum collector current is (1A).
I think this is suitable for my circuit which has ~150mA current. And also should the Maximum collector current be (+1A or -1A).
 

ericgibbs

Joined Jan 29, 2010
8,882
hi,
As it is a Negative voltage supply line to the PNP Collector, I would use the -1Amp as commonly used in datasheets for the current sense.
E
 

crutschow

Joined Mar 14, 2008
23,527
The sign in front of the current value just indicates the direction of the current.
The convention is that (+) is current into a circuit node, and (-) is current out of a node.
 

ericgibbs

Joined Jan 29, 2010
8,882
I hope I am correctly interpreting what you are saying.??

If you are saying the circuit drawing in post #15 is upside down, surely that would mean all H Bridge circuits are incorrectly drawn.
ie: with their high side PNP's upside down.
 

dl324

Joined Mar 30, 2015
9,147
I hope I am correctly interpreting what you are saying.??
You're not interpreting correctly.
If you are saying the circuit drawing in post #15 is upside down, surely that would mean all H Bridge circuits are incorrectly drawn.
ie: with their high side PNP's upside down.
I drew the schematic in #15 and it is an example of preferred style.

You misunderstood my post. Don't know how I could make it clearer.
 

ericgibbs

Joined Jan 29, 2010
8,882
This is the image that the TS posted in post #4, how is different from your corrected image.?

Thats why I queried what you were saying, do you see now how it could be considered confusing.??

pic1.png upload_2019-1-17_9-18-2.png
 
Top