# Help needed in understanding the current sense circuit using dual PNP transistor ?

#### mishra87

Joined Jan 17, 2016
1,040
Hey @all

Can anyone explain me how this circuit works ?
How can we calculate Vout in term of load current (I_Load) value.
We can assume Power Supply =12V and Rsense = 500mOhm, RL=500mA etc for analysis.

Any help will highly appreciated.

#### ericgibbs

Joined Jan 29, 2010
19,031
hi mishra,
E

#### mishra87

Joined Jan 17, 2016
1,040

#### ericgibbs

Joined Jan 29, 2010
19,031
hi m,
What is the voltage between Q1A Emitter and Collector.?

E

#### crutschow

Joined Mar 14, 2008
34,722
That circuit is incorrect (as my simulation showed).
(Where did you find it?)
An increase in current lowers Q1b's emitter voltage which turns it off.

If you interchange the power supply with the load so the current goes in the opposite direction through Rsense, and select appropriate resistor values, then it will work (sim results below):
(I leave the selection of the resistor values to the TS).

Q1a is connected as a diode to essentially cancel the Vbe of Q1b.
The sense resistor voltage drop lowers the node 1 voltage from Q1a, which generates a voltage across R2 to cause a current through Q1b and thus a voltage at Vout.

Last edited:

#### ScottWang

Joined Aug 23, 2012
7,412
(Where did you find it?)
I just used the circuit as the image to googled and found out that there are two webs -- web1 and web2.

#### crutschow

Joined Mar 14, 2008
34,722
Surprised EDN posted that circuit.
Apparently it was never built, or even simulated.

#### panic mode

Joined Oct 10, 2011
2,826
the equations in article do not match the circuit. it looks like circuit is drawn incorrectly - PSU and Load have switched places.