[SOLVED] OpAmp Voltage Regulator

[cgchas]

I am considering this circuit design for a low-cost voltage regulator:



The load resistance (R4) is expected to be about 2.5 ohms. I am considering 2N3055 since I have them on hand and they seem to be a suitable fit with regards to their power handling capability.

Can anybody explain how to determine the base current draw on the op-amp's output line so I can select an appropriate op-amp device?

Many thanks,
Charles

 

[kak111]

First we look datasheet

fig. 3 , hfe = 25..50 ,(Vce=4.0V), we take 25
Ic >> 4A , Ib >> 160mA

fig.5.& fig.7. when we go near saturation Vce<2.0V
we see Ic / Ib is about 10-

so we need for Ic=4A >> Ib= 0.4A ( R about 27 ohm )

I think , that is too much to taken from op.amp.

So we must make darlington circuit to add amplification.



If we take some smaller transistor hfe = 50
then we get amplification 50*10 = 500 and needed Ib is about 1mA.

We choose Rc 27 ohm / 5W and Rb = 470..1k for Ib 30..10mA from op-amp.
We choose Rb 560 ohm , now current Ib is about 20mA

So needed smaller transistor is hfe min. 50 Ic min 1.0A . Power dissipation about 1.5Wmax.
for ex. BD137 (BD139) ,( or other similar type)
it has TO-126 case , so you can fasten it to same cooling plate with 2N3055
(case bottom metal part is connected to collector, needs insulation )

Remember there is now 2 transistors in series , so voltage drop over
darlington is 0.8V+0.8V ( two BE diodes)

Power output voltage is 0...10.5V , when input 12.0V
Output current is able to ...10A ( remember cooling for transistors )

You can add filter capacitor parallel to zener 1...10µF.
Output filter capacitor and 100nF parallel
And of cause input voltage is filtered DC.

Hope this helps you.
KAK