Electronic – Series pass transistor, LM7812


I am making a 12 volt regulator for a solar panel to power an audio amplifier. I already have a switching regulator for most application but it is very noisy for amplifier even with a lot of filtering.
The amplifier draws over 3 amps on full load.

I want to build a regulator using 7812 with series pass transistor for higher current as shown in the datasheet, page 20.


I made the same circuit as shown in the datasheet and used the 2SD1047 as Q1



simulate this circuit – Schematic created using CircuitLab

But as I connected it to my bench power supply set to 18V I was not able to get a regulated output. It just showed ~2V less then the input voltage.

At 18V no load I got 16V output, 0.5A load 15V and 3A load 12V.
I tried different values of R1 calculated by the formula shown in the data sheet.

Formula for R1

Is the output voltage affected by the the value of R1?
Please Help.

Beta of 2SD1047
While calculating R1 I assumed the value for Ireg to be 0.1A, Beta of transistor to be ~95, and IQ1 to be 3A, when that didn't work I tried a bunch of different values of resistors with the same results.

The LM7812 works great when no series pass transistor is attached.
And the transistor is good too.

I already have a switching regulator and I also have a higher current linear regulator so don't suggest them please. I want this circuit to work, and I also want to understand this circuit further so if you could point me to other learning material concerning this circuit then that would be great. The only thing I had to work with was the datasheet. Thanks

PS: SOLVED!! My mistake, I connected a NPN BJT instead of PNP.

Best Answer

Here is the diagram from page 20 of the datasheet you linked:

Figure 13 from Fairchild LM78xx datasheet

Image source: Fairchild LM78xx datasheet, Figure 13

Notice that the pass transistor is PNP. The mistake in your design, is that you are trying to use a 2SD1047 NPN pass transistor - which won't work, as you have seen.