Simple voltage divider

[cog]

Hi,

This is my first post-- I've watched the blog on Youtube for a while.
 
I'm trying to build a simple voltage divider circuit from a schematic. I've built-traced-rebuilt this thing a half dozen times now, and yet each time I get the same result. Turning the pot gives me an output from 9V to 12V, with very little variation in between. At about ~50-ohm on the pot it just rapidly starts going up or down.

It is supposed to give an output of 1.25V to 10.50V. I've tested each of the components, they all seem OK. Here are the values for the resistors and capacitors:

C1, C2 - 100uF 25V (booklet specified 20V, but I assume 25V is OK?)
R1, R2 - 10K-ohm 0.5W resistors
R3 - 1K-ohm 0.5W resistor
R4 - 10K-ohm pot (linear)
R5, R6 - 100-ohm 0.5W resistors

Anyone have any troubleshooting tips for this?

Attached is a scan that shows the schematic, and a current picture of my breadboard. The red/black wires on the bottom are where I'm measuring my output voltage from.

This particular schematic is from a small booklet included with a Joe Knows Electronics kit. Any help would be appreciated, thanks!

Joe

[daddylonglegs]

  The MJE172 seems to be a PNP transistor, see:

http://www.onsemi.com/PowerSolutions/product.do?id=MJE172

However the schematic has this drawn and used as an NPN transistor.

  The behaviour you describe is consistent with having a PNP in the Q2 position. The feedback through Q3 and Q1 acts as positive feedback rather than the intended negative feedback. As a result, over most of the pot adjustment range the pass transistor (Q2) is held fully on and over part of the adjustment the output is unstable.

  You could place an NPN transitor in Q2. It would be up to you to test for stability and regulation over the full range of output voltages and currents. Alternatively you could look for a known good circuit from a different source.

[mikerj]

  The MJE172 seems to be a PNP transistor, see:

http://www.onsemi.com/PowerSolutions/product.do?id=MJE172

That's very odd, if you get an MJE172 from ST it's a NPN

[Marco]

It does seem you are using a PNP while it needs a NPN.

BTW, the way that book draws the circuit is silly and needlessly confusing and using a 100uF capacitor (C1) just to get the regulator stable is wasteful ...

[cog]

Thanks for the feedback, it was the MJE172 transistor. I replaced that with the NPN equivalent (MJE182) and it worked!

Frustrating that the first circuit in a booklet titled "How Circuits Work" contains such an error. Oh well, it gives me something to look at and learn from.

Also, if using C1 to stabilize the voltage is wasteful, what would a better alternative be (without completely changing the circuit)??

Thanks,
Joe