# help with adjustable power supply

#### mgh

Nov 17, 2011
8
Hello All,

Playing around with my first project. Trying to build this:
http://www.instructables.com/id/How-to-Build-a-Bench-Top-Power-Supply/

My breadboard is a mess, but I attached a picture any way.

I am using a 12V wall-wart, and I can use the pot to adjust the output, but, only when I put my negative probe on the input from the wall-wart, and the positive probe on the output from the voltage regulator. In other words, I am unable to read across the negative and positive output on the right side of the circuit. I understood that my output from the circuit should be the + and - leads to the right.

Also, the minimum output I can reach is 1.24 V. I thought the pot should be able to go to infinity resistance, to give me 0 volt output. The max output I can get is 11 V, even though I can measure 12V input.

Not sure if I am giving enough information for anyone to understand what I am saying

#### Attachments

• adj. power supply top view.jpg
44.8 KB · Views: 168

#### duke37

Jan 9, 2011
5,364
It looks as if everything is working as it shoud except that the transformer common and output common are not connected.

The 317T regulates to give 1.24V between output and reference input and this is what you get when the pot is at zero ohms.

With 12V input, you will not get much above 10V out as the 317T needs some voltage across it to operate. Look at the specification for "drop out voltage"

#### mgh

Nov 17, 2011
8
With 12V input, you will not get much above 10V out as the 317T needs some voltage across it to operate. Look at the specification for "drop out voltage"

That makes sense.

Sorry, not sure what you mean when you say 'transformer common'. The 3rd pin on the pot? Or one of the pins on the voltage regulator?

Thanks for the help.

#### jackorocko

Apr 4, 2010
1,284
not sure what you mean when you say 'transformer common'.

Your wall wart I would assume, some where there has to be a broken connection after the regulator is my guess. Where are you trying to get the measurement on the output that is not registering? Your VR needs a ground to work so it seems that you have a ground hooked to the adj pin after the resistors.

#### jackorocko

Apr 4, 2010
1,284
http://www.national.com/mpf/LM/LM317.html#Diagrams

The first picture is the typical design for a LM317 VR. See the ground connected to the adj pin, below the variable resistor? If you measure from that ground to the output you should see a voltage. If you follow that wire it connects to the neg side of the electrolytic cap. So measuring from the output pin on the VR to the neg term on the electrolytic cap will also give a voltage. I really don't see how this design would work without it being hooked up to the ground.

Last edited:

#### duke37

Jan 9, 2011
5,364
The common rail or ground is the lower wire on the circuit diagram. If you can measure a voltage between V+ out and V- in but not between V+ out and V- out, then there is no connection at some point on this rail.

Check with power off, the resistance between - in and - out.

#### mgh

Nov 17, 2011
8
Resistance -out to +out is 4.86K with power off.

I have my negative supply on the bottom of the breadboard, and positive supply to upper rail on breadboard. My positive supply goes into reg., then out to the other rail on the top of the breadboard. Negative supply is connected to pot terminal 1, pot terminal 2 is connected to adjustable reg. pin, and pot terminal 3 goes to negative rail again on breadboard. The negative of the capacitor is also connected to the negative rail.

I get good readings until I get to the output side of the pot. I can see this is where my problem is, but I do not understand the negative connections well enough to know where my problem is.

I still very much struggle going from schematic to breadboard/real-world devices and components (as you can tell).

#### BobK

Jan 5, 2010
7,682
If your breadboard is like the one I use, the top and bottom rails do not connect all the way across. The middle gap is open and needs a jumper across it to connect the rail all the way across.

Bob

#### mgh

Nov 17, 2011
8
If your breadboard is like the one I use, the top and bottom rails do not connect all the way across. The middle gap is open and needs a jumper across it to connect the rail all the way across.

Bob

That was what was missing! I disconnected pot terminal #3 from everything, and jumped across the mid-point in the breadboard on the negative rail. So I now have pot #1 connected to supply negative, pot #2 to regulator adjustable, and a jumper on the negative rail to connect the 2 halves of the breadboard. It appears to be functioning correctly.

Many thanks for the replies. Now the next step is trying it on the strip-board.

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