Help designing a voltage divider

Question

I said voltage divider on the title, but could be anything.

I have a DC source of 19.7V.

I need an output that varies from 0V to 18V.

I have:

• Resistors (1/8W);

• 1k potentiometer;

• 5k trimpot;

• 2N3904;

• LM358;

• Capacitors.

My only requirement, besides part count, is that both potentiometers be used: the 1k being the main, and the 5k acting for fine tuning.

And the adjust... not too sensitive. The more linear, the merrier, and taking advantage of the whole excursion of the potentiometer.

In the example below I could not obey the requirement of sensitivity and excursion.

The transistor I'm trying to use has a cutoff around 680mV and saturation around 820mV. Very small window. Hence, my trouble.

More:

• The output current can be small. 5mA will work;

• This whole system will control the GND pin of a LM7805, and later on, of a 7905;

• LM317 unavailable;

• The 1k potentiometer can be turned by hand and is what I have available. The 5k trimpot needs a screwdriver.

History:

At first I thought a voltage divider could do it, but in my attempts, the voltage hardly approaches 15V.

Still open for this type of design, though (just bear the requirements if you'd like to help me this way)

After giving up, I tried using OpAmp. It worked. But later I will need a negative version of this structure I'm trying to build, but 19.7V-(-19.7V) my opamps won't support.

Maybe I could reduce the supply voltage of this OpAmp, but then... part count.

Third attempt, more promising but difficult, was with a transistor.

Answer 1

^{simulate this circuit – Schematic created using CircuitLab} DONT USE ABOVE poor simulator part of EE.SE Don't burn it out by over loading the pot with 1/4W type or excess current.

Use this simulator instead

Every part serves a purpose.

If you understand this design and tried it out, say OK (+1)

Answer 2

I'll share my own solution, which loses on two aspects:

• Voltage will never reach my desired minimum of 0V;

• The part count is a little higher. Luckly, it's all resistors.

The only advantage here is the low current through the components.

I encourage those interested to enhance this model.

My next goal is a negative counterpart: 0V to -18V from a -19.7V supply

Answer 3

Is this an assignment or similar.
If so, we will still help but in a more 'tutorial' manner.

Here is a fairly complete outline.
Purposefully with no circuit and somewhat skeleton detail assuming an assignment. Ask more if wanted.

The opamp is your great friend.
An UNLOADED pot will give you the whole voltage range.
Add a trimpot at the top and you can reduce the range.
If the pot is linear (and not log or semilog) you get linear Vout with pot position.

Use the opamp to drive the transistor as en emitter follower.
Compare pot voltage and vout (scaled into common mode range of opamp) and drive transistor (drive scaled to allow for opamp not being a rail to rail one).

Divide Vout by a factor to bring it into opamp common mode range and compare it to the pot voltage. Pot Vout or pot total Voltage range can be scaled down to lower voltage to match opamp Vout related input.

Result should be reasonably superb.
Instability is possible.
Judiciously placed small cap MAY be needed somewhere if it oscillates.