Fine, coarse adjustment digitally

Question

I want to add fine and coarse adjustment to a power supply for both voltage and current control, for fine tuning I'll need four potentiometers either 10 turn or regular ones. I was thinking to trim two of this pots to reduce the cost and space and I came up with this idea:

^{simulate this circuit – Schematic created using CircuitLab}

Basically converting an 8 bit PWM to voltage and using it as voltage reference, above circuit is the same as this one:

^{simulate this circuit}

And the problem with above circuit is; if we set the coarse pot to output it's maximum voltage, it's like the coarse pot isn't in the circuit anymore and therefore we can't get any fine tuning.

In this question @Spehro Pefhany suggested a voltage divider "network" for coarse and fine tuning.

How can I combine digital(DAC) and analog(POT) to achieve coarse/fine tuning?

Answer 1

Fine, coarse adjustment digitally?

Do both in one simple digital to analog interface

The simplest method to control Voltage and current limit is to use an analog Vref derived from a DAC output with say 8 bit resolution for 256 levels.

Define the digital interface for convenience, too many ways to mention;

• Serial Async - Tx only or with feedback

  • UART with 1 byte for voltage and 1 byte for current limit followed by using Serial Data & Clock on 2 ports, a serial DAC

• Serial Sync - Clk+data depends on distance, speed and resolution

  • use a SIPO register to discrete R-2R DAC

Here an 8 bit DAC simulated with a clock and counter , add a unity gain Op Amp buffer and choose the terminator R to scale down the Vmax range. You may easily Slider change the value in this browser based Simulator.

Analog

The concept of two pots for coarse and fine must be done in series , not parallel unless they are bufference to prevent short circuiting each other in parallel. Often a fixed R is put in series to limit the tuning range. For coarse, fine the total resistance defines the load and the chpice for fine tuning % determines it's value. e.g. 1k:100R or 1M:50K for 5% fine tune range.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

It's not clear why you want to do this. The Arduino, which you mentioned in other posts, has a 16-bit PWM timer and you can learn how to use that in the answer to increase-pwm-bit-resolution.

Your cascaded potentiometer arrangement is poor. Imagine what happens when the first pot is close to maximum. The second one then spans the whole range so you have lost any advantage of having two pots.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A coarse (R2) and fine (R1) potentiometer arrangement can use standard single-turn pots. With the 1:10 arrangement shown the fine adjustment would be about 1/10 of the coarse adjustment.