For making an exact adjustment it makes far more sense to simply have the adjustment rotation cover a percentage of the initial setting.

 

For making an exact adjustment it makes far more sense to simply have the adjustment rotation cover a percentage of the initial setting.

Well, that's what the TS's original circuit had, and he apparently did not like it.

 

It still seems unusual to demand a constant voltage change per unit of rotation of the fine adjustment control. What sort of application would demand that??? The need for exact setting is why many power supplies have output voltage meters.

Hi,

It's not quite as simple as that because it is often hard to turn the pot a 'tiny' amount to adjust the output voltage up by say 1mv. You have to barely turn the knob, even with a fine adjustment. I had to increase the size of the knob of one pot on one of the power supplies I have in order to get a finer adjustment resolution. On another supply I created a special circuit that would allow me to change the voltage by 1mv with any coarse setting, which meant I could easily go from 9.000v to 9.001v, or from 1.000v to 1.001v for example. In that case I wanted to be able to have increments of 1mv regardless of what total output voltage I needed. It's not easy to get that though.

We are assuming that 10 percent is good enough, but that's not always the case. It's also a bit of a pain in the neck to have to adjust the coarse, then the fine, then find out the fine does not go far enough yet so you have to go back to the coarse and adjust it either a little higher or a little lower so the fine control then has the range you need to adjust the output to.

So a meter is not the ultimate solution. I can tell you that with confidence because most of my power supplies had meters built in, and using a multimeter doesn't help either. It's a matter of not being able to turn any pot by a very, very small degree of the total rotation. It's hard to do that.

Another solution I used a long, long, time ago was to make a GIANT knob, really big, about 4 inches in diameter. That's a big knob when most are 1 inch or maybe 1.5 inch max.
The idea here is that the human fingers sense the turn of the pot due to the outer border tangential movement of the knob. We 'feel' that edge moving and that tells us how much we turned the knob. With a very large pot like 4 or 5 inches, we get a lot more circumferential surface to work with. A 1.5 inch diameter pot has a circumference of 4.7 inches, a 4.5 inch diameter pot has a circumference of 14.1 inches which is 3 times the 1.5 inch pot. Compared to a 1 inch diameter pot is is 4.5 times more. That means when we turn the outer circumference by 0.1 inches, the rotation of the pot shaft is just 2.55 degrees while with the 1 inch pot it is 11.5 degrees. 2.5 degrees is 0.0085 of a full turn of a 300 degree pot, and 11.5 degrees is 0.038 of a full turn. That means with a 10v output the 1 inch knob would adjust the output by 0.38 volts per 0.1 inch travel of the outer tangential circumference , while the 4.5 inch knob would adjust it by 0.084 volts without any fine control. Now take the 1 inch pot knob and add a 10 percent fine adjustment, and we would see an adjustment of the 10v output adjustable by about 0.038 volts, which is about twice as better.
If we want better resolution without doing anything else, we have to try to turn the pot by a smaller amount. It is rather hard to adjust the outer circumference of the knob by an amount smaller than 0.1 inch. It can be done, but what happens is the sticking friction starts to interact with our fingers such that when we go to turn it up, it jumps up a little past the point we wanted it to, and when we adjust down it jumps a little past what we wanted it to be. The sticking friction acts as a nonlinear component that tends to make us apply more force than actually needed to turn the pot at first, and that means that once it start to turn and the sliding friction takes over the pot moves quickly up to the next place in the rotation, which appears to be a 'jump' in the rotation rather than a smooth transition.

So I think this explains why we need a fine adjustment control for power supplies. This also explains why we might even need a third pot for super fine adjustment. In one commercial power supply I know of, they use three pots: one for coarse and one for fine, and one for adjusting the operating range of those two but that's a trim pot. In the current case of this thread, we might add a third pot of maybe 100 Ohms to the other two. That would mean three pots: a 10k, a 1k, and a 100 Ohm pot.

 

The circuit I show in Post #2 can be configured so the fine control covers any desired percentage of the full scale voltage.

 

A one millivolt change is a lot less than a 1% change, most of the time. AND the original fine adjustment was good for a small adjustment as a percentage of the coarse setting. So it is actually better for making SMAll ADJUSTMENTS.That was not what was requested, which was THE SAME AMOUNT OF CHANGE for a given amount of knob turn.

 

The concept of adjustment using two series potentiometers that is; one course and one fine,
As stated above, the course adjustment will easily place the resistance in the correct range.
Here is an example using 1Volt and is a very tight design, Here the course setting is easily adjusted such that
meter reads 1.000 +/- 0.003 The finite control will easily remove the 0.003 error to 0.0000 +/- 0.0004

First the fine adjustment should be placed in the straight up, center position. The midpoint of the resistance
The knob might rotate 60 degrees in either direction from that polar reference point which is straight up.
This method of finite precision allowed instrument makers long ago to verify their math which is fundamental.

 

The circuit I show in Post #2 can be configured so the fine control covers any desired percentage of the full scale voltage.

Hi,

What do you think the advantage is over the original two-pot solution in the first post?

Also, I guess you could add a third pot to that two-pot two-resistor circuit also.

 

The concept of adjustment using two series potentiometers that is; one course and one fine,
As stated above, the course adjustment will easily place the resistance in the correct range.
Here is an example using 1Volt and is a very tight design, Here the course setting is easily adjusted such that
meter reads 1.000 +/- 0.003 The finite control will easily remove the 0.003 error to 0.0000 +/- 0.0004

First the fine adjustment should be placed in the straight up, center position. The midpoint of the resistance
The knob might rotate 60 degrees in either direction from that polar reference point which is straight up.
This method of finite precision allowed instrument makers long ago to verify their math which is fundamental.

Hi,

The unfortunate disadvantage to doing that though is that then you only get 1/2 of the total range of the fine pot for adjustment.
If you set the fine pot to the lower end and then set the coarse pot to just below the desired voltage, you get the entire range of the fine pot for adjustment.
I do have to agree though that setting the fine pot to 1/2 of the total travel does work good sometimes maybe even very often.