Figure 23 shows the adjustable voltage and current limit set up. For the feedback loop on the 741, can I change the values of the resistor and potentiometer on the divider? Can I make 1k -> 100R and the 100k pot -> 10k. It's surprisingly difficult to find 100k pots with the specs I want for whatever reason...
I took a look at the datasheet and from what I can tell, here's what is going on with that feedback loop...
Take all of this with a large grain of salt. My analysis may be completely wrong. You would definitely want to double-check my analysis and breadboard this up and test it out.First, I don't really know what R1 is there for, honestly. I'm pretty sure its value can remain as-is, though. It doesn't factor-in when determining the maximum current. From what I can tell, it is just some kind of surrogate for the current sense resistor that is normally between pins 5 and 2.
The op amp along with R2 and P1 are what set the voltage at pin 2--the current limit pin.
The op amp's output voltage--pin 2--is,
V
2 = V
o - I
o*R3*P1/R2
or
V
2 = V
o - V
sense*P1/R2
where, V
sense is the voltage drop across the current sense resistor, R3. V
o is the regulated output voltage, as labeled in the schematic. I
o is the output current.
Note that V
5, the voltage at pin 5 is just,
V
5 = V
o + V
senseVoltage limiting kicks in when V
2 is 0.45 volts below V
5. V
5 is at most 0.45V above V
2, in other words.
The maximum output current is when V
5 - V
2 is 0.45V. So,
I
o(max) = 0.45V / (R3 * (1 + P1/R2) )
You can use this equation to determine the size for R2 when the values of P1 and R3 are already set.
The larger the (user set) value of the potentiometer P1, the smaller I
o(max) will be. So if you plug P1's maximum value into the equation above you will solve for the smallest possible I
o(max).
When P1 is zero ohms then I
o(max) will basically be:
I
o(max) = 0.45V / R3
Which for the schematic in figure 23 would be 4.5A. It says 1.5A so I don't know where that comes from. This may indicate a problem with my analysis.
To set an upper limit for I
o(max) you could simply put a resistor in series with P1--call it R5. Essentially, this gives a minimum value for P1.
The equation for I
o(max) in this case would be:
I
o(max) = 0.45V / (R3 * (1 + (P1 + R5)/R2) )
So when P1 is zero R5 would determine I
o(max)'s upper limit. R5 will affect the lower limit as well, obviously.
If you choose upper and lower bounds for I
o(max), set R3 to 0.1 (as in the schematic) and P1 to 10K then you can solve for R2 and R5.
For a lower bound of 20mA and an upper bound of 1.5A...
0.020A = 0.45V / (0.1 * (1 + (10K + R5) / R2))
1.5A = 0.45V / (0.1 * (1 + ( 0 + R5) / R2))
Re-writing this as a linear system gives:
0.448*R2 - 0.002*R5 = 20
0.3*R2 - 0.15*R5 = 0
Solving this system gives the values:
R2 = 45.045
R5 = 90.0901
, which will give you ballpark values for R2 and R5. You could use E96 values of 45.3 and 90.9, for example. If you plug these values back into the equations you can figure out the exact upper and lower limits for I
o(max).
Note that this is probably why they used a 470 ohm R2 and a 100K P1 in their example. Remember, they didn't have a resistor (which we've called R5) in series with P1. That will limit the worst-case current through R2 and P1 to just less than 1 mA (0.45V / 470).
The worst case current for a R2 of 45.3 and a P1 of 10K in series with 90.9 would be 3.3 mA (0.45V / 136.2). Which shouldn't be a problem. But if you didn't have R5 then this would be about 10 mA (0.45V / 45.3).
Hopefully that gives you some idea about what seems to be going on and how to select a value for R2. R1 should be left as-is at 1K.