Hi,
I can see now that most of you know what this was already and that's good. TvTech im sure you know too but feel free to add more if you like.
First, this was a real life problem that i discovered in my power supply just recently because i wanted better regulation than i normally need with battery charging, because with battery charging i dont mind if the voltage is a little lower to start with and then picks up later when the current decreases.
Second, it's not really about the 220 ohm resistor 'value'. True it should be lower, but keep in mind that the min current spec is not a cut and dry spec it's a "to be sure over all temperature and specific part selections" min spec which means some regulators at some temperatures (like room temperature) will operate pretty much the same with either a 220 ohm, 150 ohm, or 125 ohm, or 120 ohm, except of course for the choice of R2 which also has to change to maintain the same output voltage. That said, changing the 220 ohm to 100 ohm (and R2 to the appropriate value) does not change the result as it still drops by 0.2 volts from 14.2 to 14.0 as before. However, 100 ohms will be used for the remainder of this discussion when a calculation is made but not in the schematic diagrams where it is still shown as a 220 ohm.
Third, those who stated that the problem had something to do with the layout where the physical layout position of the 220 ohm resistor with respect to the TO-220 package where correct, at least in part, and for the most part, correct. I'll explain next.
Those that said that it was a 'single wire' that was the problem were absolutely correct but this is very close to the same reasoning as the physical location of the 220 ohm resistor so the two ideas are almost the same. I'll explain next.
The main problem:
The main problem is that the wire connected to the center pin (Vout) is SHARED between the 220 ohm current AND the load current (see where Vout is taken from pin 12 of the connector). That means that any current that flows through the load also flows through this wire. Since the wire gauge is #24 AWG and the length is about 8 inches long, that puts the resistance of that one wire at close to 0.017 ohms (just 17mOhms). That is not much, and at 1 amp there is only 0.017 volts drop in that wire, which is not much. If it was just that, it would mean the output would drop by only 17mv which would not really be as much of a problem except in the most extreme cases where we need super great regulation.
But the problem is that it also drops the voltage getting to the 220 ohm resistor, and that is the resistor that should have the internal reference voltage across it at all times in order to ensure good regulation. Comparing the 17mv to the 1.25 internal voltage drop we get:
0.017/1.25=0.0136
which can be read as "1.36 percent" of the reference voltage, which still doesnt seem like much but considering this regualtor should be at least as good as 1 percent OVER THE FULL TEMPERATURE RANGE, that means we might expect another 1 percent drop which would be nasty.
But how this affects the output is quite astonishing. Since we experience a 1.36 percent drop in reference, we can expect a 1.36 percent drop in output, which means the output could drop to 100-1.36=98.64 percent. Starting with 14.2 volts that means we have as output:
Vout=14.2*0.9864=14.00688 volts !
Now we see how big a deal even a small resistance like 0.017 ohms can make in the output voltage.
The real equation is like this:
Vout=(Vref*(R2+R1)*RL)/(R1*RL+Rs*RL+Rs*R2+Rs*R1)
where R1 is the upper resistor (like 220 ohm or 100 ohm) and R2 is the lower resistor, and
RL is the load resistance and Rs is that pesky series resistance between the Vout terminal and the 220 ohm resistor when the load is drawn through that same resistance. Also, Vref=1.25 volts or close to that.
If we solve that equation for Rs, we get about 0.017 ohms, which is the resistance of that one wire.
I should also point out however that the real problem is that the one wire is SHARED with the 220 ohm current and load current. That's the real problem. If there were two individual wires, there would be no problem because then the 220 would have it's own wire for current so there would be no drop. This means the resistor itself does not have to be moved, we just need to add a new wire from Vout to the load so that any load current drops a little voltage but only in that wire not the 220 ohm resistors wire.
The fix then is very simple as many here thought: just add a separate wire to Vout that goes to the load.
The fix is shown in the new diagram, where the connector terminal 1 was disconnected and instead used for the LOAD connection for Vout (see attachment). Note i removed some of the resistors that were not being used anyway for this.
If there are any other ideas too though i welcome them as well.
For example, with some choice component selections we might be able to measure very low value resistances because we can get the output to change by 10 times the resistor R3 value. Measuring the output voltage would then tell us what value of R3 (now a resistor we want to measure) is.
Another example is if we make that resistance a low value low power resistor we can get a 'soft' output voltage regulation profile for higher currents without the need for a higher power resistor directly in series with the output.
Also, as it turns out, this isnt the only problem with these regulators, but i'll save them for next time