constant voltage drop at (relatively) high current

Question

I'm making a power supply using some decade switches and resistors. The idea is to dial a voltage, much like a resistor substitution box, but get a voltage out instead of a resistance. I'm using the LM350 (high current version of LM317), but the LM317 voltage drop makes it nearly impossible to get a circuit that will make that work (set up like a standard resistor box, but with 160 ohms, 1.6K and 16K resistors at the three decades, and a 500 ohm between out and adj. To compensate, I'm currently thinking of just adding something to drop the voltage 1.5V-2V after the regulator, and compensate with a pot in front of the switches. The problem is, a simple diode drop is too inconsistent across temperature and current, and I can't seem to find another good solution. I was thinking setting up another LM350 in constant current mode, but set to something like 50A, or set to regulate to something like 100V so that it keeps the voltage as high as it can go, minus the drop. It wouldn't actually pass that much voltage or current, it would only be given maybe 24V at a couple of amps, but because it is set so high, it should try to bring the voltage as high as possible, but I don't know if that will work. The schematic is approximately what the final circuit would be (minus filtering caps and some diodes) The three boxes with resistors are rotary switches. The part I'm asking about is basically just if there will be a 1.25V drop between N$2 and VOUT. Thus, if whatever voltage is dialed by the rotary switches (as in a resistor substitution box) will be present at VOUT.

TLDR: I need to drop a DC voltage by 1.5-2V at about 6A, the exact value doesn't matter, but it must be very constant (\$< \pm 0.02\text{V}\$) across reasonable temperatures and currents, and not overly complex or expensive. Is there any easy or "correct" way to do this?

Answer 1

The answer is simple - just connect the 'ground' end of your switched resistors to -1.25V, then the regulator's output will go down to 0V.

A stable -1.25V can be produced with an adjustable 3 terminal negative voltage regulator (eg. LM337) or two silicon diodes in series with a resistor, fed from a higher (negative) supply voltage. If you don't have a suitable negative voltage supply then you could use an ICL7662 (or equivalent) voltage convertor to produce it.

Answer 2

I'm sorry to say I'm completely flummoxed.

Can we start at the beginning, please?

Here's how an LM350/LT1084 works:

The way the regulator is designed is that you first decide what output voltage you want, (V1) then you arrange the voltage divider R1R2 so that with the desired voltage on U1's OUT pin the voltage on the ADJ pin (V2) is 1250 millivolts less. The current (Iadj) out of the ADJ pin will be less than 100 microamperes, so by making the current through R1R2 large enough, the drop across R2 caused by Iref can be swamped out and made negligible for most applications.

If not, then from the LM350 data sheet you can always use:

$$ \text{V1 = 1.25} \bigg ( 1 + \frac{R2}{R1}\bigg) + \text {Iadj R2 .} $$

To proceed without using the formula and assuming you want 10 volts out of the regulator and that you're willing to spend 10 milliamperes through the divider in order to reduce the error introduced by Iadj to around one percent, if we want a 1.25 volt drop through R1 with 10 milliamperes through it, from Ohm's law we have:

$$ R1 = \frac{Vadj}{Iadj} = \frac{01.25V}{0.01A} =\text{125 ohms.} $$

Then, to drop the rest of the 10 volts through R2, we can write:

$$R2 = \frac{10V - 1.25V}{.01A} = \text{875 ohms}$$

and we'll wind up with:

So there you have the basics.

Now for your switch...

Following is an LTspice schematic which shows a single-pole 10 position switch as a series of voltage-activated switches driven by a counter with mutually exclusive outputs, ensuring that only one switch at a time will be turned on, exactly like your manually operated switch appears to function.

The counter is used to cycle through 10 states and to connect 10 different R2's to ground, changing the ratio of R1 to R2 at each step, which will change the regulator's output voltage in a planned way if the ratios are selected properly.

Notice particularly that, as opposed to your design, each switch position relies only on the ratio of the values of two resistors to determine the regulator's output voltage.