Reduce DC voltage via simple passives BUT as input to DC-DC converter

Question

I have a bunch of cheap buck (I think) converters that do 6-20V input and 5V output; and I want to use one powered by a 24V PSU (this is for a 3D printer).

The current draw of the 5V system should be comparatively low, and constant, but the main supply will vary quite a bit in draw. as the regulator can deal with 6-20 volts I wonder if I can 'scale down' or 'clamp' the supplied power via a simple voltage divider?

I know that usually this is a solid 'No' as voltage drop varies according to current draw, but in my case:

1. I need only target a range of voltages between 6-20V versus a single stable voltage
2. The input to this is a regulated power supply, which should stabilize the voltage (which transitively stabilizes the current, right?), and I expect this should help.

What about some arrangement with Zener diodes? As I look this up mid-post, it seems along the lines of what I am looking for (i.e. something I can throw together with simple passives I am likely to have or can find in scraps): https://en.wikipedia.org/wiki/Zener_diode#Voltage_shifter

But power supplies and analog electronics always surprise me with their subtlety and I don't trust myself enough to know, so would something like the above work, or should I just buy different DC-DC converters and call it a day?

-- new edits as of 2022-12-17 --

Also, I have a bag of these 6-20v -> 5v converters, could I hook two up in series (or otherwise passively arranged) to get effectively 10v per input, but hook their outputs up in parallel?

I will have to dig up the specs on these converters and the power supply, I was more curious if there were generally applicable approaches to this problem given the circumstances (more or less regulated power on each side of the converter).

Answer 1

Anything with a voltage divider or a shunt (Zener) regulator will be very inefficient. Further, the voltage divider output will vary with your 5V loading. That may be ok or you, but not knowing your system current demand particulars it’s impossible to say.

If your overall current is low enough you could use a series regulator like an LM317 to make your 20-ish V pre-rail for the 5V switchers.

Answer 2

If the operation conditions are really constant, you can get away with inserting an appropriately sized resistor (or several to get the power rating), to burn off the excess voltage.

If a wider load current range is possible, I second the comment of @VoltageSpike to use a power transistor to burn off the excess heat. An NPN can be used for lower currents, but for higher currents and power handling capabilities, I think MOSFETs might be easier to obtain with the required power. And MOSFETs don't load the resistor R1.

Good thing with this circuit as compared to Zener droppers, is that you don't need a special valued high power Zener around but can parts that you may have in your BOM already.

^{simulate this circuit – Schematic created using CircuitLab}