24 to 12v converter 443

Did you actually *do* electronics?

The rule is the reciprocal of the additions of the reciprocals of the resistors used.

A rule of thumb is that the effective resistance resistors in parallel are always less than the smallest one. If you've got 2 resistors, one big, one small, the combined will approximate the smallest. If you've got 2 the same, the effective resistance is half the value of one.

If you've got 2 100K resistors in series, with the centre tap as the output, with no load on it, you *will* get 12v.

If you then put a 1A load (effective load resistance of 12R) into the system, you've then got a 100k resistor on top with a 12R below (I know it's not exactly 12R, but it's a close approximation of 11.998R).

The output will then drop to pretty much zero.

Given they're 1A and chunky, they're almost certainly something like a linear regulator. These are still £3-£5 as a one off from Farnell. You get soldering irons cooler than this. It means you'll need a whopping great heatsink (expensive). This will put the size up.

For more than 1A, or if it's billed as "switching", there will be a basic DC-DC converter in there. The bits will still be cheap though. efficient (so you're only having to dissipate 1.2W rather than 12W).

This means it can be smaller and lighter than the linear regulator version.

You'd need the driver IC, couple of capacitors, inductor, schottky diode, and a couple of resistors (resistors used to set the output voltage). No more than £7 as one-off components.

Multiply by the economies of scale, there won't be more than £3 of components for a small one, probably £5 for a high current version.

You've then got to add the cost of CE marking, regulations, mechanicals etc.

To keep the output of 2 100k resistors at 12v, then (as a ballpark figure), you'd need an input impedance of ~10x that of your divider resistor (This will give an output of 11.36v).

I can't actually think of anything useful that could be powered at 12v with a 1M input impedance (12uA).

You *could* use lower value resistors, but this is incredibly inefficient. Makes a linear regulator pale into insignificance.

Pete.

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