How do you reduce 100Vdc to multiple outputs of 12vdc 24vdc and 48vdc?

Answer 1

I wish I had the perfect answer for you because it's actually a very interesting question. Typical responses might include using DC regulators that cannot handle your input voltage, or perhaps Zenier diodes to clamp the voltage down. Neither is a good solution.

What you're trying to achieve has been done for many years in VCR's, TV's, and many new digital appliances. They are faced with a somewhat similar dilemma to you in that they have 120 volts AC (here in the US) and they need to reduce it down to some common levels such as +12 volts, -12 volts, and/or +5 volts.

Here's how they do it:

First, they recognize the uselessness (due to inefficiency and heat generated of just using a step-down transformer) by converting the 120 volts AC to DC (believe it or not) with a bridge rectifier. It is also filtered a bit with capacitors and perhaps even clamped a bit with a Zener diode.

The new 120 volts DC is then fed to an oscillator circuit which drives a high-powered switching device (a transistor for example) into their own custom-made transformer. The output taps for the transformer are wound proportionally to achieve the desired voltage.

Next, each output is rectified (because it's AC) to DC, filtered with a capacitor, perhaps regulated (with a regulator like an LM7812 for 12 volts). The now have their desired output voltages.

Constructing such DC-to-DC converter would be pretty much impossible unless you plan on hiring an engineer to design it and expect to make millions selling it. Yea, I know this doesn't answer your question, but that is how it's done in the "real" world.

Note that you can't use an LM317 voltage regulator because your input voltage exceeds the maximum of the device. Perhaps someone else will know of a regulator than can handle 100 volts, but I don't know of one.

I would search the 'net for DC to DC inverters and see what you find. I haven't done any searching myself, so you may find nothing.

As an interesting aside, why 48vdc? Are you building your own Telco switch? ;-)

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In many circuits, a voltage-dropping resistor or resistors are used for this function. This will work if the need is for a bias or voltage level (and not to supply power). You need to know the current to calculate the proper resistor value from Ohm's law, and also make sure the resistor can dissipate the power (heat) that may be generated.