Easy way to get 12v DC from 32v DC bus?

Question

I've got a 32 volt DC regulated bus. I also need about 30mA at 12 volts (DC). Is there an easy way to tap some power from the 32v bus?

My first thought was just put in a 12v linear regulator. But those in my parts box have absolute ratings close to (or below) 32v, and I'm not one to push the envelope.

Some divider make up using hairy arsed power resistors might work, but that's not too efficient and generates HEAT.

If the solution gets complicated (e.g. build a buck down converter), I could find a 12v wall wart and tie across the mains supply to my 32v power supply.

I know about voltage multiplier circuits. Is there a neat voltage divider trick somebody can teach a nubie?

Answer 1

this is typically handled with a "switching regulator". A "buck converter" is a type of switching regulator designed to regulate down from one voltage to another. There is any number of prebuilt modules you can find that accept 32V and regulates to 12V. Cheap ones start at a dollar and a month shipping from China, but quality costs more.

Answer 2

The linear approach

A buck converter is the most efficient way to go, and should be easy to find. Other answers cover that. I want to suggest a simpler brute-force method that you may have overlooked from the sound of your question: The good old LM317.

This is a very common linear regulator, but the "trick" compared to its fixed-output 7812 relative is that it's floating. It has a maximum voltage difference of about 40 volts. Your input-to-output difference is 20 volts.

It will dissipate 20 * 0.3 = 6 watts of heat, so it requires a reasonable heat sink, but since you have not given any more details about the project, this may not be a problem. If it is, you need a buck converter.

Answer 3

Have a look at these buck converters from TI, they're a tested, complete buck converter on a module which meet your input and output voltage and current specifications, with a lot more efficiency than a linear regulator. They're quite expensive, and you could design and implement your own buck converter for less, but they do provide a lot of convenience.

There are other parts which cost less, and are pin compatible with a TO220 linear regulator such as a 7812, but they have more restrictive maximum ratings, so you would need something else as well to lower the input voltage a little. An example of those is this module from TI.

Other manufacturers are available and found by searching for buck converter module.

Answer 4

The Art of Electronics, 3rd ed (Horowitz & Hill) 9.13.2 suggests using a depletion mode FET follower to hold the LDO's Vin a couple of volts above the regulator's required input voltage. Vgs is about -1.5v. Higher values can be obtained by connecting the gate to a resistor divider between the regulator output and FET source terminal.

Here's a portion of the circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Resistors and caps required by the LDO are not shown. I assume, but have not bench checked, the strategy would extend to other regulators.