Hey all,

Could someone tell me how to take an unregulated power supply and regulate the voltage to a steady voltage and then reduce the voltage. see below example.

Original Supply = 12V DC nominal, 10.5V min, 13.5V max
Once Regulated = 12V DC
Then Dropped = 5V DC

so my new supply would be 5V DC

 

Use a 7805 regulator (or 78L05 if you only want < 100 mA outout current), 10.5 ~ 13.5 Volt in, 5 Volt out.

__________________
Len

 

that looks like it may work great for what i need, thanks

but is there any way to figure out what to use in general? say if my power supply were 24V, or if i only needed an output voltage of 1.5V?

an answer to my current issue is great but my ultimate goal, and the whole reason for asking the question, is to understand the theory behind the scenes.

 

Variable power supplies that cover that widge a range of voltages are expensive and come in big heavy boxes that sit on a lab bench and can cost hundreds to thousands of dollars. These are the low current kind you would use to test out small circuits. High current kinds are not variable and are less well-regulated.

There are many kinds of power supplies out there. Some are efficient, some have low-noise, others boost voltage, others reduce voltage, some react to changes in power demand faster, some will not function properly in the presence of large transients (noise and spikes). As a result, there is nothing you can use in general.

When you say "learn behind the scenes", do you mean how to use the regulators? Or how to build them? Building them is an entirely different thing and is something you will have to focus on. It's not the kind of thing you can undertake (unless you quickly gloss over the theory without actually using it) while taking on other projects.

Google "linear" regulators, "charge pumps", and here is an introduction to what are called "switching" regulators:
http://www.powerdesigners.com/InfoWe...converter.shtm

Regulator and converter are also interchangeable and will change what results you come up with when you Google. Of course, a lot of this stuff may be over your head so there's foundations that have to be built.

 

Thanks dk,

I have no problem using a technology if i know it works but i am also cursed with a touch of the "why, why, why" disease.

knowing that something works and using it is one thing, knowing why is entirely different. I'm just trying to get my "why" fix here

 

Ah, well. So those articles should be great for just glossing over the theory.

Here are more articles:
http://en.wikipedia.org/wiki/Voltage_regulator
http://www.national.com/appinfo/power/files/f4.pdf
http://www.national.com/appinfo/power/files/f5.pdf

 

If you really do want to first regulate your input voltage to 12 vdc before dropping to 5 vdc you cannot do it with linear regulators with the input voltage range given. A 7805, and LM317 are examples of linear regulators.

As you will learn, a common linear regulator, used as a series regulator, might be described as a power transistor with some control circuitry. The control circuitry monitors the output voltage and adjusts the transistor to a more on or more off state to keep the voltage constant. The problem - the output of a linear regulator in this configuration will always be something less than the input voltage. To get a constant 12 vdc output you'd need to keep the minimum input above 12 vdc by a few volts - the exact amount is dependent on the regulator selected.

For many applications applying the 10.5 to 13.5 vdc to the regulator to go to 5 vdc is works fine - keeping the current limits of the regulator in mind under those conditions.

__________________
stevez