Voltage Regulator

[vission]

Hello, I know just enough to be dangerous, OK perhaps I know a whole lot less then that.

I have a regulated power supply with a 12 volt output. I want to drop the voltage to 9 volts with a total output of 1.5 amps.

I guess the first question would be should I be using a LM317 or something else? I have seen many talk about diodes in series to drop a few volts, but that seems harder to control if anything in the circuit fluctuates.

The next concern I have deals with the typical capacitors used with a variable voltage regulator. If the source of power into my circuit is already regulated and 'clean', are the capacitors still needed to support my circuit/regulator?

I guess another way to ask this would be since I have regulated and clean power going into my circuit, will I automatically get clean power out even if I don't use the capacitors? Or is there noise inherent with the LM317 which requires the capacitors. My entire circuit consists of the LM317 and two resistors to set the desired voltage.

Thanks.

 

[ronv]

The capacitors are there more for transient response rather than steady state or ripple rejection. The one on the input holds the input voltage steady if there is some inductance in the line from the bulk supply to the regulator. The one on the output supplies the load during changes in load until the regulator can respond.
**broken link removed**

 

[vission]

Hey thanks for the response, that makes a lot of sense especially on the output side since I am guessing that not much reacts faster then a capacitor. So as the regulator works at keep the voltage constant there is a delay in the time involved as the LM317 normalizes when the load changes. During this delay the capacitor helps steady the output until the regulator catches up.

I read the specifications on the LM317 you sent to me but I did not see anything about the amount of time needed by the regulator to normalize the voltage. Are these times even known or do they vary so much that it makes no sense to include them in the specifications. Knowing the response times would help in determining the size of the capacitors, wouldn't it?

I am sorry for all the questions, I am very new at all of this.

 

[crutschow]

Some linear regulators also require an output capacitor for stability (to avoid oscillations).

 

[ronv]

Page 8 has some pictures of what happens with and without the capacitors. Page 9 addresses the input capacitor - but not much detail. Also notice the capacitor on the adjust pin, but read about protecting the device when these are used.