NerdKits - electronics education for a digital generation

You are not logged in. [log in]

NEW: Learning electronics? Ask your questions on the new Electronics Questions & Answers site hosted by CircuitLab.

Support Forum » Ac Power Adapter as power source

February 13, 2011
by dgikuljot
dgikuljot's Avatar

Hey guys, I want to use a wallwart ac-to dc adapter as a power supply. so i remeber readyign that it is good to use capaciators to redeuce the noise of the aapter. I was wondering if anyone had any specifics on this. Will a 220 mf capaciator be fine. Is the capaciator supposed to be in series or parallel with the power supply? Thanks

February 13, 2011
by hevans
(NerdKits Staff)

hevans's Avatar

Hi digikuljot,

My suggestion when using a dc power adapter is to get one that outputs around 9V, and then run it through the power regulator to regulate it down to 5V, the power regulator will take care of most of the wiggles in your 9V supply.

Humberto

February 13, 2011
by dgikuljot
dgikuljot's Avatar

Ok thank you Humberto, Still i was wondering if i was going to use the capaciator to reduce the noise, would i put it before or after the voltage regualtor. also would it be in series or parallel with power supply. Thanks

February 14, 2011
by hevans
(NerdKits Staff)

hevans's Avatar

Hi dgikuljot,

That depends on where you want to reduce the noise. If you want to shield the MCU from the noise, you would put it in after the regulator. To reduce noise you want to put it in parallel with the power supply, so across the power rails.

Humberto

February 14, 2011
by Ralphxyz
Ralphxyz's Avatar

Essentially that is what the 104m capacitor you put on pins 7 and 8 when doing the initial setup does.

Adding capacitors in parallel increases the total capacitance of the circuit (Google "parallel capacitors").


Now I'll let one of the experts explain what this means (I had been meaning to ask this very question).

Does greater total circuit capacitance make the circuit more or less sensitive to noise. Is bigger always better?

I have seen suggestions to use step capacitors, real small, small, medium, bigger. But never have seen

a good explanation of what was going on.

Ralph

February 18, 2011
by hevans
(NerdKits Staff)

hevans's Avatar

Hi Ralph,

It really depends on what it is you are trying to do. When it comes to smoothing out a power supply, you are usually just looking to keep as steady a DC voltage as you can, so bigger doesn't hurt. A bigger bypass cap on your power supply will slow down how fast your device turns on, but you need a really big capacitor before you start noticing an issue there.

In other places in your circuit though, the size of the capacitor can be much more relevant. When building filters, or other circuit knowing how the capacitance you add affects you is quite important.

Humberto

March 13, 2011
by TimCole
TimCole's Avatar

One important function of bypass capacitors is noise suppression. You can think of a capacitor as a frequency-controlled shunt. Low-frequency signals (or noise) can pass through, but as frequency increases, the capacitor passes the signal (or noise) more readily. Using different sizes of and types of capacitors allows you to shunt more high-frequency noise away from your circuitry.

This sort of thing can be very difficult to model or analyze, so most designers resort to rules of thumb to choose bypass capacitors. Placing 104s across the power supply pins on digital ICs is one of those standard rules of thumb.

Post a Reply

Please log in to post a reply.

Did you know that you can connect a computer keyboard to your microcontroller? Learn more...