# Need info on part of a circuit - voltage regulator?

Discussion in 'General Electronics Chat' started by 633squadron, Nov 29, 2008.

Nov 28, 2008
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I know bits and pieces of electronics from a mis-spent youth of physics, ICs, and computer science. My practical knowledge is really rusty, though, so I've started slowly re-studying again.

I found an interesting and cheap science kit on the Make Mag website, from Sparkle labs. It's beginner enough for me at the moment, but since it's aimed at kids it doesn't explain everything.

So, the first project is to light up an LED, a good variation on the old battery and light bulb. It's basically 9V battery, LED, and resistor.

What puzzles me is some of the extras. I have attached a schematic. There's a voltage regulator and two capacitors. I only dimly understand why they're there. The regulator cuts the 9V down to 5V, but why the capacitors? And why in parallel?

I thought that maybe the capacitors keep the current steady to the LED, but if so, why not have them in series past the regulator? And what the heck is inside the regulator?

Also, I assume that the resistor is there to prevent a short circuit. Is that correct.

And finally, anyone have some good drawing tools for schematics? I tried using Visio, but it's shapes are pretty weak.

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2. ### Wendy Moderator

Mar 24, 2008
21,771
3,023
Go to my blogs, you can download PaintCAD. It's free, and pretty easy. You'll see my schematics all over this site. My cookbook is here, a lot of the circuits are duds, but it is a stash point for posting prints here as we discuss various prints.

My blogs can be accessed by clicking the colored number on the upper right hand corner of this post.

All an LED really requires is a battery, resistor, and the LED. That circuit is a major case of overkill. The 7805 is a simple 3 terminal regulator, any voltage in, 5 volts out. The caps are to keep it (7805) from oscillating. An LED does not self limit current like a light bulb, so the resistor is needed.

This is a good site to relearn from. There is also a decent book, All About Circuits, online.

3. ### leftyretro Active Member

Nov 25, 2008
394
5
Last question first. There are many free drawing programs avalible and everyone has their favorite. I use the one avalible here:

Caps in drawing? They are nothing to do with the LEDs, they are recommended by the voltage regulator manufacturer to help prevent voltage oscillations under certain conditions. The circuit would more then likely work without the capacitors installed. It's always a good idea to search for and download a copy of the data sheet for any IC that is new to you. They will explain any recommended extra components and frequently give good example circuits showing those recommendations.

What's inside the regulator? It's basically an automatic resistor that the chip controls to drop more or less voltage across such that the output voltage stays constant under any current load within it's specifications. In your example it is dropping 4 volts such that 9-4=5vdc. If the load circuit changed needing more or less current then the regulator would 'adjust' this resistor to maintain a constant 5vdc output. It's an automatic ohms law calculator that never sleeps

Make sense?

4. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Welcome back to the fray. We try to have fun here while we're learning things.

OK, the way the schematic is drawn makes it a bit unconventional. If you rotate it 90° to the left and added a ground symbol to the bottom wire, it then becomes more conventional. In this case, placing the ground symbol indicates where the 0v reference point is. Any wire or component connected to a ground symbol implies that the voltage at that given point is zero.

In this particular circuit, the 1uF capacitor on the battery side really isn't necessary unless the wires to the battery are more than about 6" long. If they're longer, you need the capacitor to help prevent high frequency oscillations due to the inductance in the wiring to the battery.

It is typical to at least have a 0.1uF (100nF) cap on the output of a voltage regulator; this helps prevent possible high frequency oscillations. Additionally, a 10uF cap will help a great deal to improve transient response.

You would not want them in series. Capacitance in series is calculated like resistors/inductors in parallel; capacitance in parallel is calculated like resistors/inductors in series.

If you took two 1uF capacitors and connected them in series, you would have an 0.5uF capacitor with double the voltage rating.

That is best determined by examining a datasheet. I've attached a datasheet for a 78L05 for your perusal. A Zener diode, 14 transistors, 17 resistors - all in one neat little package.

The resistor is there to limit the current flowing through the LED.

LEDs need to have the current flowing through them controlled. The resistor was selected to allow just a certain amount of current to flow through the LED.

See our Resources section.

Here's a good tutorial on Cadsoft's Eagle that's combined with a project so you can try out what they're teaching you:
http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=57&page=9

Linear Technology's LTSpice/SwitcherCad is good (and free) circuit emulation software, available here:
http://www.linear.com/designtools/software/

• ###### UA78L05ACLP - POSITIVE-VOLTAGE REGULATORS.pdf
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Nov 28, 2008
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I am bowled over. Most forums I work with don't provide nearly as much help, and not nearly as fast! I guess people are bored over the long holiday.

The info on the voltage regulator is enormously helpful, particularly the part about the capacitors. The kit instructions should have said that this part of the circuit is extra stuff, and then it should have explained what it does.

Thanks, too, for all the links to schematics drawing programs, etc. Very useful.

Nov 28, 2008
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SgtWookie suggested that the schematic looks a bit weird.

I see several different standards out there, alas. However, I've attached what I think he wanted for my circuit. Is this "correct"?

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7. ### SgtWookie Expert

Jul 17, 2007
22,201
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Yes, what you have now is more along "traditional" lines.

Usually, inputs and sources are on the left, processed in the middle, and outputs are on the right. It helps viewers of the schematic comprehend it much more rapidly if drawn in such a fashion; after all, one reads a book from left to right (unless you're reading Chinese) so keeping schematics organized in a similar fashion makes reading it seem more natural - at least for westerners.