All-in-1 electronic lab gives steps, explains nothing

Discussion in 'General Electronics Chat' started by darchangel, Jan 4, 2008.

  1. darchangel

    Thread Starter New Member

    Jan 4, 2008
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    I'm a programmer who recently got the itch to start dabbling in circuitry and basic electronics. I bought one of those all-in-1 electronic kits where there are components on a board with springs where you can connect wires (specifically Maxitronix Electronic Lab 75 in 1). It comes with a book telling you how to do a lot of stuff with it but there's little to no explanation about WHY they did things the way they did. In every diagram, I'm asking myself: Why a resistor there? Why do they use a resistor or capacitor of X instead of one with a different value? Why do they use 4 AA batteries in this project and only 2 in that one?

    Not only is it intensely frustrating, I also don't yet know enough to phrase my questions intelligently.

    Is there somewhere to find some of these rules of thumb? For example, unlike the resistors and capacitors, none of the LEDs or transistors have any numbers/ratings associated with them. How do you know how much or little current to allow at any given place? I'm open to any/all advice that one can give a novice.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    It is perfectly ok to be frustrated. These kits with few exceptions rarely provide the level of explanation that a truely inquiring mind wants to know. All I can tell you is that you will have to do the investigative work and ask questions to get the explanation you seek.

    A good place to start your learning experience is to review the material available here at www.allaboutcircuits.com. THere is a
    wealth of material of a basic nature to help you out.

    I don't recall ever seeing an LED that was labeled on the device itself as to the forward voltage and maximum current. You have to have the part nubmer to determine that.

    In the absense of any details on the part you can find out the basic information you need to know by measureing it yourself. This is a good way to learn more about the way an LED works. Be prepared to destroy something along the way. It is part of the initiation process.

    hgmjr
     
  3. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Reminds me of "Electron", a kit where components were fastened with springs, that also would serve to fasten wires between them. The manual also didn't explained how the circuits worked.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    For standard LED's, a general "rule of thumb" is to limit the current to 15mA.

    Red and green standard LED's are usually rated at that current somewhere between 2.2V and 2.7V.

    Let's say you're using four AA batteries in series to get +6V.
    In order to have 15mA to flow in a 6V circuit, you would need to calculate the resistance:
    R = E/I
    R = 6V/ 15mA
    R = 6V / 0.015A
    R = 400 Ohms
    So, drag out a potentiometer (pot) of say, 500 Ohms, 1K Ohms, or your smallest pot that is greater than 400 Ohms. Set it initially to read 400 Ohms, and connect it up in series with your LED (the short lead of the LED goes to negative side of the supply) along with your DVM set to read milliamps across the batteries. Decrease the value of the pot until you read just under 15mA.

    Then disconnect the batteries, remove the DVM from the circuit, set it to read Ohms (1k or 2k scale), and measure the resistance of the pot. Choose the closest fixed-resistor value that is >= the value you read from the pot for the current limiting resistor.

    While you have the pot and LED hooked together, connect them to the batteries again, and measure the voltage drop across the LED itself. Then you will know at what voltage it draws 15mA.

    When the voltage @ current rating for an LED is known beforehand:
    1) Subtract the Vled from the supply voltage
    2) For each transistor or diode between the LED and the supply voltage or ground, subtract 0.6V
    3) From the remaining voltage, calculate the required resistance to limit current.

    For example, let's say you wanted to light an LED rated 15mA @ 2.5V that was being supplied a ground via a transistor's collector, and your supply is 9V.
    9V - 2.5V - 0.6V = 5.9V
    R = E/I
    R = 5.9V / 15mA
    R = 5.9 / 0.015
    R= 393.333 Ohms
    This is a borderline case; 390 Ohms is a standard value, the next higher value in the E24 series is 430.
    Resistor series table: http://www.logwell.com/tech/components/resistor_values.html
    If we worked our way backwards, we'd find:
    I = E/R
    I = 5.9/390
    I = 15.128mA
    If we're using precision resistors, the 390 Ohms would put us about 1% over the rated current. If you like living on the edge, you could use the 390 Ohm resistor - or you could use a couple of 200 Ohm resistors in series, a couple of 820 Ohm resistors in parallel, or opt for a higher resistance.
     
  5. Salgat

    Active Member

    Dec 23, 2006
    215
    1
    I remember when I first got into electronics how all the kits I bought would give you schematics, a printed circuit board, and the parts. Never explained how the circuits did what they did though. Once you gain an elementary understanding of electronics, the circuits start to make sense. Like said hgmjr said, check out the book on this site, even I use it on occasion as a reference.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    My recommendation for getting started in electronics is the "Electronics Learning Lab" cat# 28-280, that Radio Shack carries. It's $65, and it's worth that for the workbench alone. The lab was designed and the two books written by Forrest M. Mims, and he did a really good job of it.

    In the beginning of the 1st book, he explains what each component does, and the basics of how they work. As circuits progress, the operation of each circuit is not described in detail, as he expects you to be learning from the material presented previously. However, key points of the circuits are gone over, such as the elements that control the frequency of an oscillator or a filter.

    He's also written a series of books and mini-notebooks on electronics. They're easy to read, yet cover a great deal of useful information. You can order his books online here:
    http://www.forrestmims.com/
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    I concur with sgtwookie's suggestion.

    The radioshack lab together with Forrest Mim's material is a great way to kick start your interest in electronics.

    Mr. Mims is actual a member of this forum though he has better things to do with his valuable time than to spend it here.

    hgmjr
     
  8. darchangel

    Thread Starter New Member

    Jan 4, 2008
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    0
    GREAT information! Thanks to all. Is there a standard current to not exceed for transistors?
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    Each transistor with a given part number can and will differ. All transistors should be used in accordance with its manufacturer's design specifications.

    hgmjr
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Each transistor will have a spec sheet with all the do not exceed voltages and currents spelled out. It is worth devoting some hard drive space to a collection of spec sheets on every semiconductor device you have. Can't tell the NPN's from the PNP's without 'em. Some packages, mostly TO-92, have variable pin outs which is hard to tell with the spec sheet for that device from the manufacturer who made it.

    Like getting into MS operating systems, the study of electronics at the component level takes on the aspect of a lore. And I still say 2N2222's in the TO-18 package perform better than the 2N2222's in TO-92.
     
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