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Newbie questions about resistors and measuring voltage and current

Discussion in 'General Electronics Discussion' started by misteryoyogi, Apr 11, 2021.

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  1. misteryoyogi

    misteryoyogi

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    Apr 9, 2021
    I have a simple circuit:

    - 9v Battery
    - 1 LED Light
    - 1 Resistor at 1000 ohms

    Question about Resistors

    I'm aware that polarity doesn't matter with resistors, but does it matter if the resistor is placed after the LED if we're looking at the circuit from positive to negative. Probably a foolish question, but i have to ask.

    Measuring Current with a Multi-Meter

    When measuring current, the probes need to go in series with my circuit. Does it matter which probe I connect to the positive or negative side of my circuit?

    This is my proposed way to check my current at the moment: https://postimg.cc/F7MD1mKb
    The probes are disconnected by the way.

    Also, when measuring current, does it matter where I start measuring? For example, if I measure current before (or after) the resistor, would it make a difference?

    Measuring Voltage with a Multi-meter

    When measuring how much voltage a component is drawing from a circuit, if it's an LED, can I just put the probes on the legs? Or is it recommended to place cables in parallel with the component?

    Thanks!
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    In this series connection the sequence of battery and resistor is irrelevant.
    Yes and no:
    When you connect the "+" probe towards the "+" side of the circuit and the "-" probe towards the "-" side of the circuit, the meter will indicate a positive current.
    When you connect the probes reversed, your meter will indicate a negative current.
    With old analog meters the direction was in most cases important: These meters could measure only positive quantities (voltage, current) and when connected in reverse could not indicate a value (except the more rare instruments with a center zeor indicator).
    ??? The probes are attached to cables, aren't they?
     
    misteryoyogi likes this.
  3. misteryoyogi

    misteryoyogi

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    Apr 9, 2021
    Thanks a lot for the reply Harold, that definitely answered my questions. About the probes and LED legs, I was more concerned if it was safe. I'm really, really new, so I'm kind of paranoid about everything.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,794
    2,749
    Nov 17, 2011
    With a 9 V battery you can't do much that is unsafe. Otherwise this type of battery couldn't be used in children's toys.
     
    hevans1944 likes this.
  5. misteryoyogi

    misteryoyogi

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    Apr 9, 2021
    That makes a lot of sense. Now I can lay off the paranoia a bit :)
    Thanks again.
     
  6. hevans1944

    hevans1944 Hop - AC8NS

    4,630
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    Jun 21, 2012
    When a 9V battery reaches the end of its chemical charge, there is still enough chemical activity inside to produce a small terminal voltage, although usually much less than nine volts. You can only draw a few micro-amperes of current from it before the internal resistance drops the terminal voltage to zero. So, a "dead" 9V battery is as safe as a lump of coal... or a rock.

    However, the "button and snap-ring" design of these batteries allows two or more (sometime a LOT more) batteries to be connected in a series-aiding configuration. Theoretically, one hundred of these batteries snapped together and connected in series would produce 900 volts DC. If the batteries were "fresh" alkaline batteries that would be a very dangerous situation.

    I personally have connected ten or so "fresh" 9V batteries in series, just out of curiosity, to see how much open-circuit voltage that would produce. Yep, about 90V DC there, enough to give you a nasty little electrical shock if you are careless. Moral of this tale is this: many things (like 9V batteries) are "safe" when used singly or in small numbers, but become "un-safe" or even lethal when large numbers are involved. Remember this advice when you eventually encounter diode/capacitor voltage multipliers.

    You are right to be cautious while learning about electricity and electronics, but paranoia stems from ignorance. So, ask lots of questions, here and elsewhere, while you are learning. There are no stupid questions, but you will find there are a lot of stupid answers. The monitors of these forums, people like @Harald Kapp who responded to your question, almost always give excellent and well-informed advice. Other members may do the same, but you will have to find out for yourself who they are by engaging them in a dialog (back and forth) conversation, and comparing your experience with their advice.
     
    Last edited: Apr 11, 2021
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