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this is an easy resistor question — Parallax Forums

this is an easy resistor question

RontopiaRontopia Posts: 139
edited 2007-05-17 04:40 in Robotics
so something very special has· happened to me.. yep.. I got my first multi-meter. I know its a big deal when·a man picks out his first meter..

ok enough joking around. 1 megaohm

what is a megaohm? and if I have 2.1 of them how do they work in this

··········3.0v
··· I = ----
··········2.1 Mohms or megaohm's

thanks for the time..



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IC layout designer
Austin Texas

Comments

  • SailerManSailerMan Posts: 337
    edited 2007-05-04 01:35
    M,Meg = Million



    I = 3.0v
    ···· ----
    ···· 2,100,000 Ohms

    I= 1.428uA



    Post Edited (SailerMan) : 5/4/2007 1:43:20 AM GMT
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2007-05-04 01:36
    Rontopia,

    a Megaohm... really? A megaOhm is 1 million Ohms

    Your equation is for current (I)... Again... really? I = 3V / 2100000 Ohms = 1.428uA (micro Amps) or 1428 nA (nano Amps)

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • RontopiaRontopia Posts: 139
    edited 2007-05-04 01:48
    duh.. ok thanks.

    I have never really done layout on a resistor that is in the MOhms before. I knew it was stupid when I pushed the submit button.. must be getting punchy... I should go home.

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    IC layout designer
    Austin Texas
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2007-05-04 02:36
    Rontopia,

    That would be a HUGE resistor in layout.



    Is that your RF design on the Avatar?

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2007-05-04 03:33
    RF design?... it's the Monopoly board.
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2007-05-04 03:54
    Pj,

    lol - It looks like there are two balanced coils with a third coil as a reference coil. The design actually looks familiar to me for some strange reason... When I worked at National Semiconductor, some of the Georgia Tech
    students were putting together designs very similar to the one Rontopia is using as his Avatar.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • RontopiaRontopia Posts: 139
    edited 2007-05-04 14:35
    Yes it’s a RF CDMA chip I did in England a few years back. That was a great contract for me... They paid for my Flight, car, and a hotel just out side Cambridge. It was hard to learn to drive on the wrong side of the road[noparse]:)[/noparse] I had people cussing me up and down, although I had no idea what they were saying.. Which posses a question? Is it really cussing if you don’t know what you’re saying? like "sod off you bloody tosser!! " I have no idea what that means but I was sure told to do it a lot. [noparse]:)[/noparse]
    Im actually leaving to do some work on some in NY in a few days. I hope that will be cool as I have never been there. I will only be there 4 to 5 weeks... I can’t remember the company right now... but it’s for RF and Bi-Cmos as well.
    ·
    Anyway... the question I had last night was because i had a new multi meter and I had no idea what it was saying. M to me could have stood for u or m... I just wasn’t sure. I was trying to find the resistance across a LED and I had no idea they were that resistive.


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    IC layout designer
    Austin Texas
  • Skywalker49Skywalker49 Posts: 172
    edited 2007-05-04 14:52
    resistive in one direction, much much less in the other direction. Try that one!
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-05-04 14:59
    LED's are resistive in one direction, and conduct in the other AS LONG AS you've reached 1.4 or 2.1 volts. Most Volt-Ohm Meters use a 1.5 volt battery for their resistance check function, so won't forward bias an LED sufficiently for it to conduct. For that matter, normal diodes have a 0.6 volt drop when forward biased, so Volt-Ohm meters have a special setting to be able to check them.
  • RontopiaRontopia Posts: 139
    edited 2007-05-04 15:31
    ok so.. what your saying is I need to check it how? the 2.1 MOhms would that be consistant with checking it in the reverse bias direction? its a small cheap led..I think I got 10 or 20 of them for 2.99 .

    so do you think Im checking this in the reverse bias direction? to get 2.1 Mohms?

    I have never checked an led this way, I usually just hook them up and be done with it.. but in this case I need to know the I or the current

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    IC layout designer
    Austin Texas
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-05-04 16:58
    Then you'll need to configure your meter as an ammeter -- an amp-meter -- a current meter. You'll need to put it in series with your LED and the circuit that's driving it. Then have your circuit turn on the LED, and read the current on the ammeter.

    You can't just "check" an LED using only a multi-meter -- they don't usually put out enough voltage for that.
  • Skywalker49Skywalker49 Posts: 172
    edited 2007-05-04 17:38
    Allan,

    you got me confused but I'm back in the game now. The good old simple silicon diode
    has a voltage drop op 06 - 0.7 volts when in conducting mode.

    LED are made out of other material (e.g. gallium) and the voltage drop in conducting mode is much higher.

    Thanks,
    Ed
  • UncandayUncanday Posts: 28
    edited 2007-05-17 03:22
    It doesn't really make much sense to measure the resistance of an LED since it doesn't have a linear voltage/current characteristic. To a first order approximation, in the forward direction there will be a more-or-less constant voltage drop across it. Best way to figure that out is to hook it up in series with, say, a 470 ohm resistor and maybe a 6V power supply (or whatever you have), and then measure the voltage drop across the LED with the multimeter. Off the top of my head, I think it's about 2.3 V for a standard red LED.

    To determine the series resistor you need for a given current just subtract that voltage from your supply and divide by the current you want to find the necessary series resistance.

    I'm guessing that the 2.1MOhm you're seeing in reverse bias is a function of measurement error. Are you using an analog or digital meter?

    Hope you enjoyed Cambridge, btw; I lived there for many years. Don't think I could put up with the traffic there now, though...
  • Mike GreenMike Green Posts: 23,101
    edited 2007-05-17 03:36
    The forward voltage for a standard red LED is about 1.7V. This is a function of the material the diode is made of. Germanium has a band gap voltage of about 0.3V. Silicon has a band gap voltage of about 0.7V. Gallium Arsenide has a band gap voltage of about 1.7V, etc. The band gap is a kinetic energy barrier that the electrons have to jump to conduct and it takes the band gap voltage to jump it.

    If your meter only uses a 1.5V battery for its resistance measurements, the LED will appear to have a nearly infinite resistance since the electrons can't jump the band gap barrier (except for the occasional one that might cheat and tunnel through the barrier or go around ... moisture, etc.).

    "easy resistor question" ... Hah!

    Post Edited (Mike Green) : 5/17/2007 3:41:37 AM GMT
  • UncandayUncanday Posts: 28
    edited 2007-05-17 04:40
    yes, you're right, it's about 1.7V for a standard red LED. Varies according to color, naturally, since they're made from different materials. There's a table here which may be useful:

    http://www.kpsec.freeuk.com/components/led.htm

    Regards
    Duncan
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