Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

RC Delay, Beginner 15 Year old EE, need help !

  1. Jun 29, 2012 #1
    I've been experimenting for hours, and my LED still won't light correctly when this method is used, and the fluke 87 reads voltage on the circuit in a non-logical way. Can you help? Here's what I have set up when using an online calculator:
    Time delay: 2000 milliseconds
    Power supply: 15v
    Capacitance: 100μF
    Resistance: 100KΩ
    I want to achieve a result voltage of 2-2.2V, 20ma
    Is this possible via this method?
     
  2. jcsd
  3. Jun 29, 2012 #2
    Could you please draw out your circuit and post it? Also measuring from the negative terminal of the supply (black lead) could you post the voltages (red lead) at various nodes in the circuit?
    That will help a lot.
     
  4. Jun 29, 2012 #3

    berkeman

    User Avatar

    Staff: Mentor

    Welcome to the PF.

    You have a 15V power supply feeding the RC to get a time constant of a second or two (right?). Where does the LED come into play? You can't drive the LED through the 100k resistor...
     
  5. Jun 29, 2012 #4
    Here is my circuit:
    http://caltechhelp.net84.net/ledrc.jpg
    Electricity flows from the orange (-) to the positive (+)
    Sorry for the late reply, I'm new here and had to upload the image to one of my servers (I'm a web developer)
     
  6. Jun 29, 2012 #5

    berkeman

    User Avatar

    Staff: Mentor

    Your image is getting blocked as a high-risk website. Can you just attach the JPG image to your post? Look for the paper clip icon in the edit window to upload an attachment.
     
  7. Jun 29, 2012 #6
    I completely missed that, thank you! And that's because of my hosting service..
     

    Attached Files:

  8. Jun 29, 2012 #7

    berkeman

    User Avatar

    Staff: Mentor

    Can you upload the schematic?
     
  9. Jun 29, 2012 #8
    I know the schematic is horrid, but please bear with. Additionally i noticed my cap's were to smallin size, so i changed them. The led lit very dimly, then blew.
     
  10. Jun 29, 2012 #9
    Never mind, it just quit lighting. I just tested it on another circuit. I apologize for any confusion!
     
  11. Jun 29, 2012 #10

    berkeman

    User Avatar

    Staff: Mentor

    The schematic is unvisible! :tongue2:
     
  12. Jun 29, 2012 #11
    Lol sorry XD
     

    Attached Files:

  13. Jun 29, 2012 #12
  14. Jun 29, 2012 #13

    berkeman

    User Avatar

    Staff: Mentor

    Yeah, you can't drive an LED through a 100k Ohm resistor.

    You need to do something different to drive the LED. Are you wanting to have a couple second delay after throwing a switch before the LED lights up? You would need to add something separate to sense when the RC waveform rises above some level, and then use a transistor or something to drive the LED.

    If you drive the LED from your 15V supply, you will need to choose a series resistor to drop about 13V (most LEDs have about 2V across them when lit), and set the current at 10-20mA (whatever the LED current spec is). For example, if you wanted 20mA through the LED with 13V across the resistor, the resistor in series with the LED would be 13V/0.02A = 650 Ohms.
     
  15. Jun 29, 2012 #14
    I have run the led's previously, but I want to create a time delay (preferably make two different led's flash at different time intervals).
    My supplies to achieve this are as follows:
    (2x) https://www.amazon.com/BB400-Solderless-P…
    https://www.amazon.com/Joe-Knows-Electron…
    https://www.amazon.com/Joe-Knows-Electron…
    https://www.amazon.com/Solderless-Flexibl…
    https://www.amazon.com/Joe-Knows-Electron…
    http://shop.vetcosurplus.com/catalog/pro…
     
    Last edited by a moderator: May 6, 2017
  16. Jun 29, 2012 #15

    berkeman

    User Avatar

    Staff: Mentor

    Look at using 555 timer chips or similar to generate the delay functions you want, and then drive NPN transistors as switches to turn on your LEDs. Just Google 555 timer LED driver, or similar to see various circuits that you can use.
     
    Last edited by a moderator: May 6, 2017
  17. Jun 29, 2012 #16
    I've come across those alot.. I guess I'll have to tough it out and start buying diods and such? Lol, can you provide me with links to good components for beginners, with a quantity high enough to blow a few or more of each? XD Thank you!(:
     
  18. Jun 29, 2012 #17

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    None of those links work. Would it be possible to repost them?
    True, especially when there are capacitors in series with that resistor :confused:
     
    Last edited by a moderator: May 6, 2017
  19. Jun 30, 2012 #18

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Google search on reading schematics

    My thought is, if you read through or watch some of sites on that list you will then be able to draw us a schematic diagram. We need it to help you.

    Also it may help you with getting your circuits working.
     
    Last edited: Jun 30, 2012
  20. Jul 1, 2012 #19
    That circuit looks an awful lot like a circuit for driving an LED using alternating current. The LED would rectify the small alternating current and the resistor would limit the current so as not to burn out the LED. The capacitor would couple the AC to allow enough current to flow to allow the led to light.

    If you use DC then you would not get any current through the capacitors. I supposed if you had a very unfiltered DC power supply the ripple might cause the LED to light.
     
  21. Jul 9, 2012 #20
    I am using a 15v dc 30mA power supply, which i found laying around and soldered/shrink wrapped breadboard jumpers on, for easy experimentation. Is it not possible to power though a capacitor this way?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook