Power Supply and Ohm values Help....

Discussion in 'The Projects Forum' started by Needs2TinKer, May 1, 2016.

  1. Needs2TinKer

    Thread Starter New Member

    Apr 26, 2016
    4
    0
    Hello all,

    First and foremost I will say I'm new to the electronics field, I've only had real experience in machining and fabrication.
    So apologies in advance for any silly mistakes. I have tried to immerse myself in as much as I can understand concerning electronics.
    I just have a few questions in some areas that don't make sense and would appreciate any input

    My project is a home built AC unit using rather basic means. Will my project be that efficient? I doubt it but I will have fun making it and learning
    Ive taken into account of all parts I need but Im having a few issues with understanding some terms and wasn't able to find an answer that is clear to me.

    So:
    I'm going to be using a 12V 8.5Amp, 102 Watts DC power supply to power to 4 devices which require a total of 73.2 watts combined.
    Ive taken into account the resistors I will be needed( If I've done my maths right) but when looking at resistors and their values, this additional info is part
    and parcel of the resistor "1/2W Watt 0.5W Metal Film Resistor ±1%". From what I can gather it states the obvious of 1/2 a watt of power but I don't
    understand how that applies so a device that may be using 12 V at 5amps for 60 watts ...........?
    Please see the attached PDF where I have input details and done a very basic design of my per say circuit. Im just not sure if the "1/2W Watt 0.5W Metal
    Film Resistor ±1%" values are the correct ones for this project

    And any recommendations to parts to use would be greatly appreciated

    Many thank.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,978
    744
    Draw the circuit diagram out so we can see what you need.
     
  3. Veracohr

    Well-Known Member

    Jan 3, 2011
    550
    75
    For each resistor the power it will dissipate is P=I^2*R. Use a resistor with higher power rating.
     
  4. Needs2TinKer

    Thread Starter New Member

    Apr 26, 2016
    4
    0
    I've done the circuit as accurate as to how I would understand it to be. I have also added the aspect of a potentiometer to control fan speeds of the radiator fans. I do apologize for my lack of understanding and if the design is wrong, I'm currently finishing night shift and haven't really slept much. Any and all criticism is welcomed and I will endeavor to learn from your instructions and what I can learn on my own part.

    Many thanks again.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,978
    744
    Don't understand why you want to use resistors, if the items run on 12v dc just put them on the supply...
     
  6. Needs2TinKer

    Thread Starter New Member

    Apr 26, 2016
    4
    0
    I would have thought that I would have needed to limit the current as the devices use less that 8.5amps and mainly 150ma to 350ma to 5/6 amps. (I wanted to limit the peltier to 5 amps opposed to 6amps)

    Well if that's the case great, less fiddling about and would it be okay to use a 12v 8.5amp 102w DC supply to power devices that require a total of 98.4 watts or should I use one with more power? I did forget to add another fan motor........ Also what about a potentiometer to control the fan speeds ? would you be able to recommend anything on that note ?

    Thanks again for the info, its much appreciated
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,978
    744
    Controlling fan speeds is done with pwm, most pc fans are dc brushless motors, and dont work well slowed down, personally i would use a psu with 50% more power.
     
  8. eetech00

    Active Member

    Jun 8, 2013
    649
    112
    What are the resistors for?
     
  9. Needs2TinKer

    Thread Starter New Member

    Apr 26, 2016
    4
    0
    So I'd be fine using the DC 12 v 8.5 amp to power the peltier at a working current: 4.3-4.6 A ? without it going over its max of 6 amp ?
    If so would the use of a 2.4 ohm 1/2W Watt 0.5W Metal Film Resistor ±1% would be the correct one to limit it to 5 amps of current?

    http://www.ebay.co.uk/itm/New-1-2W-...hash=item1ea8f94617:m:mNyLewwndV1vFLMfWyh2DHg

    And the rest of the devices I can leave them as they are without any need or resistors and they should be fine. I was under the impression from what I have read that the resistors would limit the flow of current through the device. Or was I mistaken :( I just didnt want to over load the devices.
     
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