12v to 5v +mini Peltier fridge

Discussion in 'General Electronics Chat' started by pete1986, Oct 19, 2013.

  1. pete1986

    Thread Starter New Member

    Oct 12, 2013
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    just wanting some experienced users advice as im new. i have a 5v lcd temperature screen. the sort you can connect to pc's. anyway i have built one into the door of a mini fridge ( you no the camping/bedroom Peltier type)

    this is just a bit of fun/learning curve plus ill be using it to store all my flux/soldering stuff that needs to stay chilled. the mini fridge temperature is 10c which is perfect range just for doing this.

    im looking for a simple but safe way to convert 12v to 5v to power the little screen and found this cheap device and wondering if it was ok

    New 6.0V-12V to 5V AMS1117-5.0V Power Supply Module AMS1117
    http://www.ebay.co.uk/itm/2PCS-New-6-0V-12V-to-5V-AMS1117-5-0V-Power-Supply-Module-AMS1117-/200915902888?pt=LH_DefaultDomain_0&hash=item2ec78561a8

    also i fixed the 12v power supply by replacing some bad caps but there is a resister that seems to look in bad shape but i am struggling to identify it so i can replace it. if i upload a picture would anyone be willing to help me i.d it please ? thanks for any reply :)
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Yep upload a photo....also why dont you use an LM7805 voltage regulator to drop the 12 to 5V?
     
    Last edited: Oct 19, 2013
  3. pete1986

    Thread Starter New Member

    Oct 12, 2013
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    i was just looking for small board to buy so i could just pop it in without much work.plus i would buy a couple for future projects. they do sell boards with them on with heatsinks. would that be a better way of doing it ?

    iv taken some pictures. under the resister the silkscreen says R7 the resister is sort of brown looking butt he board round the resister is slightly burned due to heat
    [​IMG]
    [​IMG]
    [​IMG]
     
  4. eyesee

    Member

    Oct 19, 2013
    33
    3
    The AMS1117 board will be fine - it will give you 5V out @1A. Try cleaning the resistor with some IPA so that you can read the colour bands. It may be one of the resistors associated with the UTC3842. Have a look at the datasheet to see if you can work it out from the PCB connections and other resistors connected to the IC:

    http://www.datasheetarchive.com/dlmain/Datasheets-6/DSA-104182.pdf
     
  5. pete1986

    Thread Starter New Member

    Oct 12, 2013
    15
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    you are correct. i have followed the tracer from the resister and it goes to the UTC3842. and the other end of the resister goes to the + of the 100uf 400v cap
    [​IMG]
     
  6. eyesee

    Member

    Oct 19, 2013
    33
    3
    The resistor is connected to pin 3 which is a current sense terminal on that IC.

    Have you tried cleaning the resistor?

    Try desoldering the resistor and measuring it out of circuit.

    The value may be marked on the PCB under it.
     
  7. pete1986

    Thread Starter New Member

    Oct 12, 2013
    15
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    iv pulled it out and iv put it under a magnifying glass and from what i can work out it has brown band each side of the resister with a band of green in the middle. thats all i can see.im not sure but i think i also see 1 gold band. the heat has cracked the colour marking so if i attempt to clean it it will just fall off. also im not sure if my multimeter has the option to test it ? under the resister on the pcb it ways R7
     
  8. eyesee

    Member

    Oct 19, 2013
    33
    3
    Most multimeters have a resistance setting:

    [​IMG]

    If it is indeed Brown Green Brown with a Gold band for tolerance, it'll be 150 ohms +/- 5%.
     
  9. pete1986

    Thread Starter New Member

    Oct 12, 2013
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    just tested it and im not getting any reading from it. iv tested the multimeter on other resisters and i do get a reading so the multimeter is ok. iv tried 2000k 200k 20k 2000 200. im pretty sure im doing it right
     
  10. pete1986

    Thread Starter New Member

    Oct 12, 2013
    15
    0
  11. eyesee

    Member

    Oct 19, 2013
    33
    3
    Looking at the datasheet for the UTC3842, the low value resistor may be R marked in the 'Current Sense Circuit'. It probably is 150 ohms (+/- 5%) and forms an RC filter to the current sense input pin 3. Make sure you choose a resistor of the same or higher wattage rating when replacing it.

    The sense resistor marked Rs in the diagram in the datasheet, is probably one of the larger resistors in that area on the circuit board.

    Here is a better version of the datasheet which shows the resistor symbols properly:

    http://www.unisonic.com.tw/datasheet/UC3842A_3843A.pdf

    If there is blackening around and on the resistor, some other components may have been damaged too.
     
    Last edited: Oct 19, 2013
  12. eyesee

    Member

    Oct 19, 2013
    33
    3
    To get some more experience fault-finding and with your multimeter, test some of the resistors, diodes and capactors and compare them with the markings on the component.

    You can find useful video tutorials on how to use a multimeter on YouTube.
     
  13. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Here is a circuit using a UA3842, the sense resistor is on pin 3 0R27 ohms, it monitors the Fet current, it may have blown as well, check the D to S pins on the Mosfet, if in doubt you can omit it and connect pin 3 to pin 5 to run without current sense, if it works you can lower the output voltage by changing the feedback resistor R1, on pin1, the lower the value the lower the output voltage.


    Looking at your pcb it has an opto coupler to sense the the feedback from the 12 v output , this will be used with a TL431 zener possibly?
     
    Last edited: Oct 20, 2013
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