12v to 5v +mini Peltier fridge

Thread Starter

pete1986

Joined Oct 12, 2013
20
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 :)
 

Thread Starter

pete1986

Joined Oct 12, 2013
20
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


 

Thread Starter

pete1986

Joined Oct 12, 2013
20
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
 

eyesee

Joined Oct 19, 2013
78
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.
 

Thread Starter

pete1986

Joined Oct 12, 2013
20
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
 

eyesee

Joined Oct 19, 2013
78
Most multimeters have a resistance setting:



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

Thread Starter

pete1986

Joined Oct 12, 2013
20
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
 

eyesee

Joined Oct 19, 2013
78
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.
 
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eyesee

Joined Oct 19, 2013
78
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.
 

Dodgydave

Joined Jun 22, 2012
11,285
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?
 
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