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R3 and R4 of 5V power supply
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R3 and R4 of 5V power supply

by Bandit on Tue Oct 20, 2009 3:25 am

The resistor values quoted in the instructions yeild an output voltage of around 13V when it should be 5V. The datasheet advises 13.3K Ohms for the lower resistor so I choose 12K. The equation the datasheet provides was used to calculate the value of the other resistor, aprox 39K Ohm to get 5V output, but this doesnt seem to work.

What values for R3 and R4 have you used to get 5V on the main board? Thanks.
Bandit
 
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Re: R3 and R4 of 5V power supply

by Bandit on Tue Oct 20, 2009 8:44 am

I got it working, I used a 39K resistor with the 12K:)
Bandit
 
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Re: R3 and R4 of 5V power supply

by Andyroo56 on Mon Feb 11, 2013 8:17 pm

What value of capacitor did you use for C20? My R3, R4 and C20 are: 36.5k, 12k and 590pF, but my regulator isn't working. It seems to start off kind of ok (4.8V), but then slowly dwindles until it settles around 3.3V.

Any suggestions? I'm getting really frustrated with this darn regulator.
Andyroo56
 
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Re: R3 and R4 of 5V power supply

by Andyroo56 on Mon Feb 11, 2013 9:12 pm

I saw on a lot of other peoples designs that they completely omitted C20. This capacitor is used to prevent oscillation according to the datasheet, but should not affect the output voltage.

I removed it and changed the 36.5k resistor with a 39k resistor. The regulator started giving me a reasonable voltage (~5.3V) that it MAINTAINED for a while, until about a minute later the voltage jumped up to ~12.5V and eventually fried the 100uF tantalum capacitor. I replaced the tantalum and dropped the 39k resistor back down to 36.5k but I am scared to turn it on again as I am worried that it will fry this tantalum (it is the last one i have).

Any suggestions are appreciated, this is rather frustrating.
Andyroo56
 
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Joined: Thu Sep 06, 2012 1:03 pm