LM338 gives output voltage input

N

nohutpilav

New Member
Hello guys. I am using LM338 regulator for my project. But when I build the circuit, the output voltage is equal to the input(I tried it on breadboard). There is no short circuit in the LM338 legs. The voltage is collected directly on R1 and the resistor gets very hot. There is no heating in the regulator. Vadj voltage, that is, the voltage on R2, is 0V and as far as I observed, the resistors show different values when I energize the system. What could be the problem in my circuit? This system is designed to step down 12V to 3.3V. The current it draws is not even 1A. Does this circuit work on the pcb board or are the LM338s faulty?
 

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I measure my output across capacitor C1
 
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From the post #5 pic of the underside of the board it looks as though you have some crossing wires which could be touching and causing a short circuit.
 
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If the Pot is open or not in the circuit, it will produce the results you saw.

There must always be 1.25V approx across R1 when R2 exists. I propose R2 is not connected or was burnt out... hmm.

The output voltage is a result of the same current 1.25/R1=Ipot through the pot, plus 1.25V.

For more accuracy include a small uA current from Vadj when higher R1 values are used as per the datasheet.

If you intend to draw 1A, you may want consider a 10W heatsink or more for the dissipation and heat rise from the (Vin-Vout)*Iout * Rja
 
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