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Basic Electronics » What is the purpose of the resistor in this power supply?

April 17, 2011
by rajabalu21
rajabalu21's Avatar

I am not sure I understand the reason behind the 1 ohm resistor in series with the output capacitor in this circuit.

Power supply

I have seen this in many places. Can any one explain the purpose of the 1 ohm resistor in the above circuit.

-Raja

April 17, 2011
by bretm
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It's usually not needed because the output capacitor will have a sufficient Equivalent Series Resistance (ESR). But if you choose a capacitor where the ESR is very low, the capacitor in parallel with the regulator can cause oscillation because the regular can act like a virtual inductor. The series resistor damps the oscillations.

The datasheet for the MC33269DT says to use an output capacitor with an ESR of at least 0.2 ohms. The 1-ohm series resistor ensures this.

April 17, 2011
by rajabalu21
rajabalu21's Avatar

Thanks for the clarification.

-Raja

April 17, 2011
by rajabalu21
rajabalu21's Avatar

I have checked the capacitor specs and found out that the ESR is 45 mOhm. As it is less than 0.2 Ohm, it makes sense to include that 1 ohm resistor. Learned something new today. Thanks.

-Raja

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