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The ST L78L33ACZ 3.3V voltage regulator has 5.028V in the Vin pin but the Vout pin is giving 4.765V.

The datasheet states that the dropout voltage is 1.7V. Shouldn't 5.028V Vin be sufficient to regulate the voltage to 3.3V? Or is there another problem?

Photo + Update:

May be hard to identify in the photo, but I did make the necessary cuts to the traces.

Vin is now 3.927V after adding the load and using a new regulator. Have not used the capacitors yet, maybe thats why its at 3.9V? Or maybe the load (Xbee) is not pulling enough current?

enter image description here

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  • \$\begingroup\$ Sounds like there's another problem. What's the load on the output? \$\endgroup\$ – John D May 9 '14 at 17:27
  • \$\begingroup\$ Are you sure you wired it correctly? \$\endgroup\$ – Cornelius May 9 '14 at 17:28
  • \$\begingroup\$ you may have simply fried the one you're using. After double-checking your connections, try connecting another one. \$\endgroup\$ – sbell May 9 '14 at 17:29
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    \$\begingroup\$ The input and output capacitors are not optional, they are required. Without them, the regulator may oscillate. When it oscillates, a dc voltmeter can not be trusted to read the output voltage. This is probably the problem you are having. \$\endgroup\$ – The Photon May 9 '14 at 17:41
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    \$\begingroup\$ Without the caps, there's very little reason to even measure. \$\endgroup\$ – Scott Seidman May 9 '14 at 23:37
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Although the datasheet doesn't explicitly state a minimum load current, it does give the output regulation rating for 1 mA to 40 or 100 mA, implying to me that there is a minimum load required for the part to regulate (and I recall the older 78xx regulators specifiying a minimum 5 mA load current for proper operation).

Put a 1 mA or more load on it and see if it works.

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The dropout voltage is only "typically" 1.7V, there is no guarantee of how bad it can be other than that implied by the 40mA out at 5.3 in (so it's not a very good regulator for this application), but that does not explain why you're getting higher than 3.3V.

Assuming it's the correct part number, and is not damaged, the simplest explanation is that \$V_{OUT}\$ and \$V_{IN}\$ are swapped and you're reading the voltage drop of a diode with very low current through it. Or the GND pin is not connected to GND.

enter image description here

Note that the pin order on the TO-92 is reversed compared to the familiar TO-220 78xx regulators.

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    \$\begingroup\$ Stackexchange should allow betting using reputation points :-) \$\endgroup\$ – gwideman May 10 '14 at 6:41
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A 3.3V regulator with a 1.7 dropout voltage should begin regulating at 5V. The datasheet shows that given an input of 8.3V, expected output voltage should be between 3.036 and 3.564V, with a typical of 3.3V.

Your measurement of 4.765V is outside these specs, so either the regulator is defective, it's wired incorrectly, the measurement was not correct, or something else is contributing to the value.

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Input ripple. Is your input supply actually a stable 5.028 volts? Are you using a multimeter to check or a scope? A multimeter is not good for measuring an unstable supply, it will average out what it sees.

Input capacitor. Is it good? Within the reg's specs? Input and Output capaciters are not always optional. It can depend on your input supply stability or distance. It can depend on your output load. It can depend on your loads operating frequency.

Output capacitor. Consistance loads can deal withou a output cap, but variable loads might require multiple, on top of the regulators required cap if it has one.

Minimum regulating load. Some regulators require higher minimum output loads for prper regultion. Some can be 20mA to 100mA. The bigger the regulator was designed to handle, the higher that minimum tends to be.

Output ripple. Mainly, bad/ovr/undersized caps or insufficient/excessive loads on the output can cause ripple, which will give you the wrong idea of the output voltage with a multimeter. An oscillascope is needed to see what's actually on the line.

Finally, there are tolerances in the typical specs given. Just because it is typically 1.7V drop out, it could actually be 1.55V or 1.85V instead. While a manufacturer tries to meet the typical, different batches might not meet all typical specs and different circuit desgins will change them.

Frankly, giving the reg only 0.028V over the dropout voltage is asking for trouble. Never push a regulator to the line on any spec. A 5.5 or 6V supply, or a 1v dropout ldo would do better.

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