LD1117 not providing fixed 3.3V

Question

It is my first question on this site and I'm probably doing something wrong, try to be gentle! haha I want to provide 3.3V to my ESP8266 module from my Arduino, but people say this WiFi module requires more current than that 3.3V pin can provide. So I'm getting 5V from Arduino and turning it into 3.3V through LD1117 (just as it is done here http://iot-playground.com/2-uncategorised/17-esp8266-wifi-module-and-5v-arduino-connection).

^{simulate this circuit – Schematic created using CircuitLab}

This is the little guy. The problem is I can't get it to provide 3.3V. With no Resistor Load, output is 4.2V. With low Resistor Load (1k Ohm) just for the purpose of testing voltage does not keep at 4.2V, it drops, but it shouldn't, right?

^{simulate this circuit}

I'm starting to think that maybe providing 3.3V directly from the Arduino isn't that bad of an idea. Can someone guess what I am doing wrong with this LD1117? Thanks in advance!

Answer 1

Due to the constant revision of part numbers and datasheet for the ST LD1117 over several years, it's hard to 100% prove that you have the Adjustable Version. The current Datasheet shows the topcode LD1117AV as the TO-220 package, but has only shows the 3.3V fixed regulator available in that package.

But some googling found an example of the 3.3V fixed regulator.

Notice the obvious LD1117AV33 on the IC, meaning this is the LD1117 To-220 Fixed 3.3V regulator option, likely made in Week 49 of 2013.

As such, you have the adjustable version, and by tying the "GND" pin, really the "ADJ" to ground, it defaults to the 1.25V internal reference voltage, and regulates to 1.25V ± 0.07V, under load (suggested as 10mA minimum. A 1KΩ Resistor is not enough for 10mA, so you might see some ripple or out of regulation voltage.)

To make it 3.3V, you need two resistors. A 1K from VOUT to ADJ, and a 1.64K from ADJ to Ground would give 3.3V out. Use a LD1117 calculator if you want to use different value resistors.

Answer 2

That's most likely the adjustable version. This is denoted by a lack of a voltage specified on the front - the 3.3V would typically have "LD33" printed on it. This is backed up by the fact that you appear to be powering the regulator from the 5V output of the Arduino board, and the no-load Vout of the regulator is ~0.7V below that.

Have a read of the datasheet, in particular page 12, "LD1117A adjustable: application note". It tells you precisely how to use just two resistors to set the voltage of the output to whatever it needs to be.

The maths involved isn't very difficult, but I do find that I'm always fighting a losing battle of what resistors I need vs. what I actually have in my pocket. To prevent you pulling even more hair out, take a look at this calculator and the associated table of voltages available using standard resistors. It's saved me hours of time! Don't worry that it's for the LM317, they're interchangeable as far as a basic voltage regulator goes.