When connected to 3.5v - 5v DC, my LM1117 will show 2.68v at Pin4 whixh is also the metal tab on the back which I usually thought was just a heat sink.

Why does that tab (aka Pin 4) have a positive voltage?

Thanks

 

you should check datasheet, it tells that tab is Vout

 

Have you read the data sheet? The tab may be Vout. I see that was covered.

Ron

 

I have the data sheet open now. It doesn't explain the purpose of it.

V out doesn't mean much to me. Kinda new to electronics. But working on it.

Why would it have two V outs? Isn't the one at the designated pin sufficient?

What would be the purpose of having the heat sink tab as another Vout?

And why would the voltage be different since they are both Vout ?

 

tab is meant as a way to dissipate heat - handy when devices are getting smaller and smaller.
while it is possible to have tab insulated that has negative effect on thermal conductivity

 

What you are seeing is not unusual and you will find the same in assorted components like transistors. The tab or can is normally an element. Why? To facilitate different designs. Vout is Vout and whatever one reads the other should read. Matter of fact if you measure the resistance they should be about zero ohms. The voltage should not differ at all. Ground to either point should read the same.

The chip itself is a voltage regulator which comes in several packages as well as voltages. Which exact version do you have. For example LM1117-adj is an adjustable voltage regulator, LM1117-3.3 is a 3.3 volt regulator. Next look at your data sheet as to the dropout voltage? The LM1117 is an LDR meaning Low Dropout Regulator. I see the dropout voltage is between 1.3 and 1.4 volts depending on the load. This means if for example you have a LM1117-3.3 (a 3.3 volt fixed regulator) the minimum input voltage should be 3.3 + 1.4 = 4.7 Volts or greater.

Voltage regulators are a world unto themselves and require hours of study to understand. Things like an output capacitor are important. The most common mistakes with these and similar devices is getting the pinout wrong.

Ron

 

Thanks for the great info.

As pointed out...the Vout from the Vout pin is different from the Vout from the heat sink / tab.
About 1 volt difference.

I wonder if that means it's bad?

 

As pointed out...the Vout from the Vout pin is different from the Vout from the heat sink / tab.
About 1 volt difference.

I wonder if that means it's bad?

I would have thought they were the same bit of metal so I reckon it is a measurement error. Check for good connections of your meter probes.