Issue with LM317 and current draw

[EasyGoing1]

I'm supplying a LM317 that's configured for 5.0V output with a source voltage of 7 volts. The LM317 is supplying power to a 5 volt sensor. The sensor is drawing 110ma of current, but the LM317 gets really hot and when i check the current draw from the 7V supply to the LM317, the LM317 is drawing a little over 400ma ...

This means that the LM317 is consuming almost 3 times the energy that it's supplying.

Is this normal for a LM317?

[budm]

Re: Issue with LM317 and current draw

Can we see you setup, schematic, and the pictures and how amp meter are setup to measure the Output current and the input current?

[eccerr0r]

Re: Issue with LM317 and current draw

One time I got really wonky behavior with a 3 terminal regulator... when I hooked up the pins incorrectly... it's ADJ-OUT-IN ... or was it OUT-GND-IN?

Dang 78L05 got me all messed up, it had to be different and all...

[redwire]

Re: Issue with LM317 and current draw

what you are describing is impossible - LM317 has a resistor to GND so extra 300mA is not possible

[budm]

Re: Issue with LM317 and current draw

what you are describing is impossible - lm317 has a resistor to gnd so extra 300ma is not possible

+ 10

[EasyGoing1]

Re: Issue with LM317 and current draw

I figured it out ... well ... it could be one of two things:

1) I am using a two channel digital POT to set the R1 / R2 values, and I just figured out that the POT chip I was using isn't quite working right. The R values from the wiper to terminal B (which is what I was using in both channels) would in fact adjust properly and the R values would read correctly but when i connected them to the LM317, it would exhibit the problem.

I started thinking maybe the digital POT was somehow malfunctioning so i used standard resistors and the LM317 functioned normally. SOOoo... a little probing on the resistor pins on the chip gave me more odd behavior stuff like infinite resistance between pins A and B when that should have read 50k.

I swapped the chip out and now that problem is solved.

I want to say that the second possible cause of the problem is that I was using R values that were too high, but the more I think about it, i don't think that's possible ... I think that bad chip was sourcing current to ground through the digital POTS which was PARTIALLY shorting either pin 1, pin 2 or both to ground, causing the LM317 to draw too much current.

With the new digital POT, everything works great. the LM317 draws no current even when powered when the sensor is not connected and when the sensor is connected, the LM317 is only drawing 110ma from the 7V supply which is only the current that the sensor is drawing from the LM317, so all is well.

Thank you ... all of you who took the time to reply.

[redwire]

Re: Issue with LM317 and current draw

Digital potentiometers are limited for working voltage. Most are low voltage parts. The resistance elements with 5V Vcc is about 5V max. so the LM317 could not be dialed too high, maybe 7V max. before the EEPOT shorts.
Usually an op-amp is used after the EEPOT with a gain of 4 so 0-5V becomes 0-20V control.