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LM317 - what happens if there isn't enough voltage in?

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joecool85

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I'm working on a LM317 based project and I'm curious what happens if the resistor network asks the LM317 for more voltage than it can put out? IE: the resistors call for 18v out but the circuit is only getting 13.5v in. From what I understand, 12v is the max the circuit could do. Does it just "hang" at 12v or does it shut itself down or something? I'm hoping for the former not the latter.

I've designed a basic thermally controlled fan controller for my wood stove using a LM317 and a thermistor and it will run 12v at 140F or so (wall temp a foot away or so), if it gets past that I don't want the fan to shut off, I just want it to stay full speed.

**broken link removed**
 
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the output will sit at (Vin - dropout voltage). The dropout voltage is the minimum input voltage in excess of the output voltage needed to keep the output in regulation. The LM317 is about 2V, so if you have 13.5V in, the output will sit at about 11.5V
 
The LM317 needs a minimum headroom voltage of 1.25V to regulate. If Vin is not high enough to satisfy that requirement, then it will just do the best it can and Vout will be Vin - 1.25V

If your Vin varies, than Vout will vary when Vin is less than target vout + headroom. When Vin is above Vout + headroom. the output will regulate at Vout as set by the resistors.
 
Thanks for the help guys. My fan draws 111ma @ 12v as measured by my voltmeter. The LM317 will always be "room temp" so if I have my input voltage 13.5-14v it should work perfectly.
 
How will you cool your LM317 to room temperature? It will heat up due to the power it dissipates.

The LM317 needs a 120 ohm resistor from its output to its ADJ pin, not what you have.
 
Attached dropout voltage vs. current drawn and temp.

There is also rollback if too much current is drawn.

Also rollback if die gets too hot due to current draw times input to output voltage drop.

The Adjust pin pulls about 50 uA's to ground and Vout to Vadj must regulate to 1.25vdc.

The purpose of the relatively low value resistor across the Vadj to Vout is to swamp out the 50 uA's pulled from Vadj to ground. The desired voltage is set by resistor from Vadjust to ground.

Figure the current through the resistor from Vadj to Vout based on 1.25v/resistor value. This current plus about 50 uA from Vadj is put through resistor to ground. Vout with then equal 1.25v plus current pushed through ground resistor times resistor value. Lowest output voltage is 1.25 vdc when zero ohms from Vadj to ground is used. You want the current flowing through the resistor to ground to be much greater then 50 uA so variations in the Vadj current due to load and temp are swamped out.
 
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The regulator needs a minimum load of 10 ma. That's why you usually see them with 120 ohms from output to adjust. (1.25/120) But in your case the fan is always there for a load so no biggie. ;)
But, I would go back and review the hook up and the math. If it is a NTC (lower resistance at higher temperature) I think you want it as the bottom resistor and the pot as the top resistor.
 
How will you cool your LM317 to room temperature? It will heat up due to the power it dissipates.

The LM317 needs a 120 ohm resistor from its output to its ADJ pin, not what you have.

Not worried about the resistor because as mentioned by ronv, it will always have the fan on it so it will have a load. As for how to cool the LM317, it seems obvious to me that it will need a heatsink, unless you have a better way to cool it.
 
The regulator needs a minimum load of 10 ma. That's why you usually see them with 120 ohms from output to adjust. (1.25/120) But in your case the fan is always there for a load so no biggie. ;)
But, I would go back and review the hook up and the math. If it is a NTC (lower resistance at higher temperature) I think you want it as the bottom resistor and the pot as the top resistor.

I used this calculator for figuring the LM317 math: **broken link removed**

According to that, as the temp goes up (and resistance goes down in R1), the voltage will go up at Vout. I was using an inexpensive Epcos thermistor from mouser as the 10K NTC.
 
Sorry, It is in the right place, it's just that Vo is around 5 volts not 12 volts.
 
Sorry, It is in the right place, it's just that Vo is around 5 volts not 12 volts.

Sometimes yes. It can be adjusted from fan turn on at 64F and max speed at 131F up to turn on at 104F and max speed at 167F with "turn on" being 4.5v and full speed being 12v.
 
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