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Author Topic: LM317 regulating voltage but no current!  (Read 1296 times)
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I have used the typical LM317 circuithttp://www.ee.buffalo.edu/courses/elab/LM117.pdf with a salvaged, 30v printer power supply. It adjusts voltage but there is absolutely no measureable current. Can't turn a small motor that runs easily on a 9v battery at 53 mV. If I replace this circuit with the current limiting configuration, won't the amp burn components? 
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Should have added that I want to use this circuit as a benchtop adjustable power supply for a variety of applications. 
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I have used the typical LM317 circuithttp://www.ee.buffalo.edu/courses/elab/LM117.pdf with a salvaged, 30v printer power supply. It adjusts voltage but there is absolutely no measureable current. Can't turn a small motor that runs easily on a 9v battery at 53 mV. If I replace this circuit with the current limiting configuration, won't the amp burn components?  

If you have built the circuit correctly from the design but end up with an adjustable output voltage but can supply no load current (?) then you have either made an error in building it or have a defective component(s).
Or perhaps you are failing to stay within the voltage/current/heat dissipation limits of the regulator, which will shutdown in some situations to protect it'self.

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Can't turn a small motor that runs easily on a 9v battery at 53 mV.


Typically a 9V battery runs at 9 volt, no 53 mV, but that is either not what you meant to write or there is more to the story?

Lefty
« Last Edit: February 04, 2013, 09:05:29 pm by retrolefty » Logged

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Sorry, I am not experienced. The 53 was the load taken while running it off the battery.
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Sorry, I am not experienced. The 53 was the load taken while running it off the battery.

I'm sorry, that statement is also not clear. The term "Load" is usually measured as a resistance or the current consume at a specific load resistance, or a direct measurement in amps or milliamps of the 'load current'.

Lefty
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I set the multimeter to measure the resistance (through the multimeter) while under load. In any case, my circuit is adjusting voltage, but killing current. After reviewing the tutorials, there seems to be no mention of how to fix the current while regulating the voltage.
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I set the multimeter to measure the resistance (through the multimeter) while under load.
That doesn't work.  You can only measure resistance in an unpowered circuit.

If you measured while powered, your meter won't work for very long.
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I should have avoided details about the motor because they digress from the immediate problem: Why is the circuit regulating voltage but killing the current? I'm using a 240 ohm resistor and 5k pot.
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I've had similar problems where I could only get about 9mA from a 317. Turned out that the 317 was blown.
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It adjusts voltage but there is absolutely no measureable current.

...

I should have avoided details about the motor because they digress from the immediate problem: Why is the circuit regulating voltage but killing the current? I'm using a 240 ohm resistor and 5k pot.

...

I set the multimeter to measure the resistance (through the multimeter) while under load. In any case, my circuit is adjusting voltage, but killing current. After reviewing the tutorials, there seems to be no mention of how to fix the current while regulating the voltage.

I simply don't understand what you are talking about. What circuit? What do you mean by killing current? Should we call the police? You can't kill current.

How do you "fix the current"?

Can you post your circuit, and your measurements, that lead you to believe current is being "killed"?
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If you have built the circuit exactly according to the "sample" circuit in the datasheet, then the only conclusion I come to is that your active component (LM317) is broken.  Double check that you have wired it correctly, have use the right components and values... making sure you have specified the correct pin assignment.

I have use this circuit in a home built lab supply since the early 1980's.  The LM317 is very robust if wired correctly.
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The terminology here is confusing, and therefore I suspect the test methods to be faulty.  You should definitely NOT have left out the part about the load because it has given us some clues into your line of thinking.  Be patient.. you're not getting exact answers because you're not defining the problem well enough yet.

These may be obvious, but perhaps not:

- You can't regulate both voltage and current; only one or the other.  Current is demand-based, as determined by the impedance of the load.  Voltage is supply-based, as determined by the supply.  (Assuming a voltage regulator and not current regulator.  In a typical variable supply, you want a voltage regulator.)
- You can't measure resistance directly in a working circuit.  You can however measure a voltage drop across a component, and hence calculate its resistance.
- You can measure current consumed, but it is slightly more involved.  It's not as easy as probing two parts of a circuit and seeing a number.  The current either has to flow THROUGH a meter, or you need a current sensing resistor with a known value, and then you measure the voltage drop across it.

Now, instead of accusing the regulator, describe the symptoms exactly.  E.g.:  You connect a load to a 9v battery and it does its thing as expected.  You connect it to the regulator and it does nothing, or acts like it's running from a dead battery.  Something like that...?
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Remember the golden rule: If so many things are wrong that you fail to understand how they can be so wrong, check the ground connections smiley-mr-green

Also, make sure the regulator isn't getting hot when it's sitting regulating voltage w/o load (and with load as well) because the LM317 has thermal overload protection to protect fools from themselves. If it is getting hot, use a heat sink, even an alligator clip on the top may do the trick! Remember, the bigger the difference between the input and the output voltage, the more heat is going to be dissipated. So, if you're regulating 30 volts down to, say, 9 volts, you'll be putting out a lot of heat even under a small load.

One more thing, look at *THEIR* example circuits on the data sheet ("they" being the makers of the voltage regulator) and compare them with yours.

If all else fails and you're sure your circuit is correctly wired (or you've corrected any "mistakes") then it's possible that the regulator is broken (age, prolonged overheating, incorrectly wired on power-up, etc.)

Often when I'm shopping for electronic components, if they're cheap enough, I'll get one more than I need in case I break one.  smiley
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