Hello again,
This might be the most difficult problem to answer, but when you dont have the answer yet i guess that's the way many problems can seem. I dont have the answer for this one either yet so lets see if anyone can come up with something interesting.
In the diagram the circuit is a very simple one LM317 circuit set up as a 1.25v regulator. The input voltage range is about 5v up to about 25v, so it's not a dead set value this time, but can be varied. That's to allow various input voltages to power the circuit, and since it only has to put out 1.25v (or close to that) it should be able to work down to 5v input, and i am pretty sure it does.
So the question is, can you find the circuit flaw in this circuit?
If i were to take a casual look at this circuit i would say, "There is no flaw really". But as luck would have it, that's not correct, and as of yet, i still dont have the answer.
If you would like to find the flaw yourself, then dont scroll down. If you want to see what the flaw is, then scroll down. I advise you to scroll down though because this one is entirely different than the other problems where looking at the schematic might reveal the answer. Looking at this circuit doesnt seem to help unless you've already encountered this problem in the past.
Scroll down...
The problem:
When the input is at 5v, the output is 1.25v (or close to that) and relatively stable. When the input voltage is raised up to around 7v, still stable and at 1.25v as expected. But once we get to around 10v or so (may be a little higher as per package) the output starts to drop down until it reaches maybe 1.22v.
Note that if we add a bottom resistor of 120 ohms the output goes to 2.500v (or close) and stays there pretty solid over 7 to 25v input.
This 1.25 to 1.22 volts decrease might not seem like much, but that's around 3 percent which is much more than the data sheet indicates, and i have not tested to see if it is stable with load.
So the real question is, does anyone have any information on why the voltage might drop when the regulator is used as a 1.25v reference?
This might be the most difficult problem to answer, but when you dont have the answer yet i guess that's the way many problems can seem. I dont have the answer for this one either yet so lets see if anyone can come up with something interesting.
In the diagram the circuit is a very simple one LM317 circuit set up as a 1.25v regulator. The input voltage range is about 5v up to about 25v, so it's not a dead set value this time, but can be varied. That's to allow various input voltages to power the circuit, and since it only has to put out 1.25v (or close to that) it should be able to work down to 5v input, and i am pretty sure it does.
So the question is, can you find the circuit flaw in this circuit?
If i were to take a casual look at this circuit i would say, "There is no flaw really". But as luck would have it, that's not correct, and as of yet, i still dont have the answer.
If you would like to find the flaw yourself, then dont scroll down. If you want to see what the flaw is, then scroll down. I advise you to scroll down though because this one is entirely different than the other problems where looking at the schematic might reveal the answer. Looking at this circuit doesnt seem to help unless you've already encountered this problem in the past.
Scroll down...
The problem:
When the input is at 5v, the output is 1.25v (or close to that) and relatively stable. When the input voltage is raised up to around 7v, still stable and at 1.25v as expected. But once we get to around 10v or so (may be a little higher as per package) the output starts to drop down until it reaches maybe 1.22v.
Note that if we add a bottom resistor of 120 ohms the output goes to 2.500v (or close) and stays there pretty solid over 7 to 25v input.
This 1.25 to 1.22 volts decrease might not seem like much, but that's around 3 percent which is much more than the data sheet indicates, and i have not tested to see if it is stable with load.
So the real question is, does anyone have any information on why the voltage might drop when the regulator is used as a 1.25v reference?