voltage regulator

[eblc1388]

Also - you were wrong in stating that the transistor on the left is not conducting - it is.

He said it does not conduct when the BE voltage is less than 0.54V and conducts when it is higher. This is how the circuit regulates its current.

Now referring to what's you have said earlier:

Regarding LM317's - as long as you pick the right ratio for your resistors, and keep your input voltage above 1.25V above your output voltage (in yourcase you need to make sure your input doesn't go below 4.25V if you want 3V output), then you will be fine.

I have checked. The input voltage needed to maintain regulation is at least 1.5V at output of 20mA and more than 2.0V at 1 A, according to the LM317 datasheet. So one would need at least 5V for a 3V output. Please quote any reference to your above comment of the 1.25V higher input voltage requirement.

 

[eblc1388]

Huh? This transistor supplies the LED current, not the 27 ohm resistor.

Yup. Acute Current Mirror Syndrome.

 

[audioguru]

Thanks, L. Chung. I would hate to try explaining the operation of a complicated current mirror. This one is easy.

Even more information about the dropout voltage of an LM317 is the condition for the excellent regulation spec's on the datasheet: a minimum voltage across it (from input to output) of 3.0V. Any voltage that is less then the spec's aren't guaranteed.

Your observation of the curves for dropout are correct. But they apply only to typical LM317s, individual ones vary and could be worse.
Also, the definition of "dropped-out" in the curve's text explains an output voltage drop of 100mV which is pretty darn poor regulation.

On another electronics forum I recommended the LP2951 low-dropout variable regulator. Its typical dropout voltage is 240mV at 20mA. It even has a pin (it is an 8-pin DIL) that goes low during dropout to give a low-batt indication or an indication when power is applied that the voltage is up and running. Unlike most regulators, it is unusual to need some ESR in its output filter cap. I added a 0.22 ohm resistor in series with it to be sure. :lol:

 

[eblc1388]

Yep, I pick the voltage from the curves, too lazy to read all the small text.

Regarding the current source, I'll try my way of explaining it.

A guy standing at the doorway is letting people through one by one with his boss watching his back. If his boss notices that the guy is letting people going through too quickly, the boss would call/divert the attention of the guy so he is letting fewer people through while busy replying his boss.

 

[audioguru]

Hi L. Chung,
Your explanation is nice and clear, try this one:

Many people think of electricity as water in a hose.
1) A spring holds a water valve open and water comes out of the hose at full flow. The spring is the resistor providing base current to the upper transistor.
2) The water is directed onto a vane on the valve to close it. The vane is the 27 ohm current-sense resistor and the lower transistor.
3) The force of water on the vane equals the force of the spring, therefore the water flow is regulated. :lol:

 

[audioguru]

Hi again, L. Chung,
I had a second thought about your analogy of a constant current regulator.
The guy's boss distracts him to prevent him from allowing people to pass too quickly. But the people stampede and push the guy and his boss out of the way. Thermal runaway! :lol: :lol:

 

[eblc1388]

But the people stampede and push the guy and his boss out of the way. Thermal runaway! :lol: :lol:

Yeah Audioguru nice description. The thermal runaway could occur even the current is constant.

Edited: Thermal runaway can happens.

 

[audioguru]

If you are worried about thermal runaway occuring in junction transistors then use Mosfets instead. When they heatup they conduct less which allows them to cool. :lol: