7806 voltage regulator

[wacki]

I've been playing around with avr's, pics, and BasicX-24 stamps lately. But one thing I don't understand is how the 7806 voltage regulator bleeds off the extra voltage so perfectly. How does it get the perfect 5 volts no matter what voltage (within reason, of course) I run through it.

 

[Geniere]

It has an internal zener reference diode.

 

[DerMonkeyhauser]

It uses a zener diode inside. A zener diode acts like a normal diode (letting voltage flow only one way).
Put it in backwards, though, and the voltage has to exceed the "zener voltage" of the diode (giving
it its name), at which point the diode begins conducting. This, with a low-value (say 680 ohm) resistor to
limit the current through the diode (you don't want to shunt your entire source power supply to ground!),
connected power->resistor->diode->ground, gives you a nice, stable (but very-low power, on the order of
.25 watt or less) voltage regulator. If you want to up the power to a useful level, you connect the diode from
the base of an NPN transistor to ground, and a low-value resistor from collector to base. Pump in as much
voltage as you'd like (within the specs of the transistor, anyway) and it'll give you a constant-voltage output
on the emitter, at the zener voltage of the zener diode. Congratulations, you've just built a linear
regulator - the most basic form of power supply. A 7805 (or 7812, or any number of similar chips) just gloms
it all into one package.

As an aside, if you need a hefty power supply, try this:
12v 30A power supply
You can replace the 7812 regulator with anything you'd like (including the schematic I described above, and
any value of zener), and get as much or as little voltage as you want. I recently built a very hefty 48v power
supply out of that - I just left off the input transformer, and bought capacitors and transistors rated to 200VDC
(if line voltage scares you, don't try that at home!). I haven't tested its amperage capacity yet, but I estimate it
at around 20A (I used transistors with somewhat different ratings).

 

[FrankSchwab]

DMH -
Now that's a power supply!.

A simpler answer to wacki (I answered similarly in the Flashlight Regulator question, so though I'd throw it in here too...)

A 7805 or others of that family can be viewed as a variable resistance and a voltage sensor. The voltage sensor constantly looks at the output voltage coming out of the device, and tells the variable resistance to increase it's resistance if the output voltage is too high, or decrease the resistance if the output voltage is too low. The net effect is that the extra "voltage" is turned into heat by the variable resistance. You can calculate this by measuring the difference in voltage between the input and output, and multiplying by the current flowing through the device/circuit. Assuming you have a 9V input (normal 5V wall-wart), and are pulling 0.2 amps through your circuit, a 7805 will be dissipating (9V - 5V) * 0.2 = 0.8Watts. It'll get warm.

The Variable resistance technically isn't much more than a transistor - change the drive to the base, you change the resistance across the transistor.

The Voltage sensor is a zener diode. This is just a normal diode that has been constructed to have a very specific breakdown voltage. This provides a very stable, precise voltage reference that the output voltage can be compared to.

Hope this helps.

/frank

 

[wacki]

wow, thank you guys, that was a big help!