I need an explanation about the current and voltage through the zener diode.
The 3,3 Zener in series with a 300 ohm resistor is giving me a 2,7 volts reading.
When I replace the resistor with a greater one (1K), the zener is giving me a reading that I was expecting (3,4 - 3,5V).
Whats the trick with the "weak" resistor ?
I want to power a LED with 3,3V , 20mA that is in series with a Zener. How do i get that current, if i have to use a 1K ressistor to get the expected voltage with the zener ?
Its a blue LED so it has a forward voltage of around 3.3V.
The source is a 9V battery.
"Why don't you just use a current limit resistor ? " - im just studying, and I wanted to make a voltage regulator with a zener.
I know they are a very bad choise as a regulator, but i wanted to experiment with it a little
LEDs have a working voltage that depends on LED colour,
temperature of the chip inside, and the current through the LED.
So the voltage across a LED at a certain current is not constant.
Therefore we try to power LEDs with a constant CURRENT (we want that to be constant).
Current through the LED can be controled/limited with a resistor in series with the LED.
The LED then drops the voltage it needs (a LED acts like a zener diode).
Vf (working voltage) for a common 5mm blue LED is about 3.3volt, and absolute max current is 20mA.
Say we're going to run it at 10mA (more than enough), on a 9volt battery.
The resistor in series with the LED needs to drop 9-3.3= 5.7volt.
The resistor for 0.01Amp (10mA) calculates to 5.7/0.01= 570 ohm (560 ohm will do).
Leo..
Dacha011:
Ahhhh , and yes, is there an explanation why the zener is not giving me the correct voltage when i use a low resistance resistor ?
Check the data sheet of your specific zener for its characteristics.
They're not accurate, the voltage depends on the current. First conductance starts at a voltage higher than rated (which indeed you see at low currents - large resistor value).
If you want an accurate voltage, look for precision reference diodes. Those are designed for just that purpose.
Low voltage zeners aren't very good regulators at all, but at higher voltages (10 and up) they can be
reasonable. Shunt regulation with a zener isn't useful very often as it relies on the load not changing
too much, and wastes power. If you want voltage regulation use a voltage regulator.
Zeners are very useful for level shifting and transient protection though.
The 3,3 Zener in series with a 300 ohm resistor is giving me a 2,7 volts reading.
When I replace the resistor with a greater one (1K), the zener is giving me a reading that I was expecting (3,4 - 3,5V).
Merits of the circuit aside, I wonder if someone can provide a plausible explanation of why the voltage across the parallel zener & LED go down with the lower value resistor. The lower resistance would imply higher current, hence increasing voltage across those devices. This doesn't seem possible barring some sort of negative resistance behavior.
Usually when faced with something like this my first reaction would be to go back and retake the measurement.
Ahhhh , and yes, is there an explanation why the zener is not giving me the correct voltage when i use a low resistance resistor ?
Assuming you are measuring across the Zener/LED it doesn't make sense!
There might be something wrong with the measurement, or maybe the resistors values are not what you think.
Did you measure the resistors with your Ohmmeter?
...You might try putting the resistors in parallel (less resistance) or in series (more resistance). The easiest thing would be to connect the 1K resistor and connect the meter "permanently" across the Zener/LED. Then while watching the meter, connect the 300 Ohm resistor in parallel with the 1K resistor. That gives you 231 Ohms and we would expect the voltage to stay the same or increase slightly.
...If it's a actually 300K resistor, you'd have a parallel-combination of 997 Ohms and no change.
