shaneshane1
New Member
Hi, i have a 3.3v Zener diode in series with a current limiting resistor (any value within range) but its output is about 3.8volts? can anyone shed some light on WHY its not 3.3volts?
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yes... i know how to measure voltage!, and i am measuring over the diode... with no load at all!!!
hi eric, its 1N4728...
Hi,
Zeners are not perfect voltage regulators. The voltage may go higher than the rated value for some current levels. The lower voltage ones are the worst.
You might find some data looking up the part number that may help.
hi,
Checked the d/s for that 1 Watt zener, it states 3.3V ok.
Tried a few test currents with a 1N4728 using LTspice, all around 3.3V.
What supply voltage and resistors have you tried.?
hi, you may well be right there... but i have tried 10mA to 70mA and the voltage output is always around 3.8V...
Shane,
The d/s says Standard Vz tolerance 5%, not ~15%.
My first reaction was that the characterisic curve of a low voltage zener was very soft, as intimated by Mr Al.
Many years ago I had a bad experience with low voltage zeners due to this very problem.
However, looking at the datasheets and from your experience it looks a though these zeners are much better than those from the "middle ages"!
It sounds as though you have a cheap imitation or a reject which has been labeled in a 3rd world back street knock off sweat shop!
JimB
It sounds as though you have a cheap imitation or a reject which has been labeled in a 3rd world back street knock off sweat shop!
JimB
Does Jaycar sell poor quality "failed" parts like RadioShack did?
I agree that a low voltage zener diode has poor voltage regulation. Its voltage also changes as its temperature changes.
Here are some graphs that show impedance and the voltage increasing as the current increases:
LEDs set their own voltage which is a range of voltages. You need to limit their current, not their voltage.
If the "3.3V" LEDs are 3.0V to 3.6V and the car electical voltage is 13.8V, you can connect one to three LEDs in series and in series with a current-limiting resistor.
Maybe the car switches in a lower voltage to dim the "globes" at night then three LEDs in series will not work.
Use simple arithmatic to calculate the value of the current-limiting resistor.