Hello,
it's the first time i write here.

I'm trying some simple circuits and one of them is not working the way i think it should.
Dc source is about 8V and zener is 3,36V. The problem is that zeners voltage seems to depend on the resistor value. If i take small value, like 200ohm, zener takes 3,6V which is good. If i take resistor of 20kohm, i get only 2,25V on zener and don't understand why, because i thought that resistor only consumes extra voltage which is left from zener.

Could somebody explain why? I added the scheme

 

Hello,

I can not read your schematic, but the voltage accross the zener diode depends on the series resistor AND the load.
See this page from the eBook for more info:
http://www.allaboutcircuits.com/vol_3/chpt_3/11.html

Bertus

 

Hi bertus,

i was actually using the scheme from that page. I'm missing something obvious or understanding some fundamentals wrong. Why does voltage depends on load, i mean load voltage is the same as zener voltage(they are parallel)?

 

Hello,

The current through the series resistor will split into the zener diode and the load.
The zener will need a certain amount of current to regulate the voltage.
When the load is taking to much current, the voltage will drop.

Bertus

 

I see, i thought that a certain amount of voltage is enough no matter how much current is passing. So, if something(big value resistor) is slowing down the current too much, zener will not get saturated even if there is enough of voltage. Will it be in some sort of semi-awake situation?

 

I think there are some zener diode charastics that you need to realize. One is the Izk(or the minimum zener current to sustain zener breakdown) In your circuit you were using a 20K resistor in series this equates to 8-3.6=4.4V/20K or 220 uA. This may be too small for the zener to regulage. Not knowing if the zener is 1/2W or 1W and if it is a suffex A that means 5% of the nominal value.
Consider the resistor in series with th zener and the load in parallel. If the load current increases the zener current will decrease and versa visa.

 

Look up the data sheet for your zener. It should show the dynamic curve for voltage vs current. As you will see, it is non-linear and even more so at low current levels.

 

It's always safe to run a zener at half its rated wattage. That pretty much tells you the maximum current you can allow for the load. As with many things electronic, you start at the end (the load) and calculate backwards to get to the resistor.