# Zener regulator question

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#### caringin

##### Junior Member level 1
Hi,

I use Zener 5.1 V in my recent design. At the beginning, my design works succusfully, but yesterday, it didn't work. I looked for the error and I tried to change the Zener and it now works. My question is, what is the maximum voltage can be given to Zener 5.1V?

#### IanP

Have a look at the zener diode characteristic (pictures below) ..
Once you pass the Pmax point, and this can be easily done by trying to apply to high voltage, the zener diode will be killed ..

Zener diodes are usually rated for 0.5W, 1W ,...5W, ..
For example, at 5.1V and 0.5W the maximum zener current is Imax≈98mA.
Your task is to make sure that with or without load the zener current is always smaller than Imax ..

In other words, don't apply a voltage directly to a zener diode; use serial resistor which will limit maximum current below Imax ..

Regards,
IanP

### caringin

Points: 2

#### Verba

##### Member level 5
you could have probably killed it by applying forward voltage.
if you use a serial resistor, make sure it can dissipate (Vin_max-Vz)^2/R

### caringin

Points: 2

#### sabu

##### Junior Member level 3
u choose the voltage level (un reg) such that the currt through diode never exceeds (Izmax).ok.

g luck
sabu

### caringin

Points: 2

#### Kral

caringin,
Ultimately, the important parameter for reliable operation of o Zener is its maximum allowable junction temperature. Let Rtheta be the total thermal resistance from junction to ambient temperature. Then the junction temperature rise DeltaTj will be Rtheta X Pz, where Pz is the power dissipated by the device. Let Ta = the ambient temperature. The Ta + DeltaTj must be less than the maximum allowable junction temperature (usually 175 Deg C).
Regards,
Kral

### caringin

Points: 2

#### caringin

##### Junior Member level 1
Hi,

Unfortunately, I didn't know the power can be delivered to the Zener. The store didn't tell me anything about it. I just asked for Zener 5.1 V. Maybe I have blown the Zener up because I once directly connected the Zener to battery 9 V. I didn''t know that it's forbidden, but I don't want to make the same mistake now. I'll try to put a resistor in series combination with the assumption that the Zener I bought has 0.5 W and with the hope that it won't be damaged anymore.

### caringin

Points: 2

#### Davood Amerion

I dont know what is your circuit, but minimum value for 9V input and 400-500mW, 5.1 volt Zener is 47 ohm.
I recomend you choose larger resistor (eg 100 ohm).

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