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zener and pn diode

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The transfer characteristics of zener and pn diode looks same in reverse bias, i have asked few why in many rectifier circuits zener is used.i got answer like, it wont burn up in high reverse current like pn diode . why zener is special than pn diode.
 

Tony Stewart

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" Since the diodes will break down in the regions where the breakdown field is reached first, one has to take into account the radius of curvature of the metallurgical junction at the edges. Most doping processes including diffusion and ion implantation yield a radius of curvature on the order of the junction depth."

upload_2015-3-17_2-33-8.png
 

Tony Stewart

Well-Known Member
Most Helpful Member
" Since the diodes will break down in the regions where the breakdown field is reached first, one has to take into account the radius of curvature of the metallurgical junction at the edges. Most doping processes including diffusion and ion implantation yield a radius of curvature on the order of the junction depth."

View attachment 91383

Zeners are made more rugged by the above methods. But in low current situations, I have used for example the Vce max as a zener for regulating the HV in a horizontal sweep in late 70's when our colour TV broke.
That worked.
 

KeepItSimpleStupid

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the OP said:
why zener is special than pn diode

In a Zener diode, the breakdown is the controlled parameter. Also remember that there is Zener and avalanche breakdown mechanisms. I believe at 5.1 V both are going on simultaneously. Breakdown isn't destructive if the current is limited.
 

Tony Stewart

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You can use a White and HB Red LED in series to make a 5.1 V zener too. ESR is around 15 Ohm @ 100mW , so 2 LEDs in series ~ 30 Ohms (+/- 50% typ) at 20 mA

Thus is bias current changes +/-10% voltage changes 2mA * 30 Ω = 60mV
or a 0.06/5.1 = for a 1.2% change in voltage ,
so bias regulation sensitivity 1.2%/10% = 12% but accuracy is very high when bias current is lower due to lack of ESR drop and low variation in Vthreshold ( < 2%) same as a zener.
So I would choose 2~3mA

A typical 5.1V Zener is 60 to 80 Ohm 4.980 5.200 @ 5mA 80Ω max. but much cheaper in volume.
**broken link removed**
 
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