Zener diode and current limit - please help !

[yashwant]

Hello folks,

I am trying to understand simple zener regulator.

Consider a standard zener voltage regulator with Rs=615 ohm as series
resistance between supply voltage Vs and the zener. RL=1k ohm as load
resistance across the zener. zener gives reference voltage =10 V and
requires 3mA current to maintain breakdown state.Maximum power
dissipation in the zener is 500mW. The Vs is 18 V. This are standard
parameters. According to calculations RL draws 10 mA and zener draws
minimum 3 mA for its operation in the breakdown region.So Vs should
provide minimum 13 mA.

Now my question is 'what would happen if the Vs has current limit upto
12mA.
Will zener stop working in breakdown region?


Thank you,

yash

 

[Spehro Pefhany]

Hello folks,

I am trying to understand simple zener regulator.

Consider a standard zener voltage regulator with Rs=615 ohm as series
resistance between supply voltage Vs and the zener. RL=1k ohm as load
resistance across the zener. zener gives reference voltage =10 V and
requires 3mA current to maintain breakdown state.Maximum power
dissipation in the zener is 500mW. The Vs is 18 V. This are standard
parameters. According to calculations RL draws 10 mA and zener draws
minimum 3 mA for its operation in the breakdown region.So Vs should
provide minimum 13 mA.

Now my question is 'what would happen if the Vs has current limit upto
12mA.
Will zener stop working in breakdown region?

Is this a homework question? A real-life Zener diode does not suddenly
stop working when the current through it drops below a certain
current. The zener voltage will be a bit less with ~2mA than at 3mA
(see the "typical" curves for the device) but it will continue to
conduct.

Best regards,
Spehro Pefhany

 

[R.Legg]

Hello folks,

I am trying to understand simple zener regulator.

Consider a standard zener voltage regulator with Rs=615 ohm as series
resistance between supply voltage Vs and the zener. RL=1k ohm as load
resistance across the zener. zener gives reference voltage =10 V and
requires 3mA current to maintain breakdown state.Maximum power
dissipation in the zener is 500mW. The Vs is 18 V. This are standard
parameters. According to calculations RL draws 10 mA and zener draws
minimum 3 mA for its operation in the breakdown region.So Vs should
provide minimum 13 mA.

Now my question is 'what would happen if the Vs has current limit upto
12mA.
Will zener stop working in breakdown region?

If Vs was current-limited, you wouldn't need Rs.

The characteristic dynamic impedance of the zener at it's knee voltage
is specified as a maximum value in most device data sheets. This will
give you a maximum dV/dI around a specified bias current.

RL

 

[pingpong]

Hi,
good questions and answers.

I would like to supply a single cmos 74hc123 from 12 V using a zener regulator (variation in vin from 9 to 14 V).
I have a board with a processor that is separately supplied from 5 V and sink less than 1 mA. Unfortunately seems that minimum current to make a zener regulate is some 5 mA.
The cmos chip that I have to supply absorb a fraction of a mA.
Is it normal I have to waste all this current and power in the regulator?
Could I drive the zener at less than 1 mA in this application?


> Hello folks,
>
> I am trying to understand simple zener regulator.
>
> Consider a standard zener voltage regulator with Rs=615 ohm as series
> resistance between supply voltage Vs and the zener. RL=1k ohm as load
> resistance across the zener. zener gives reference voltage =10 V and
> requires 3mA current to maintain breakdown state.Maximum power
> dissipation in the zener is 500mW. The Vs is 18 V. This are standard
> parameters. According to calculations RL draws 10 mA and zener draws
> minimum 3 mA for its operation in the breakdown region.So Vs should
> provide minimum 13 mA.
>
> Now my question is 'what would happen if the Vs has current limit upto
> 12mA.
> Will zener stop working in breakdown region?[/color]

If Vs was current-limited, you wouldn't need Rs.

The characteristic dynamic impedance of the zener at it's knee voltage
is specified as a maximum value in most device data sheets. This will
give you a maximum dV/dI around a specified bias current.