# Why zeners are generally employed in regulators?

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#### maxwell_28

##### Newbie level 2
why zener's are generally employed in regulators

Re: regulators

Hey....

#### k_0009

##### Newbie level 2
Re: regulators

see dude as far as know,

zener has excellant propety of withstanding as constant voltage source in breakdown region.this property is very very useful in maintaining constant voltage so by coupling it with transistor we can make voltage stability up to the marks.so may be thats y its in use.

#### Kral

Re: regulators

maxwell_28,
Zeners are used in two ways.
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The simplest is to use the Zener directly as a shunt regulator. Zeners have the property that the voltage is relatively independent of the current through the Zener.
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The Zener can also be used as the reference voltage for an active regulator. The error voltage, which is the dfference between the reference voltage and the output voltage, is used to regulate the output through a pass transistor. In a switching regulator, the error voltage is used to control the duration during which the switching element is turned on.
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Other devices, such as voltage reference ICs are often used in place of Zeners, because of their superior regulation and lower temperature coefficients.
Regards,
Kral

#### v_c

Re: regulators

One of the limitations of zener regulators is that you cannot get low zener voltages. I think the lowest voltages are in the 5-8V region. Unlike normal diodes the zener has a positive temperature coefficient. The zener can be used along with a normal silicon diode (which has a negative temperature coefficient) to make zener-based references with lower temperature coefficients since one has a positive and the other has a negative temperature coefficient. I think the overall result would be around 100ppm/degree C. This was used in the 1N821-1N829 range of diodes know as temperature compensated zeners.
Also zener regulators may be noisy due to the zener breakdown.

Best regards,
v_c

#### vicky

##### Full Member level 4
Re: regulators

hai
the basic quality of zener is tha break down region as the voltage more than the capacity pf zener are transferd to ground. if zener is of 5 volts than as a reference when input increases from 5 its goes to break down region and the over voltages goes via reference and our circuit safes from big loss.

#### gunsuresh

##### Member level 2
Re: regulators

When zener diode is reverse biased it offers constant voltage irrespective of the current flowing through it. hence it can be used as a regulator.

#### sivani_vs

##### Junior Member level 3
Re: regulators

it is very clear that stability and accuracy is generally good enough for the zener diode to be used as a voltage regulator device in common power supply circuit.for more information u see this website

#### A.Anand Srinivasan

Re: regulators

it is because zener has the property of remaining in a constant voltage in reverse bias which is the basic requirement of a regulator

#### E-design

Re: regulators

Zeners at 5.6V normally have a low coeff while > 5.6 have a positive coeff, and < 5.6V a slight negative coeff. That is the reason why you will often see 5.6V zeners used in voltage reference circuits.

Also there's a rule of thumb to never design with zeners at operating currents below about 5mA. If you do you may get large LF variations in zener voltages (1/f noise). Also in some cases a sawtooth noise waveform may result when a capacitor is connected in parallel with the diode.

#### hiyan

##### Member level 5
Re: regulators

zener diode work on break down voltages or region and it make zener a good regulator .

#### v_c

Re: regulators

I thought that they could not make zeners below about 6V. I know we have voltage references like the TL431 with a reference voltage of 2.5V, but I think that is based on a bandgap reference, isn't it? Am I missing something here?

Best regards,
v_c

#### vsgiri

##### Member level 5
regulators

Hi v_c

The principle of a bandgap zener is different.
It can be summarized as follows:
A forward biased diode has a negative temperature coefficient ~ -2mV/deg C.
When you have two forward biased voltages with different current densities in them, delta(vbe) has a positive temperature coeff.
This negative and positive temperature coefficient are appropriately summed to get a flat curve.
With lots of compensation and trimming upto 5ppm accuracy has been achieved!

I hope you got the general picture!

#### v_c

Re: regulators

vsgiri,

I understand all that about the bandgap. Generally, you take the Vbe of a transistor which has a negative tempo (as you mentioned) and you add it to a scaled version of devices with different emitter densities ($\Delta V_{BE} = \frac{kT}{q} \ln \frac{J_{E1}}{J_{E2}}$, with $J_1 \approx 10 J_2$) which has a positive tempco. The addition results in a relatively flat curve which varies < 0.5% over a large temperature range. I guess I don't consider the bandgap as a "zener" although I don't see anything wrong with doing that.

I was referring to discrete, two-terminal zener diodes (something like a 1Nxxxx). I was just trying to say that I have not seen anything below 6V that has good performance.

Best regards,
v_c

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