Is there any advantage of Zener Diode as regulator over LDO other than BOM cost? - Page 1
Forum section: electronics / beginners

author: RoadRunner
author_rank: Frequent Contributor
timestamp: June 01, 2018, 10:50:48 am
content: i am currently looking through frequency counter stuff, there is one little prescaler circuit which divides couples of Ghz down to Tens of Mhz. and in this section circuit has simple Zener Diode with and resistor to power presclaer chip with around 5V and current less than 10mA.i am thinking why did the person who designed it, didn't used a simple LDO. was BOM cost only concern?is there any advantage of Zener diode and resistor based regulator vs LDO , other than cost ?any help will be highly appreciated.

author: Cliff Matthews
author_rank: Unknown
timestamp: June 01, 2018, 11:10:06 am
content: Where you checking without an active input? Try measuring >1 Ghz..

author: bd139
author_rank: Super Contributor
timestamp: June 01, 2018, 11:17:25 am
content: Zener blows off more power to regulate the same load. Plus the regulation is worse even if you get the zener on the lowest part of the dV curve which usually requires a lot of current. Also if zener goes pop, the circuit will fail to the input voltage likely destroying everything after it.

author: RoadRunner
author_rank: Frequent Contributor
timestamp: June 01, 2018, 11:20:36 am
content: Quote from: Cliff Matthews on June 01, 2018, 11:10:06 ami am not doing any measurement right now. i have only schematic not the board itself. Measurement is not my primary concern i just wondered about question why a zener not a LDO.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2018, 11:21:52 am
content: Why make things complicated when they can be simple?With a zener diode, there is none of that nonsense about frequency compensation. Noise is good. Line and load regulation is not as good at low frequencies but can be better at high frequencies.

author: coppice
author_rank: Super Contributor
timestamp: June 01, 2018, 11:38:11 am
content: One key thing a zener diode, or other type of shunt regulator, gives you that an LDO doesn't is supply clamping. If energy can get into your Vcc rail, then with an LDO there is nothing to stop the voltage rising to levels which can cause damage. With a shunt regulator the Vcc is clamped. This can be important if your product is susceptible to discharges, RF and other external sources of energy getting into the heart of your electronics. Decoupling caps are usually enough to absorb brief shots of external energy, without a dangerous rise in the Vcc voltage. To withstand a sustained attack you need to clamp Vcc.

author: Zero999
author_rank: Super Contributor
timestamp: June 01, 2018, 12:08:02 pm
content: Yes, as mentioned above, clamping is about the only advantage. Quite often you'll find both a voltage regulator and a slightly higher voltage zener diode, which performs the clamping function. If a zener on its own is sufficient to regulate the voltage, then why have an additional regulator?If there was a voltage regulator, what makes you think it would need to have a low drop-out voltage? If the input voltage is always more than a couple of volts above the output, then a standard regulator would do a better job.

author: RoadRunner
author_rank: Frequent Contributor
timestamp: June 01, 2018, 12:55:59 pm
content: Thank you all for comments. i think it is quite clear now.

author: bd139
author_rank: Super Contributor
timestamp: June 01, 2018, 12:57:26 pm
content: Why would you use a zener for clamping though? You can get LDOs with clamps and overvoltage protection built in

author: Zero999
author_rank: Super Contributor
timestamp: June 01, 2018, 01:28:52 pm
content: Quote from: bd139 on June 01, 2018, 12:57:26 pmCost, stability? If a cheap zener is adequate, then a fancy IC is a waste of money.Another thing to consider is the age of the device being discussed by the original poster. It might be from an era when IC regulators with built-in clamping weren't widely available.

author: bd139
author_rank: Super Contributor
timestamp: June 01, 2018, 01:31:18 pm
content: Fair points!

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2018, 03:37:51 pm
content: Quote from: bd139 on June 01, 2018, 12:57:26 pmWhich LDOs have clamps and overvoltage protection on their output?

author: coppice
author_rank: Super Contributor
timestamp: June 01, 2018, 03:40:15 pm
content: Quote from: bd139 on June 01, 2018, 12:57:26 pmCan you name some of the parts you are thinking of? Most LDOs with extended features don't quite do what the headline seems to suggest, or require additional parts like a external current bypass FET.

author: dcarr
author_rank: Regular Contributor
timestamp: June 01, 2018, 03:51:34 pm
content: Zener regulators also provide effective protection from reversed input polarity. A LDO might just go pop.

author: C
author_rank: Super Contributor
timestamp: June 01, 2018, 07:08:07 pm
content: Most are thinking that Voltage regulation is the main point, it might not be.People go crazy for voltage regulation, before doing this would be good to know the voltage range the load needs.It could very well be that noise isolation was the main reason to select the cheap zener.The isolation by the series resistor is bidirectional.This load from main supply orMain supply from this load.Think most will find that a resistor has a better frequency response then a LDO.And un like a LDO a zener is a change in load regulator.To state it a different way if you load current varies by 10% the zener is only regulating that 10% change.With a load change a LDO creates a load change on main supply.With a load change a zener creates a constant load on main supply.C