Zener regulator.

[aargee]

Hi all,

I'm using a 2.2K resistor and a 4.7V zener with an assortment of decoupling caps to regulate a car voltage (+12-15V) to drive a simple PIC circuit that powers a relay. Currently it is operating at 3.7V due to the low current draw of the setup, which is fine and well within the PIC supply criteria, the zener is really adding protection against over voltage and spikes etc. Non life critical application.

The question is not about the circuit design but the operation of the zener away from its 'knee' point. Are there any issues I should watch for in doing this... actually as I write this I think probably not but I'll post it anyway. 

- Rob.

[sleemanj]

I would posit that you do not have a 4v7 zener, or that you circuit is not correct. 

Post your circuit diagram.

Sent from my HTC Dream using Tapatalk 2

[georges80]

2.2k ohms.... If you are drawing 5mA, that's a 2200 x 0.005 = 11V drop across the resistor....

MUCH better to be using an automotive rated regulator (that handles reverse polarity, load dumps etc).

cheers,
george.

[SirNick]

In my very humble opinion, Zener regulators are really only suitable as a voltage reference, where you can expect the current draw to be minimal and somewhat consistent.  You can use an opamp buffer to amplify the current, and then you have a basic linear regulator.  (Or you can just use a linear regulator.)

This does not address the plethora of potential issues with an automotive supply, as georges said.  I designed a generic 3.3v / 5v PSU for this purpose once.  It uses an MCP33036 switching regulator after hefty input protection:  A fuse, (reverse-polarity protection) diode, wire-wound high-current low-value resistor (or MOV), a TVS, choke, and bulk cap.

[jamesglanville]

A bridge rectifier with some decent caps and a 7805 would be my preference, maybe with a 20V zener across the output of the rectifier. That way you'll always get 5V out, no polarity reversal problems, voltage spikes are dealt with and it's cheap.

As to your circuit though, you could just drop the resistor value to get the voltage to be stable and higher, but it's not a particularly nice solution.

[gardner]

I would also add big-ish series inductor on both input legs.  Maybe after the fuse and MOVs

[georges80]

I really wish folk would stop using/suggesting the 7805 regulator family. It is antiquated junk that was 'ok' back in the 80's. There's many excellent automotive rated regulators that take care of things a 7805 requires a multitude of bandages for and still doesn't do the the job right.

Most modern automotive regulators are stable with minimal input/output capacitance and are designed to deal with the nasty stuff on a vehicle's power rails.

I have several automotive designs that are mostly on motorbikes (even nastier in many ways), so have some experience here.

cheers,
george.

[SirNick]

Got any favorites?  I've seen quite a few "automotive grade" regulators lately, but I never really knew how much faith to put in that without the same bullet-proofing I'd give to a commodity part.

[georges80]

Old-ish standby I often use is the LM2931 - NS/TI and Onsemi make them.

There are a few others I've used, but the LM2931 is easy to use and available in packages that may be more 'hobby' friendly.

cheers,
george.

[Rufus]

MUCH better to be using an automotive rated regulator (that handles reverse polarity, load dumps etc).

A zener shunt regulator also handles reverse polarity, load dumps, and transients limited only by the zener dissipation and dropper resistor dissipation and voltage rating.

Cheap and extremely robust providing your current requirement is low. An LM2931 needs a zener shunt regulator in front of it except you would call it a Transient Voltage Suppressor diode. 

[georges80]

Ok, whatever you say - feel free to use that zener shunt regulator to feed your vehicle ECU...

cheers,
george.

[fcb]

Ok, whatever you say - feel free to use that zener shunt regulator to feed your vehicle ECU...

cheers,
george.

If the OP knows what his load parameters are and the quality of supply regulation isn't an issue (5V PIC's tend to be very forgiving), then I can't see anything wrong with a resistor and zener.  If the zener and resistor are big enough (wattage) to deal with load-dump, etc.. go for it.

[Jeroen3]

Get a proper LDO and some TVS diodes. And if there is space, use a fuse. Automotive usually has a large lead-acid battery, so you have "unlimited" current available for a short time until the fuses blow.

[opty]

I really wish folk would stop using/suggesting the 7805 regulator family...
..
I have several automotive designs...

I think most beginners choosing 78xx chips don't even know alternatives. It would help them a lot if you gave your preferred regulators.

I'm recently using MCP1702-xxxx (up to 250mA 0.4%) but that is not automotive grade...

Opty

[dannyf]

It is antiquated junk that was 'ok' back in the 80's.

If it gets the job done, who cares if it is junk or antiquated.

[hamdi.tn]

sure 7805 is cheap and get the job done, but i think you should use an automotive class  step-down circuit, better performance.

[dannyf]

better performance.

What if you don't need the better performance?

[hamdi.tn]

that's why you don't have to use those converter, however it will be interesting to try some advance tech rather than an old 7805.

[Peter1964]

I agrre with "georges80" - use a LM2931 for automotive applications.
You do not need a zener on the input just as stated in the documentation and you are home free.
The LM2931 is designed for that kind of use the old 7805 is not.
As you can see in the documentation you can get it in different packages and even a regulated version with on/off pin.
All in all you can get away with a very neat small design with no problems.
Take a look: http://www.ti.com/lit/ds/symlink/lm2931-n.pdf

[Rerouter]

Element14
1903040 - Recom 5V regulator

We religiously use them to replace the old bi-metalic regulator in automotive gauges, and so far in 4 years have never had one come back, if your only pulling uA then it may not be the most effective choice but we run them with no input or output capacitance our minimum load right into a resistive load with no issues

[rob77]

It is antiquated junk that was 'ok' back in the 80's.

If it gets the job done, who cares if it is junk or antiquated.

+1   couldn't agree more.

btw.. i like the robust "ancient" components - they're cheap & reliable

but regarding the OP's questions:

1. yes that zener regulator is just fine if it does the job well
2. operating the zener at a different than the "knee" point on the V-A chart is safe if the heat dissipation is within the specs.

the good way to calculate the zener regulator is to measure the max current of your load - calculate the resistor for the needed voltage drop at that current, and choose a zener capable of handling that current and related heat dissipation. it's not 100% accurate, but you'll be covering the worst case scenario as well (disconnected load).

[Rufus]

I agrre with "georges80" - use a LM2931 for automotive applications.
You do not need a zener on the input just as stated in the documentation and you are home free.

Until for example you get the standard contact break transient specified by Ford in their EMC-CS-2009.1 requirement document which is a decaying sinewave -230v to +150v peak at about 2kHz. Pretty much guaranteed to destroy an LM2931 with its -50/+60v transient rating.

[aargee]

Thanks for all the replies, particularly the ones that answered my question.

You know who you are!  .

[lewis]

No issues at all running a zener below its knee.

And a resistor-zener regulator is one of the better ways to power non-fussy low-current circuitry in an automotive environment. It's extremely cheap, reliable, you get transient protection, reverse polarity protection and current limiting for free, and the series resistor and bypass capacitor helps filter out supply noise. In fact I consider the circuit extremely elegant. Of course it's not ideal for all situations, but there is absolutely nothing wrong with it for many low power applications, and I use it all the time.

The ISO7637 load dump pulse will destroy most transient voltage suppressors, let alone a series voltage regulator. Read more here: http://www.radiocad.co.uk/files/1413/6921/7114/LoadDumpPaper-final.pdf