Voltage reg and Zener

[Screech]

I'm building an automotive circuit.

I had the idea of using a 16 volt zener with an 8 volt voltage reg for my circuit.

Problem is zeners are only 1 watt.
my circuit uses up 200 milliamps (6 watts).
Are there any 6 watt zenners around?
don't I have to use a 6 watt resister with that Zener?

same for the reverse polarity diode. it's only rated at 1 amp.
what amperage rating for the reverse polarity diode do I use?


do I really nead to use the Zener and resister in automotive circuits?


Thanks

 

[Miles Prower]

I'm building an automotive circuit.

Then what do you need a 16V Zener for?

I had the idea of using a 16 volt zener with an 8 volt voltage reg for my circuit.

Since this adds up to 24V, I suspect that you are fooling around with aircraft power systems. Now, I don't know what the regs in your country are like, but in the USA, the FAA will demand to see what's called a "Supplemental Type Certificate" for any new equipment you add to any aircraft.

Getting these are costly, and time-consuming.

Unless I know more about what you're trying to do, I won't help you.

 

[Nigel Goodwin]

Yes you can get high wattage zeners, they are usually stud mounting, and require fitting on heatsinks. However, you would not usually design a circuit so crudely, a properly designed voltage regulator would only use a small zener, and a power transistor would dissipate the heat.

But your question is far too vague to make any suggestions!.

 

[Screech]

I hope the pic is not crude.
do i really nead the zener for automotive applications?
I would rather do without it.

the electronic components are leds, some cmos chips, 555 times, a few transistors , all consuming 200 milliamps.

Thaks for your replys.

 

[Sebi]

The zener and serial resistor not really needed. ( except when some spikes existiert over 16V.) The 7805 require heatsink, or direct mounting on chassis.

 

[Nigel Goodwin]

For in-car use the zener and resistor is probably a good idea, and would almost certainly be included in any properly designed commercial equipment - it might also include a LC filter on the incoming supply as well?.

The electrical circuit in a car is a VERY noisey place!.

 

[Screech]

sebi,is that a 10 ohm, 10 watt resistor?

I got a automotive peformance electronic magazine, and nearly all the circuits in it, use a 10 ohm resistor(dont know the wattage), a 1 amp diode, and 16 volt zener(1 watt).

Thanks for your reply.

 

[Screech]

For in-car use the zener and resistor is probably a good idea, and would almost certainly be included in any properly designed commercial equipment - it might also include a LC filter on the incoming supply as well?.

Damb, another thing to confuse the hell out of me. :roll:

 

[Nigel Goodwin]

sebi,is that a 10 ohm, 10 watt resistor?

I got a automotive peformance electronic magazine, and nearly all the circuits in it, use a 10 ohm resistor(dont know the wattage), a 1 amp diode, and 16 volt zener(1 watt).

He labelled it 10 ohms, but it rather depends on the current required by the load. The 7805 is rated at 1A maximum, so if the 10 ohm is passing 1A it will drop 10V, which of course means you won't have 5V coming out of the 7805 anymore!.

 

[stevez]

Several years ago I read some applications notes for automotive use and the comments suggested that good design practice would allow for as much as 60 volts. Some of this had to do with alternator regulator failure. I would agree with the other comments that the 16 volt zener and resistor is good practice.

LC filter refers to coil/capacitor to knock down or smooth out transients.

 

[audioguru]

Instead of adding a pile of parts to protect an ordinary regulator, why not use a regulator that is designed to take care of itself and its load in a car?
Here is one that I use all the time. It is a little low-power one but bigger ones are available. As a bonus, it is also low-dropout, so is also excellent to use with a little 9V battery.

 

[www.elektrotech.info]

re

LM317!! is good

 

[instruite]

Automotive circuits have surge ratings and the voltage in those ratings can go upto +/-150 V for very short time durations
Screech, I have a doubt regarding your circuit,
First of all is this circuit going to be used comercially?
If yes, then you will have to use ICs which are of automotive Grade, I dont think 7808 is an automotive grade IC
If you are design this for comercial purpose then you should check with your customer reagrding various other specifications like surge specs, dump surges (it might include alternator failure also but that surge also depends on the alternator type (protected/unprotected) )
If you can let me know your exact application I can suggest you something.

Instruite (am an automotive design engineer)

 

[Screech]

Instead of adding a pile of parts to protect an ordinary regulator, why not use a regulator that is designed to take care of itself and its load in a car?

That makes a lot of sense.
Just found that national makes 1 amp ones. They are Lm2940 series. They come in 5,8,9,10,12,15 volt.

I will most likely use them with my fingers crossed.

If you can let me know your exact application I can suggest you something.
Instruite (am an automotive design engineer)

I'm still trying to design a simple efi system.
I've got it working, and it has never misses a beat, but I would like it to be super reliable.

thanks guys

 

[audioguru]

Hi Screech,
The LM2940 will work fine in a car. Notice that a minimum value of 22uF is required as an output cap, like most low-dropout regulators.
It is interesting that its quiescent current spikes as it approaching a low-dropout condition with a heavy load, 'cause I guess its poor transistors are starving for some voltage. :lol:

 

[Oznog]

You size the zener to what the regulator can handle for input, by spec sheet. If you have a reg which is spec'ed as 30v Absolute Max Voltage, then use that for the zener. 16v is conducting at too moderate of a voltage.

Here is a much more proper way to protect the input:
1. Replace the zener with a transorb. These are basically zeners but sized to take huge surge currents.
2. Replace the input resistor with a self-resetting polymer fuse. Those transorbs can sustain large surge currents, so even a polyfuse in the range of many amps is fine. A polymer fuse will reset itself in a few moments as it cools, and can do so an indefinite number of times.

That guarantees the reg won't see anything above the transorb voltage, and shuts off the power rather trying to dissipate it. It is essentially bulletproof.

I have become quite fond of this form of protection. The only downside is a polymer fuse does have a resistance which may not be trivial. The smaller the current rating, the higher the resistance. It can cause a slight voltage drop which will only hurt you if the circuit is really sensitive. But if you're trying to do this with a transorb rather than protect against circuit failure causing overcurrent, you can use a polyfuse sized many times higher than the max current the circuit could use so you get the lowest resistance part.

 

[Screech]

So much to learn! :?


Audioguru, they only specifying a .47 Uf cap on the input of the LM2940?

Can I put a bigger cap then the .47 uf one there, or beside it?

Thanks :mrgreen:

 

[instruite]

Scrrech,
The capacitor is basically used to maintain the voltage constant when there are transistions in the voltage.
In this kind of regulator the output capacitors determine the load and line regulation characteristics. the recomended minimum value is to get the performance as per specified in datasheet.
The input capacitor is basically just to remove transistions from reaching the input of the IC (that is it helps in line regulation) but if you are using a battery source or constant DC source then the making the value of input capacitor high wont make any effect on the circuit.
It will just increase the cost and space on PCB

 

[audioguru]

The datasheet says that the 0.47uF input capacitor is needed if the regulator is located far from the power supply filter capacitor (of the rectifier). Like all high gain regulators it keeps the circuit from oscillating. It is best to use it anyway. Of course it still needs a big main filter cap.

The datasheet also says that the minimum value and low ESR of the output capacitor is extremely important over the full temperature range expected to keep the circuit from oscillating. Since its transient response isn't great anyway, it is best to increase its value more than minimum. Look at the curve that shows the output impedance skyrocket up with transients above only 6kHz.

Low-dropout regulators use a common-emitter slow PNP transistor with gain as the pass device. It has a high phase-shift above 6kHz therefore these regulators need the output capacitor for stability.
Ordinary regulators use a common-collector (emitter-follower) fast NPN transistor without gain as the pass device. They don't have much phase-shift. :lol:

 

[Screech]

Cool!
I"ll keep all that info in mind, and will use it.

Thanks a lot guys.

Just a pitty my supplier cant supply me with the 8 volt one.
Will have to use the 9 volt one.