Electronic Circuits and Projects Forum

Graham,

Here's the zener pics I mentioned.

NOTE the current and voltage numbers. You can see how the zener's current increases (as its resistance decreases) with increased VS1 voltages, "bleeding off" the excess VDC will maintaining (pretty closely) the 5.1 VDC the zener is rated at.

BASIC ZENER 12VDC.JPG BASIC ZENER 18VDC.JPG BASIC ZENER 24VDC.JPG

Just thought to add this about the zener examples above:

Most circuits assume a fairly stable power supply. It would be a rare beast that experienced the voltage swings depicted in the examples I gave. As a result, the zener circuit would never be subjected that sort of voltage instability, and would be able to maintain its output very close to the specified level. In this case, 5.1 VDC.

Morning CBB

ok, I have been doing a bit of exploring this morning

Lets see if my understanding is any better now

So basically we can use a zener diode to regulate voltage by using & staying within the parameters of it's reverse breakdown min/max, thus giving us a fixed voltage despite changing loads, they can be used as you were saying to operate a transistor to operate a larger current than would be possible still maintaining the required voltage, going even further we can add current limiting into the equation too, meaning that should current be exceeded putting the zenner outside it's parameters, the circuit would simply bypass the zener thus protecting it & shutting down devices, so my understanding now is that zener can be used as a voltage regulator as you explained, but not as a surge/spike protector.

so this would be a useful addition to my power supply box for my 5V requirement but a TVS diode would still be needed to deal with the spikes.

How's that?

Yes.

Within the confines of the zener's specifications, what you suggest is valid.

Originally Posted by cowboybob

Yes.

Within the confines of the zener's specifications, what you suggest is valid.

trying to look at datasheets to get a better understanding it seems 5.1V zeners come at 500mW, which is not enough for 2A is it or have I totally misunderstood, if I'm right it would would have to go through a transistor to provide 2A, if not, I've totally missed what I'm trying to grasp

Edit: so now I have a datsheet that we can work with to try & understand this better, if I change the diode on the schematic you sent to a IN5231B, I can try & figure out resistors needed for a 2A load & figure out what I'm missing on calculating R1

so if I work on a 13.7 voltage - 5.1V I need R1 to be equivilent to 70 ohms giving 5.1V across zener, basically after playing I'm talking nuts again aren't I, it can't be done with a 2A load, I would have to reduce R1 to 5.8 to give me load which messes the V up, is this where the transistors come in

Please forgive me if I'm hard work

ok, after another look I'm back to your test of IN4733 which looks possible so basically I have learnt a bit more about zener diode but your already 5 million steps in front of me, lesson for today, listen to your tutor

where were we can we try your test with 13.7V & see where that takes us

One of the problems with using a zener as a voltage controller is that at the higher current ratings, it takes a greater minimum current through the zener to get it into "steady" or specified VDC output mode. As the load draws additional current away from the zener, the available current for the zener to maintain its "steady state" output voltage is reduced, eventually to the point of no longer working as specified.

That's why a voltage regulator chip, like what you spec'ed out at the beginning, is a better choice for your needs. It can handle the additional current requirements of the load without losing the ability to maintain the appropriate voltage level(s).

ok, being totally nieve has it's advantages here, because I'm guessing I have lots more possibilities until those ideas get shot down in flames
So I see two possibilities

Originally Posted by cowboybob

One of the problems with using a zener as a voltage controller is that at the higher current ratings

I'm thinking along the lines of 2A max current

Originally Posted by cowboybob

it takes a greater minimum current through the zener to get it into "steady" or specified VDC output mode.

greater than what, a chip

Originally Posted by cowboybob

As the load draws additional current away from the zener, the available current for the zener to maintain its "steady state" output voltage is reduced, eventually to the point of no longer working as specified.

Isn't that why you need a higher working voltage than zener voltage

Originally Posted by cowboybob

That's why a voltage regulator chip, like what you spec'ed out at the beginning, is a better choice for your needs. It can handle the additional current requirements of the load without losing the ability to maintain the appropriate voltage level(s).

failing the zener being the right product, is it possible to make one, it would be a lot more fun afterall, they are about £4, I reckon we could make a schematic, buy parts & build one for about £150 including time edit: ok, I accept at the speed I learn it might be £1500.00