Zener diode question

[MrAl]

Hi again,

I forgot to mention, at this low voltage you can even try several series connected
SS diodes to act as a zener clamp.

Another way is to use a single SS diode from the divider to the +5v or +3.3v
supply. When the divider tries to go higher than say 5v the diode conducts
and limits it to 5.5v or so. A Schottky would work good here i think.
One catch though is you have to make sure the impedance of the network
is high enough and the 5v supply has enough load so that the network does
not overdrive the 5v supply and cause it to rise up above 5v. An extra 100
ohm resistor from +5v to ground helps here.

Willbe reminded me of something else i forgot to mention...
Since your basically detecting dc and it probably doesnt have to be
super fast, a small capacitor should also be used from the divider
output to ground. This will help block the steep spikes from
reaching the AD input.

 

[Willbe]

a small capacitor should also be used from the divider
output to ground. This will help block the steep spikes from
reaching the AD input.

Yes.
The voltage across a capacitor can only change instantly by supplying infinite current to it. Most transients cannot do this.

 

[stuhagen]

I thought I would throw this circuit out here as well, This is from another thread, but the application is the same, I think. In this circuit, I tried throwing a 3.3v Zener so I could limit the "reference" voltage to about 3.3v. Since the car's voltage can swing, it impacts the voltage divider's reference. But as mentioned here, this circuit will not function at all with it in there. As stated above, the zener characteristics do not apply well here. Maybe if I threw in a resistor, it may do what it is suppose to do. I think I might not even need it. I worry that the voltage could rise and create almost a 4v reference voltage, which is getting close to my "tripping" voltage of about 4.8to 5.0v. I can't really find the correct info on this comparator to know all of the thresholds. Not sure how close the voltages can be to each other before the circuit quits working. If all I need is.1 or .2 voltage differences, then all might be ok.

Stu

 

[audioguru]

Low voltage zener diodes make poor voltage regulators. The voltage decreases when the current decreases and when the temperature increases. The voltage increases when the current increases and when the temperature decreases. 5.6V to 6.8V zeners are good.

You have a 1N5225 which is 3.0V at 20mA and at room temperature. You have it operating at only 2mA so its voltage might be only 2.9V. Its tolerance might be 10% so it might be only 2.6V.

 

[Willbe]

Low voltage zener diodes make poor voltage regulators. The voltage decreases when the current decreases and when the temperature increases. The voltage increases when the current increases and when the temperature decreases. 5.6V to 6.8V zeners are good.

There is supposed to be a zener with a naturally occurring zero tempco, around 6.8v, and I think National has even given it an LM number.

 

[stuhagen]

Low voltage zener diodes make poor voltage regulators. The voltage decreases when the current decreases and when the temperature increases. The voltage increases when the current increases and when the temperature decreases. 5.6V to 6.8V zeners are good.

You have a 1N5225 which is 3.0V at 20mA and at room temperature. You have it operating at only 2mA so its voltage might be only 2.9V. Its tolerance might be 10% so it might be only 2.6V.

That's exactly the problem I had. I was reading about 2.7v on the divider with the Zener in, and 3.3v with it out, which was making the comparator be "on" all the time. I was baffled for awhille until I read this post. So is there a proper sized Zener that will allow at least 3.5v and clamp anything above lets say 3.8v-4.0v?
As I said, it may not be needed anyways. It is a safety net in the event the input voltage goes way high and the voltage divider's output goes way up.

Stu

 

[audioguru]

Like Willbe says, use a voltage reference IC instead of a lousy zener diode.
An LM4041-ADJ is pretty good and is available in a through-hole TO-92 package.