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Old 10-31-2001, 01:14 AM   #1
Kcz
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Question Zener Diode?

Here's the deal. I know many of you use this as a voltage clamp for the boost fuel cut. Now, I constantly get a CEL caused by the second O2 sensor reading too clean. I'm trying to clamp the voltage on that sensor so the ecu would think it's normal and no CEL. Would this work? If so, how would I do it? Thanks in advance.
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Old 10-31-2001, 01:18 AM   #2
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IIRC, the fix is getting a TEC-II
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Old 10-31-2001, 01:57 AM   #3
Kcz
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$2500 to get an O2 sensor to read the correct voltage seems kind of rediculious. Surely there is an easier way.
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Old 10-31-2001, 02:45 AM   #4
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ya, cut the wire
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Old 10-31-2001, 02:53 AM   #5
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So then I'll have a CEL and a cut wire. At least now it has a reading, I'm sure there will still be a CEL if I cut the wire and it's reading no voltage. Please, correct me if I'm wrong.
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Old 10-31-2001, 03:04 AM   #6
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Sounds likes the voltage clap will work to me. Go for it, and let us know how it turns out.
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Old 10-31-2001, 03:10 AM   #7
Kcz
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Yeah, I think it will work too. My problem is that I use a J&S Fuel cut defenser for the boost fuel cut and I'm not sure how the Zener diode works. This is why I posted. I was hoping someone w/ first hand experiance could understand what I'm trying to do and tell me how I could get this to work.
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Old 10-31-2001, 03:38 AM   #8
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Just wire a resistor in series w/the o2 sensor.
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Old 10-31-2001, 03:55 AM   #9
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the zener just limits voltage going in one direction. say you have a 5v zener diode, it will not let anything less than 5volts pass through it, so if the signal is more than 5v you will get the higher voltage to pass through it.
someone help me out, i am not an computer engineer, but i am pretty sure that is how it works.
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Old 10-31-2001, 04:31 AM   #10
Kcz
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That sounds perfect. I'm not any sort of electronics engineer myself. The voltage is reading too low. If I can keep it at what it wants and above, It should keep the CEL off. I think this would be useful for anyone that has a CEL caused by something unharmful. Austin, wouldn't a resistor drop the voltage even more. I need to raise it. If you have an idea please post. I need to keep the voltage within a certaian range. I want to keep the ECU happy.
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Old 10-31-2001, 09:27 AM   #11
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I thought your rear o2 sensor volts were too low, but now that I reread your post, I see they're too high... I didn't think that could produce a MIL.
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Old 10-31-2001, 11:27 AM   #12
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A long time ago rallispec posted up some really good instructions on how to do this, try doing a search.

Brian
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Old 10-31-2001, 11:38 AM   #13
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I meant cut the CEL wire. If you have a turbo, you should have enough guages to know what is going on with your motor. Once the turbo goes on, the CEL is worthless (unless you like the yellow glow). Now, if you could modify it to say, Check Out My Engine, that would be worth keeping.
Kevin
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Old 10-31-2001, 11:58 AM   #14
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Quote:
Originally posted by Perrin Subaru Performance
the zener just limits voltage going in one direction. say you have a 5v zener diode, it will not let anything less than 5volts pass through it, so if the signal is more than 5v you will get the higher voltage to pass through it.
someone help me out, i am not an computer engineer, but i am pretty sure that is how it works.
That's not right. A diode has a turn-on voltage after which point it starts flowing current. This has the effect of having a maximum voltage drop across the diode, which is the turn-on voltage.

So if a diode has a 5v turn-on voltage and it is wired between a signal wire and ground, then it will clamp the signal wire to 5v.

I'm assuming the reason everyone is using a zener diode is because of their varying turn-on voltages. I think a lot of "normal" diodes (at least the ones I've used back in school) have a turn-on voltage of 0.7v. I think LEDs are around 1.4v. So you could actually get a voltage clamp of around 5v by wiring 7 0.7v diodes in series.

A zener diode has the added feature of being able to be reverse biased as well, which is probably not important for what everyone is using it for... unless you're using the reverse bias voltage as the clamp and hooking the diode up backwards.
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Old 10-31-2001, 02:43 PM   #15
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Some relevant info about zener diodes:

http://www.impreza-rs.com/forums/sho...5&pagenumber=2


Regards,
Ed.
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Old 10-31-2001, 05:01 PM   #16
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Quote:
Originally posted by iodine23

...
A zener diode has the added feature of being able to be reverse biased as well, which is probably not important for what everyone is using it for... unless you're using the reverse bias voltage as the clamp and hooking the diode up backwards.
Reverse biased is actually the normal use of a zener diode, and the one that people should be putting it to for this application. The reverse breakdown voltage is the main controlled characteristic of a zener. I.e., a "4.7V zener" has a reverse breakdown voltage (i.e., "zener voltage") of 4.7V.

The problem of using zener diodes alone for this purpose is that it is required that a certain amount of current be flowing through the diode (i.e., the "zener current"). Below this threshold, the reverse breakdown voltage is lower, and not very well characterized. This is why some people have had success using 5.6V zeners to clamp a signal to 4.85V or less. And the uncontrolled nature of this region may be why some other people using the same part have not had as much success. Most zeners are characterized for zener currents of 50mA or higher. Some, however, like the 1N4688 mentioned in earlier posts, are characterized at a very low 50uA. This should be low enough that a 4.7V rated zener diode actually clamps at somewhere around 4.7V, even assuming a very small sensor output current.

You definitely DON'T want to hook it up forward biased, as this essentially shorts out the signal you're trying to measure.

In other words, the anode end should be hooked to ground, and the cathode end (the one with the band) should be hooked to the signal output.
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Old 11-01-2001, 12:31 AM   #17
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i was close........sort of.
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