OP
The Chairman
Chairman of the Board
Great article Jax... Thanks to everyone, but special thanks wedivebc and Zung!
Electronics help... esp with a diode.
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Pretty good discussion on identifying unknown Zeners: Build a Zener Diode Identifier : Identifying an Unknown Zener
Looks like fun to build the tester.
The article states the voltage drop is dependent on the "ohmage" of the resistor and that just ain't so. The whole point of a zener is to supply a constant voltage drop over varying input voltages. The op amp based tester is a nice project but I have tested hunderds of zeners with a voltage divider circuit and as long as you don't exceed the maximum current of the diode the voltage drop across the diode is constant. That's what they do.
OP
The Chairman
Chairman of the Board
While I understand (and have for a long time) what a diode does (one way gate), it's always intrigued me why there are so many types: switching, power, Zener, Schlotsky, etc. For those trying to also get a handle on this device, here is the best description of a diode and a bridge I have seen so far:
The problem I am having is with an oxygen analyzer I own that stopped reading. I bought a new cell and it didn't come back on. I pulled it apart and was surprised to find it had only eight discrete parts: a battery holder, a switch, a Lascar SP200 panel meter, a 1.5K resistor, a 5k pot, the cell and this mysterious diode all mounted on a PCB. Mind you, I saw the issue with the circuit was corrosion and swelling on a few of the solder joints and rust on the battery holder. Poking around with my DVOM verified that these were problems. I soldered in a new battery holder, refreshed a few solder joints and ran a wire to replace one of the PCB runs that was not showing continuity. The analyzer works perfectly until I put it into the housing then it goes into a countdown from 199 to -199 and starts over again. So with just a few fairly inexpensive components, I am trying to reverse engineer so I can have my analyzer back, and I don't mind having a better idea how it does it's thing at the same time.
So, I have the circuit built out on a breadboard. I carefully pinned out the panel meter and have dutifully followed the circuit board path. I had some electronics hanging around and the other pieces were easy and cheap to get. A replacement panel meter was the most expensive part and it's only $30. Since I had no idea what diode I had, I simply got a variety of them:
For $18, shipped to my door, I thought it was a deal. So, I pinned everything out on a breadboard and guessed at which diode to use. When I switched it on I got a -1 on my display, even when I flipped the polarity. So, I go to the next diode and try again. Some of the diodes clearly aren't what I need, like the big power diodes or the bridges. I tried each and every diode, even reversing polarity with no joy. The more I think about this, the more I don't think it's the diode. Consequently, I'm going to re-suss my pin-out.
I haven't made much progress because a friend from my childhood has come down to visit me. We went diving one day and then hiking to see Key Deer yesterday, so I was pretty tired when I got home last night. It's always fun to reconnect with the past, and Joe and I had a lot of memories to relive and some of them are embarrassing. However, I'm feeling fresher this morning and I have an hour or so to retrace the circuit before I drive him to the airport, hopefully before he reveals too many secrets. Heck, I might even snap a pic or two of the project.
The problem I am having is with an oxygen analyzer I own that stopped reading. I bought a new cell and it didn't come back on. I pulled it apart and was surprised to find it had only eight discrete parts: a battery holder, a switch, a Lascar SP200 panel meter, a 1.5K resistor, a 5k pot, the cell and this mysterious diode all mounted on a PCB. Mind you, I saw the issue with the circuit was corrosion and swelling on a few of the solder joints and rust on the battery holder. Poking around with my DVOM verified that these were problems. I soldered in a new battery holder, refreshed a few solder joints and ran a wire to replace one of the PCB runs that was not showing continuity. The analyzer works perfectly until I put it into the housing then it goes into a countdown from 199 to -199 and starts over again. So with just a few fairly inexpensive components, I am trying to reverse engineer so I can have my analyzer back, and I don't mind having a better idea how it does it's thing at the same time.
So, I have the circuit built out on a breadboard. I carefully pinned out the panel meter and have dutifully followed the circuit board path. I had some electronics hanging around and the other pieces were easy and cheap to get. A replacement panel meter was the most expensive part and it's only $30. Since I had no idea what diode I had, I simply got a variety of them:
For $18, shipped to my door, I thought it was a deal. So, I pinned everything out on a breadboard and guessed at which diode to use. When I switched it on I got a -1 on my display, even when I flipped the polarity. So, I go to the next diode and try again. Some of the diodes clearly aren't what I need, like the big power diodes or the bridges. I tried each and every diode, even reversing polarity with no joy. The more I think about this, the more I don't think it's the diode. Consequently, I'm going to re-suss my pin-out.
I haven't made much progress because a friend from my childhood has come down to visit me. We went diving one day and then hiking to see Key Deer yesterday, so I was pretty tired when I got home last night. It's always fun to reconnect with the past, and Joe and I had a lot of memories to relive and some of them are embarrassing.
However, I'm feeling fresher this morning and I have an hour or so to retrace the circuit before I drive him to the airport, hopefully before he reveals too many secrets. Heck, I might even snap a pic or two of the project.
I would guess it may be a Zener so as to have a constant voltage reference to be used as a comparator. Also constant voltage would mean more a more accurate baseline for the readings. I have a couple of older electronic textbooks from when I went to Penn State if you are interested. Let me know and I'll get them to you.
OP
The Chairman
Chairman of the Board
That would be too much information for me. I don't mind playing a bit with this stuff, but I'm not making a career of this.
I don't mind playing a bit with this stuff, but I'm not making a career of this.
flots am
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Good luck with that.
You could figure this out with a variable power supply and a meter, although it's probably easier and cheaper and faster to just buy what you need.
If there aren't any markings your best bet is to see if someone is familiar with the equipment it came from and if they know what it's supposed to be.
flots.
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-look for a hairline crack / broken wire on one of the printed circuit traces or a cold solder joint that is opening from mechanical stress when you re-assemble everything. By broken wire I mean look to see if the electrolye wicked up into a wire's insulation and caused damage to the thin copper conductors (is not obvious), very common problem. Corrosion is the key word that we didn't have.
A zener usually fails as a short, when failed, it looks the same in both directions. If the original conducts in one direction and not in the other (with your ohm meter) then just put the original one back, it is probably fine.
-may not even be a zener, could just be a simple diode that keeps you from frying the DVM display if you touch the battery contacts the wrong way while attaching a new battery...
mrfixitchapman
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The 'diode check' function of the multimeter indicates the voltage that is required to 'forward bias' the diode. Most silicon diodes will require .400V - .500v to forward bias, (begin to conduct). This meter reading will also correspond to the voltage loss across the diode when the diode is conducting. So if you applied 3.0V across the diode and it had a forward bias voltage of .450V then you would see an output voltage of 3.0V minus .45V = 2.55V When you reverse bias the diode by connecting the leads 'backwards' the meter should read 'infinite' because there should not be a path for current to flow through the diode since it is acting as a open circuit.
TLDR= You should get a voltage reading one direction and 'infinite' reading the other. If you get voltage reading both directions the diode is shorted and it has failed. If you get 'infinite' readings both directions the diode has failed 'open'
PS the 'swing needle' meters used to be pretty reliable diode testers, but digital multimeters use such a small amount of current while measuring resistance that they are not well-suited for that type of test.
PPS the end of the diode that has the stripe across it is the negative end. It helps to know that when you are deciding which way is 'forward'.
DC
TLDR= You should get a voltage reading one direction and 'infinite' reading the other. If you get voltage reading both directions the diode is shorted and it has failed. If you get 'infinite' readings both directions the diode has failed 'open'
PS the 'swing needle' meters used to be pretty reliable diode testers, but digital multimeters use such a small amount of current while measuring resistance that they are not well-suited for that type of test.
PPS the end of the diode that has the stripe across it is the negative end. It helps to know that when you are deciding which way is 'forward'.
DC
OP
The Chairman
Chairman of the Board
So, who has a great oxygen analyzer circuit they would like to share?
Rooster59
Contributor
Can you 1) sketch a schematic of the circuit 2) upload a photo of the diode
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