How to detect the type of diode?
john_s
Posts: 369
Lets say I have an unknown device that looks like a diode...
Now, how would you figure out WHAT type of diode is it - Schottky, Zener, or "plain" (... and without destroying it during test
)
Now, how would you figure out WHAT type of diode is it - Schottky, Zener, or "plain" (... and without destroying it during test
)
Comments
Here's a similar discussion with some tips:
https://community.pennfoster.edu/thread/11159
... and I'm trying to figure out that "complicated" part of this task
1) is it a diode ?: connect 10k resistor to 9v battery minus. The other end to the device lead with a ring (cathode) and the other end (anode) to the 9V plus.
2) what's the voltage accross the diode? 0.2V to 0.35V ~ germanium (rare)--- 0.2V -0.35V Schottky--- 0.5V-0.7V = silicon / zener
3) reverse the diode, does voltage go to 9V? No, then its probably a zener. Yes, then plug in 3 more 9V batteries in series to get 36V. (or use a power supply). Does it zener now?
4) Look at it under a microscope or loop. Can you see a chip cap in there. They usually don't have cathode rings.
A few times I was able to read the end of the PN using a microscope. 1Nxxxx without the 1N. Adding 1N gave me a real diode number and that led me to the right datasheet.
Schottkys tend to destruct with reverse voltage over their limit, which tends to be lower than silicon. 50V or less maybe. Using a 10K resistor limits current to 1ma - 4ma to help protect the part.
Even after you figure out a few things you won't know the device breakdown voltage, reverse leakage current, wattage, the forward dropV/current, max current, capacitance, switching speed and other design considerations found in a datasheet. Good Luck.
Dom..
1. If the forward drop is around 0.6V and the reverrse drop is infinite, it's probably a plain silicon diode.
2. If the forward drop is around 0.6V and the reverse drop is measurable, say less than 30V, it's likely a zener.
3. If the forward drop is 0.5V or less it's either a Schottky or, if it's really old, a germanium diode.
But these readings tell you nothing about the PIV or current ratings, and you will not be able to measure those nondestructively. so I'd still just toss it.
-Phil
All that to be measured at what V and I to play it safe yet get the meaningful results ?
-Phil
Make like a Curve Tracer
Forward direction up to 100mA should be safe for all but the smallest diodes.
In Reverse start with 1uA / 10uA / 100uA and check the reverse voltage.
Silicon diodes have much lower reverse leakage than Schottly diodes, and Zeners have a soft knee at those currents, so once you figure it is looking like a Zener, you can try the more usual 5mA or 20mA reverse.
Do you have a short list of possible suspects ? ( or is this some bulk special buy ? )
Yeap, the first one is SMD type, glass, MELF diode with a single blue band.
It's inside the PLC' power supply made in Germany.
Love your curve tracer idea
It is pointless to create a curve tracer and to apply voltages if you don't take into consideration the amount of watts the device is limited to. At some point, you must limit current or voltage in relation to the power capacity of the diode.
This may also give you an immediate indication that the diode is just too small for whatever project you are working on.
Color codes are often proprietary and ordered by the manufacturer (maybe the military). Management of such junk tends to cost more in time than it is worth. It is also likely that there are newer better products that have overtaken the market.
This has a color code. A single Blue band indicates a Zener diode.
That alone creates yet another nightmare. Are you going to calibrate, catalog, and store a full range of zener diodes. The range of voltages is quite wide.
As others have said, it is just easier and more pleasant to buy what you need when you need it. Of course, if you are trying to reverse engineer for a repair, the zener may be damaged or out of spec and you have arrived at a dead end.
This one looks like a good starting point - thanks.
I'll look into this - still remember Huntron tracer and Tek HUGE one
I google and also found these old goodies...
So true, but not all that much more difficult if you have a prop and an adc.
An oscilloscope is really the 'swiss army knife' tool of electronics. (I do admit I am a bit old school analog about this.)
No arguments from me on that point. A good scope is one of the best tools available. OTOH sometimes you either have to make do with what you have at hand, and sometimes a scope cannot do what you need to do. A good scope is great for analog signals, not so good for things like serial data or parallel bus signals.