Diode Test Circuit?

[vigilante397]

The school at which I study electrical engineering regularly cleans out some of the "old junk" (also known as vintage goodies) lying around, and I recently had the opportunity to pick up a decent supply of assorted diodes, transistors, and lovely capacitors.

The transistors, capacitors, and silicon diodes are all labelled, but there are a handful of Germanium diodes that are unmarked, and I have no idea what they are or what they do.

Does anyone have a schematic for a test circuit that might let me figure out what these things are? It's a shame to just have them sitting on my desk staring at me.

Thanks 

[Seljer]

Hook the diode up to a power supply through a 10kiloohm resistor. Measure the voltage across the diode. That's a rough measurement of the forward voltage and should quickly let you ballpark what type of diode it is.

If you want to be fancier you can take a set of voltage measurements at different currents by using different resistors or a potentiometer or a variable power supply and then drawing full current-voltage characteristic. The most simple way is to hook up a second multimeter in series to measure current, but if you only have one, you can use ohm's law to get the current from the voltage drop over the resistor.

[duck_arse]

show us some photos. us old blokes love looking at stuff from our yoof.

[vigilante397]

Hook the diode up to a power supply through a 10kiloohm resistor. Measure the voltage across the diode. That's a rough measurement of the forward voltage and should quickly let you ballpark what type of diode it is.

Exactly what I was looking for I don't know when I'll get to this as finals week at school is next week which means no sleep this week, but hopefully I can do it soon 

show us some photos. us old blokes love looking at stuff from our yoof.

I can definitely get some pictures up when I get home today

[vigilante397]

So here's a picture of some of my lovely unlabeled finds. The transistor on the left may have been marked at one time, but even held up to light I don't see traces of writing. 99% sure it's germanium though, largely because it has the transparent top. The diodes in the middle seem to be germanium as well, and also look as though they never had a marking on them. The cathode is marked rather nicely though, leading me to believe they're not that old. The red diodes on the right are silicon, but they have copper leads which I've never seen before, and there is no apparent marking of the cathode. Is there such a thing as non-polarized diodes? 

[PRR]

Clear-top transistor is likely a Photo-transistor.

My vote on the red things is capacitor or exotic resistor--- what makes you say they are Silicon diodes?

[vigilante397]

Clear-top transistor is likely a Photo-transistor.

My vote on the red things is capacitor or exotic resistor--- what makes you say they are Silicon diodes?

A quick google search for phototransistor leads me to believe you're right about that one.

But now that you mention it the only reason I was assuming they were diodes was because they were in a bag with other diodes (not exactly scientific reasoning), so it didn't even cross my mind that they could very well be something else

A quick check with my multimeter told me basically nothing. If they're resistors then the resistance is above 200M, which I find unlikely. The capacitance meter jumped a little bit when I set it at 2000pF, but I've never really trusted the capacitance meter on my multimeter.

[Jdansti]

Check the resistance and then swap the leads. If they're diodes, you should get low resistance in one direction and very high resistance (infinite/open) in the other direction.

[vigilante397]

Check the resistance and then swap the leads. If they're diodes, you should get low resistance in one direction and very high resistance (infinite/open) in the other direction.

I just tried that, and it's giving me open on both directions :-/

Should I cut one open and see what's on the inside?   Not really sure what that would get me, but it's an option

[Mario44]

Middle ones are germanium diodes.
I've got the same in my mxr dist+.

[vigilante397]

Middle ones are germanium diodes.
I've got the same in my mxr dist+.

Yeah, I'm positive they are germanium, I'm just not sure exactly what exactly they are or what they do.

[Jdansti]

Check the resistance and then swap the leads. If they're diodes, you should get low resistance in one direction and very high resistance (infinite/open) in the other direction.

I just tried that, and it's giving me open on both directions :-/

Should I cut one open and see what's on the inside?   Not really sure what that would get me, but it's an option

Maybe they're capacitors. Do you have a capacitance meter?

[GibsonGM]

Middle ones are germanium diodes.
I've got the same in my mxr dist+.

Yeah, I'm positive they are germanium, I'm just not sure exactly what exactly they are or what they do.

Well, they're Ge diodes )  What they do?  Same thing all other *regular* diodes do....allow current to flow in only one direction. 
Doing the above test for voltage drop, or "Vf", will let you know.....the Vf...which will help you decide what to do with them. 

I recommend you use them for CLIPPING in really cool distortion circuits!   The Vf will be lower than silicon diodes (oh, roughly .3V vs .5V), making them clip earlier and, some say, more 'aggressively'...let your ears to the talking...beyond that, there isn't much to know about them, IMHO, YMMV.    They also make great detectors for crystal radios.

The red guys really do look like caps... you could try this:

Battery "+" >>  meter red probe, set to mA >> meter black probe >> 1Meg resistor >> cap >> battery "-".    See if you can get a curve going on the meter....a "fast-ish" rise in current that then slows down and settles.   An ANALOG meter is great for this, but it CAN be done with a DMM.  Short the cap leads together before re-testing.

If it's a diode, you'll get either NO current flow or one level of constant current.     If not much happens (too slow) decrease the resistance.  Anyway, a slowly rising current that seems to slow but never end means they're caps.  Hope that makes sense...

[duck_arse]

I've got some germanium diodes exactly very similar looking to your middles shown. my yellow banded ones have "60" marked on, so they are OA60, and that means the green banded ones would be most likely "OA47".

but don't quote me.

[armdnrdy]

Scroll down on this page and you'll find some useful information on identifying the green banded diodes:

http://wiki.waggy.org/dokuwiki/crystal_radio/detector

[vigilante397]

Maybe they're capacitors. Do you have a capacitance meter?

My DMM has a capacitance meter, but either I don't know how to use it properly, or it doesn't work.

The red guys really do look like caps... you could try this:

Battery "+" >>  meter red probe, set to mA >> meter black probe >> 1Meg resistor >> cap >> battery "-".    See if you can get a curve going on the meter....a "fast-ish" rise in current that then slows down and settles.   An ANALOG meter is great for this, but it CAN be done with a DMM.  Short the cap leads together before re-testing.

If it's a diode, you'll get either NO current flow or one level of constant current.     If not much happens (too slow) decrease the resistance.  Anyway, a slowly rising current that seems to slow but never end means they're caps.  Hope that makes sense...

This does make sense, and I think I will give it a try this evening.

Well, they're Ge diodes )  What they do?  Same thing all other *regular* diodes do....allow current to flow in only one direction. 
Doing the above test for voltage drop, or "Vf", will let you know.....the Vf...which will help you decide what to do with them. 

I recommend you use them for CLIPPING in really cool distortion circuits!   The Vf will be lower than silicon diodes (oh, roughly .3V vs .5V), making them clip earlier and, some say, more 'aggressively'...let your ears to the talking...beyond that, there isn't much to know about them, IMHO, YMMV.    They also make great detectors for crystal radios.

I was trying to think of a distortion to test them in, then I remembered my Proco RAT build that's waiting for an IC I already have silicon diodes in it, but I've seen a couple distortions that have switchable silicon/germanium clipping sections, so I think I'll try that out 

[vigilante397]

Scroll down on this page and you'll find some useful information on identifying the green banded diodes:

http://wiki.waggy.org/dokuwiki/crystal_radio/detector

This is incredibly useful, thank you So according to this it looks like they are a couple of "Modern Germanium 1N34A," but I haven't gotten around to checking forward voltage yet. Finals week is next week, so it's not exactly a priority 

[PRR]

> Check the resistance and then swap

That works for old-school meters (mostly needle-meters).

Most DMMs limit resistance voltage to 0.199V, so are useless for Silicon and dubious for many Germanium junctions.

A needle-meter is good to have. For this chore, the $9.99 one in the home center is fine. (Not so good for reading voltages in electronics.)

These mostly have 1.5V battery for Ohms. A 0.6V forward Silicon junction will swing the needle not quite half-scale. A ~~0.2V Germanium junction will sit closer to the "zero" end of the Ohms scale.

It is also practical to use a battery and resistor and DMM to check forward/reverse drops. Best might be a 1.5V cell and 1K resistor. A 9V and 10K will sort diodes, but the 9V may break-down transistor junctions, give unexpected readings, and change the transistor's fine properties.

[vigilante397]

So I bought myself a transistor/diode/capacitor tester, and it turns out the red ones are indeed 0.47 nF capacitors.