What Happened to my Optos?!

[YouAre]

Hey Guys,

Have a bit of a phenomenon here with all of my LDR's/optos.

I've been using them successfully for some time in my Seek Wah Clone and successfully built a Tap Trem, and have been getting proper values from them. Depending on the LDR/Vactrol, I'll get proper ranges like 3-8Megs Dark Resistance or 300k-500k (depending on product of course), and like 5-10k Light Resistance, pretty typical values and close to what is documented for the products.


Now all of my Optos, Vactrols, and LDR's are not within any sort of proper range at all. Dark Resistances are in the range of 16k-23k, and light resistances are 600ohms-900ohms.

The Optos that are behaving like this are: VTL5c3, VTL5c9, and the 8002 and 8102 from http://www.smallbearelec.com/Detail.bok?no=711 .

What could cause all of my opto's to be misbehaving like this?

It should be noted that my DMM seems to be working, as it measures resistors and potentiometers accurately. I am testing the Vactrol with a simple voltage divider/pot setup that functions perfectly with an LED and gives a nice even linear performance for Off to Brightest on the LED. I test the LDR's the same with, having sealed them with an LED inside of heat shrink. I also test the LDR's with my desk lamp by covering the cell and by exposing it to light. All these test yield similar results as to what was posted earlier.


Side notes:

I first noticed this when i was tweaking the Tap Trem to have less noise, and I heard a pop. I assumed it was either a static pop, or the power wire must've shorted out on something on my prototyping setup. I assumed I damaged the Opto, so i replaced it with another one on the desk. Similar results.....so then I replaced it with an opto from my parts bin on the opposite side of the room. Same results. I measured all my optos, and they all gave the numbers i posted above.


Thanks again!

[darron]

heard a pop? well of course half of a vactrol is an LED, and like any with too much current can burn out, or half burn out and be damaged. but i would expect this to cause a higher resistance in the LDR side, not lower as you are experiencing. i'll watch for replies with interest...


of course i assume you're not reading the resistances now that they are plugged into the circuit? i bet you didn't but just in case... i'm not quite sure if you removed them while in the voltage divider setup...

[petemoore]

  If there's a measurable change in LDR R, ...when the LED has current going through it the LDR/LED must be working.
   "Not the typical values'' of LDR Resistance would be caused by the opto's LED circuit [1rst guess]  not driving the LED to bright/ off.

[YouAre]

heard a pop? well of course half of a vactrol is an LED, and like any with too much current can burn out, or half burn out and be damaged. but i would expect this to cause a higher resistance in the LDR side, not lower as you are experiencing. i'll watch for replies with interest...


of course i assume you're not reading the resistances now that they are plugged into the circuit? i bet you didn't but just in case... i'm not quite sure if you removed them while in the voltage divider setup...

The LED's seem to function in the vactrols because the resistance isn't just reading as ~5megs (dark resistance), and the light resistances go to ~.8kohms. So applying current has an effect. And you're right, the resistor side is not in any circuit.

  If there's a measurable change in LDR R, ...when the LED has current going through it the LDR/LED must be working.
   "Not the typical values'' of LDR Resistance would be caused by the opto's LED circuit [1rst guess]  not driving the LED to bright/ off.

The opto's LED circuit works fine. It's a basic voltage divider i rigged to test LED's from off to bright. The main problem is, i'm getting really low Dark resistances (<100k) and extremely low light resistances (<1k). The latter shouldn't be possible based on the provided specs for each product. I don't think they're defective from the manufacturer, because they gave their correct resistances before, but don't work now.

[petemoore]

  If the R's sweep value, the LDR's must be doing something, an LDR's LED could fail, and have different current needs compared to the LED.
   1k is low, the battery in the DM meter is good and reads ~1k across a 1k fixed resistor ?
  When the LDR is in the circuit, the measurements should show the parallel LER//circuit, which can measurably change [reduced] the R reading.
   What is the Dark resistance with no power supply to the circuit/LED ?

[YouAre]

  If the R's sweep value, the LDR's must be doing something, an LDR's LED could fail, and have different current needs compared to the LED.

I believe the LDR's are doing something, just not in a usual range. This is also with non sealed LDR's, like from the link I posted earlier. So testing under a lamp removes the LED from the equation, and i still get the same results.    

1k is low, the battery in the DM meter is good and reads ~1k across a 1k fixed resistor ?

The DM seems to work. I didn't test a 1k resistor, but I don't see why it wouldn't work when i measured pots and other value resistors, and they worked fine.

When the LDR is in the circuit, the measurements should show the parallel LER//circuit, which can measurably change [reduced] the R reading.

The LDR is not in any pedal circuit. It is being manually controlled by a voltage divider with a pot.

What is the Dark resistance with no power supply to the circuit/LED ?

Depending on the product, <100k for the most part.

Thanks for all the help thusfar!

[amptramp]

The sensing element in the LDR is usually CdS or CdSe which is notoriously susceptible to moisture contamination.  If this happens, you will get a substantial reduction in resistance and the resistance will not be stable - it will continue to change.  Incidentally, the first active matrix panel used CdSe MOSFET's as the switching medium and the name, "active matrix" was invented by the founder of the transistor array that is now used in most LCD displays, Peter Brody.  I have seen an electroluminescent television first built using CdSe high-voltage transistors vapour deposited on glass at his house in Pittsburgh in 2001.  This display was almost thirty years old at the time and had numerous lineouts due to the primitive processing, but could still be viewed.  I worked extensively with CdSe for many years for LCD displays.

[YouAre]

The sensing element in the LDR is usually CdS or CdSe which is notoriously susceptible to moisture contamination.  If this happens, you will get a substantial reduction in resistance and the resistance will not be stable - it will continue to change.  Incidentally, the first active matrix panel used CdSe MOSFET's as the switching medium and the name, "active matrix" was invented by the founder of the transistor array that is now used in most LCD displays, Peter Brody.  I have seen an electroluminescent television first built using CdSe high-voltage transistors vapour deposited on glass at his house in Pittsburgh in 2001.  This display was almost thirty years old at the time and had numerous lineouts due to the primitive processing, but could still be viewed.  I worked extensively with CdSe for many years for LCD displays.

It has been disgustingly humid in jersey the past couple of days....could that be a factor? It should be noted that some of the LDR's were sealed in plastic bags inside a plastic parts bin.

[amptramp]

Humidity seems consistent with this problem.  The conductive element on these devices is on the order of maybe hundreds of nanometres thick, which may translate into a few dozen atoms.  And humidity can go through a plastic bag easily and any parting line in the plastic casting of the photocell is also suspect.

[YouAre]

Humidity seems consistent with this problem.  The conductive element on these devices is on the order of maybe hundreds of nanometres thick, which may translate into a few dozen atoms.  And humidity can go through a plastic bag easily and any parting line in the plastic casting of the photocell is also suspect.

Ahhh crap. I'm going to have to buy more opto's to test this, aren't I?


Wait, what about the vtl5c3 and vtl5c9 vactrols? Wouldn't the hermetic sealing protect against this?

[amptramp]

It does not appear to be something like surface leakage - that would give you a much reduced dark resistance but not much of a change in illuminated resistance.  Surface leakage certainly could not bring the 5K to 10K illuminated resistance down to 600 to 900 ohms, so it appears to be internal.  It would be interesting to put thses devices in dry heat or a heated vacuum to see if they recover - but they would still have to be sealed in order to be used even if they did recover.  It does appear to be a defect, because photoresistive elements are used in a lot of places where this kind of deterioration would cause immediate problems.

[YouAre]

It does not appear to be something like surface leakage - that would give you a much reduced dark resistance but not much of a change in illuminated resistance.  Surface leakage certainly could not bring the 5K to 10K illuminated resistance down to 600 to 900 ohms, so it appears to be internal.  It would be interesting to put thses devices in dry heat or a heated vacuum to see if they recover - but they would still have to be sealed in order to be used even if they did recover.  It does appear to be a defect, because photoresistive elements are used in a lot of places where this kind of deterioration would cause immediate problems.

I just find it really odd that all of my photocells, stored in different parts of the room are all behaving this way. Most of them were used/tested and worked find previously

[petemoore]

  Been fairly humid, never had a photocell act wierd that I remember.
   Especially not a 'batch', whenever I see 'a whole batch of 'should work' parts seemingly earining failing marks, the suspicion is directed to whatever they all have in common, ie the 'peripheral test stuff' power supply, DMM, anything 'else' besides all the LDR's going bad at one time.

[YouAre]

  Been fairly humid, never had a photocell act wierd that I remember.
   Especially not a 'batch', whenever I see 'a whole batch of 'should work' parts seemingly earining failing marks, the suspicion is directed to whatever they all have in common, ie the 'peripheral test stuff' power supply, DMM, anything 'else' besides all the LDR's going bad at one time.

You're right. I guess the best way to test it is replace the PS, the voltage divider, and the DMM. The only thing i can't replace is the DMM. I'll report in a few with my findings.

[YouAre]

Ok.....

New Power supply, different breadboard, new multimeter....same results!

Here I've plotted the input current vs resistance for the vtl5c3 vactrol, and conveniently added R', which is what the resistance "should" be around. May not seem like much, but these are pretty significant differences in what we can expect in variance of vactrols.

Here's the graph:


And here's the rough data.


(all resistance in K)
I   R   R'
0.1   470   1500
0.2   210   450
1   15   40
2   6   15
3   3.8   10
4   2.7   8



For reference, here's the datasheet for all the vactrols of this company:

http://www.datasheetcatalog.com/datasheets_pdf/V/T/L/5/VTL5C9.shtml

Scroll to page 45 for the I vs R chart.


What do you guys think? This doesn't seem like damage to the LED portion, but to the photo resistor. The results for the vtl5c9 were even worse, so odd I didn't bother graphing them, but the raw data was:

I   R   R'
0.03   30   
0.1   2   30
0.5   0.46   1
1   0.19   0.4





If I can't get any insight, I'll have to contact smallbear about replacements :/

[YouAre]

Polite bump.


Anyone have any experience with optos drastically drifting out of their spec'ed range?

[YouAre]

Also, I am open to suggestions for alternative ways of testing my opto's resistance range. Alternatively, I'd be willing to ship my optos to someone to check them out/pay for return shipping. I am THAT vexed by this situation!

[PRR]

What is this "breadboard"? Specifically: how do you set/measure LED current? What are you using to measure resistance?

[YouAre]

What is this "breadboard"? Specifically: how do you set/measure LED current? What are you using to measure resistance?

Breadboard looks kinda like this:

http://www.dbmsignaldynamics.ca/products/images/breadboard.jpg

Not using the banana plugs for rails. The rails are wired straight to a standard power jack, powered by a One spot and a Godlyke Powerall.

The 9volts feeds a 5k trimpot setup as a voltage divider. Output of that goes to a 1k resistor. 1k resistor goes to LED portion of Vactrol. Negative leg of Vactrol's LED goes to DMM setup as current meter. Output of current meter goes to positive leg of another LED. Negative lead of LED #2 goes to ground. This way, I can tell my vactrol's are passing current. Another DMM reads the resistance of the vactrol's output