Glass diode photoelectric effect - Page 1
Forum section: electronics / projects

author: GK
author_rank: Super Contributor
timestamp: May 11, 2013, 12:29:23 am
content: Last night I spent around 20 minutes trying to track down a source of leakage current in an integrator circuit. The integrator circuit in question is a design for my analog computer project and I was just making some final tests on the prototype before going ahead and making another 29 of them. The problem was, in the "hold" mode of operation, in which all input sources are switched off, I was getting a steady and constant output voltage drift in the negative direction at about 0.5V per second. With a 10nF timing/integration capacitor switched in, this corresponds to a positive input current of ~5nA - way higher than the input bias current of the jfet op-amp that forms the front end of the composite +/-100V integrator.For protection the circuit has a pair of BAS45A low leakage diodes in anti-parallel at the input. At first I thought I must have a severe case of board surface leakage, but all voltage sources anywhere near the integrator input were negative, which means if that was the case, the voltage drift at the output of the integrator would have been in the positive direction rather than the negative. So, ruling that out, I remembered that silicon diodes packaged in clear glass can exhibit sensitivity to light. however until now I have never measured it and did not know to what magnitude.Well, I reached up and turned off the desk lamp illuminating my PCB on the workbench and sure enough the integrator drift magically stopped. Turned the light back on, and it was back. Turned it off again and..... yada yada.... Of course, the source of the troublesome 5nA input current was one of the ant-parallel input protection diodes.The pics attached show a quick measurement of the phenomenon. A BAS45A low-leakage, glass-encapsulated diode is simply connected to a x10 CRO probe. Illuminated with light, as shown on the oscilloscope trace, I get a whopping -400mV, which, into the 10M input resistance of CRO probe, corresponds to a photoelectric current generation of 40nA.The rectangular wave response shown on the oscilloscope trace, with the timebase in "roll" mode, was generated simply by turning the desk lamp illuminating the diode on and off. Note that the anode of the diode was connected to the probe ground clip.Well there you have it; something to be aware of when applying unpainted glass diodes in certain applications!

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 12:35:52 am
content: Does anybody know why, since this is specifically meant to be a "low-leakage diode", they would even put it in a clear glass package in the first place? Surely it wouldn't cost too much to put it in a plastic DO-41 like the 1N400x series, or even a TO-92 or something if size is an issue?

author: GK
author_rank: Super Contributor
timestamp: May 11, 2013, 12:38:45 am
content: That's a good question! On that topic can anyone suggest a low-leakage black paint that adheres well to glass?

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 12:46:27 am
content: I've actually run into the same issue before. I had some ofthison hand (the kind in a jar, not the spray can). Nice, thick, rubbery coating that basically turns them into DO-41s, and it's quick to apply. Leakage current tested under a minimum resolution of measurement of 1nA at 100V once dry and it appeared to do a perfectly fine job blocking every bit of light.

author: branadic
author_rank: Super Contributor
timestamp: May 11, 2013, 12:49:11 am
content: Glas packages are some sort of hermetical sealed packages instead of the plastic version.

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 12:54:36 am
content: True, but why couldn't they just give the glass package a plastic dip afterwards?

author: pickle9000
author_rank: Super Contributor
timestamp: May 11, 2013, 01:30:31 am
content: Use a layer of heat shrink then foil then heat shink. No dry time or risk of conductivity.

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 01:40:48 am
content: Quote from: pickle9000 on May 11, 2013, 01:30:31 amGood idea. +1

author: GK
author_rank: Super Contributor
timestamp: May 11, 2013, 02:43:25 am
content: Actually it's not particularly important in this application as when housed the boards will be in the dark anyway; just good to know. The schematic of my integrator module is attached. The "hold" mode occurs when both the !Compute and !I.C. (initial condition) logic control inputs are high and both JFETs Q1 and Q2 are off. D5 and D6 then become the photoelectric current generators. It could be argued that low leakage diodes aren't even required here as being at virtual ground they only effectively see Vos of U2 plus the small "error" voltage, but it doesn't hurt to pick them just for good measure anyway.

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 02:45:07 am
content: What software are you using to prepare the schematics? They look very nice and I don't recognize the appearance.

author: GK
author_rank: Super Contributor
timestamp: May 11, 2013, 02:50:34 am
content: Protel 99SE. I drew my own component library and print to CutePDF.

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 02:54:16 am
content: Quote from: GK on May 11, 2013, 02:50:34 amIt looks very professional. I never used to care for the style, but I'm really starting to like the "textbookish" look of the bold lines around the semiconductors and such. Nice work

author: SeanB
author_rank: Super Contributor
timestamp: May 11, 2013, 04:51:28 am
content: The light sensitivity is actually noted somewhere in a data sheet, and is known. As you found out most of the time the cure is to put it in a shielded case ( done for RFI normally, but gets rid of the light as well incidentally) or to pot the section of board top and bottom in a black potting compound. The reason for using glass is that then you are sure that the die cannot be exposed to moisture, as it was well dried during the application of the glass, so there is no surface leakage path inside. Epoxy is slightly porous to water vapour.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 11, 2013, 06:08:35 am
content: For the record, I am strongly against relying on the enclosure to block light. It is too easy to have a problem down the line where you need to troubleshoot a circuit and find that its behavior changes when you open the lid to probe it. It is easy enough to fix with heat shrink tubing

author: smashedProton
author_rank: Frequent Contributor
timestamp: May 11, 2013, 06:29:28 am
content: You should play around with different wavelengths of light in order to find the work function of the material. Show us rhe results

author: SeanB
author_rank: Super Contributor
timestamp: May 11, 2013, 07:07:22 am
content: As it is silicon it will respond to everything above 1100nm, so is pretty much going to give a result for all visible light.

author: free_electron
author_rank: Super Contributor
timestamp: May 11, 2013, 08:32:57 am
content: Any semiconductor is light sensitive. Even non-silicons.You can use an led as photodiode...