Thermal specs for LDR - Page 1
Forum section: electronics / projects

author: ciccio
author_rank: Frequent Contributor
timestamp: February 25, 2019, 11:44:10 am
content: I'm redesigning a custom device that must replace a similar one designed and manufactured years ago by another company.I must maintain compatibility with the older one, and Imustuse two Light Dependant Resistors coupled with an incandescent lamp as a dual tracking, insulated attenuator.My problem is that I cannot identify the LDRs used in the old design (the previous manufacturer has disappeared) and all the LDRs I could find show a temperature coefficient that makes them unusable, because the lamp will heat the LDR's.I have in my junkbox some very old (30 years) LDRs that were made in Europe (maybe by Siemens) that seems to be less temperature dependant, but they are unmarked and I do not have two with the same value for a quick test.Does anybody has some information about LDR's thermal coefficient? Manufacturers do not give this value.I've tried with LED couplers : some commercial Vactrols and similars, some built by me with a LED and a LDR, and I've seen that, due to lower heat generated by the LED the temperature effect is negligible, but the transfer function (LED current vs LDR resistance) is not suitable for the job.I need a lamp, and the lamp and the two LDRs must be encased in a small box to keep them in the dark.I even thought about a thermostatic control of the box's temperature, but it will increase too much the complexity an the cost of the product.Thanks to anybody that will post some suggestion and/or some information.Best regards

author: tggzzz
author_rank: Super Contributor
timestamp: February 25, 2019, 12:45:44 pm
content: What leads you to believe that the original LDRs have a lower tempco, or that their tempco isn't taken account of elsewhere in the system?Having done zero research, I would presume that all LDRs use the same fundamental materials and therefore have the same tempco defined by physics.Different encapsulation might mean the time constants are different, of course.

author: ciccio
author_rank: Frequent Contributor
timestamp: February 25, 2019, 03:22:00 pm
content: Thank you for your attention.I'm correct: there is a temperature coefficient, and it depends on the resistive material.I found an old Perking Elmer datasheet for LDRs and Vactrol Optocouplers where they specify that different resistive materials (they list many) have different thermal specifications.I found it, after a long search, onhttp://www.alldatasheet.com/datasheet-pdf/pdf/84389/PERKINELMER/VTL5C3.html?.Perkin Elmer does not manufacture LDRs anymore, it's spin-off (Excelitas) does not, but I've found some pieces of NOS from a British supplier.They can be good for some tests, but I'm afraid to use an obsolete component that has non second source..I took apart the original unit: it had very limited thermal drift, and only at start-up.There is no hint of any thermal compensation in the circuit.I tried other LDRs, replacing the originals in the same circuit, and they drift.I'm baffled..best regards

author: StillTrying
author_rank: Super Contributor
timestamp: February 25, 2019, 04:10:14 pm
content: Quote from: ciccio on February 25, 2019, 03:22:00 pmHow do you know it's not the bulb or its supplied power that's drifting.Do both LDRs drift the same.Is there much current through the LDRs to cause them to self heat.Perhaps a 3rd LDR looking at the bulb could adjust the bulb to cancel the 3rd LDRs drift and hopefully the other 2 LDR's drift.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2019, 05:41:55 pm
content: There are a few different LDR materials, but by far the most common is CdS. Due to RoHS it's already difficult to get new ones of this type.The best chance is probably looking for a little more thermal isolation between the incandescent lamp and the LDR. The CdS LDRs usually main react to yellow and shorter wave length light, while the lamp will produce mainly IR light that only heats the LDR. So an IR absorbing filter / glass and maybe a little more distance (e.g. 5 mm) could help a lot.A thermostat to the LDRs may not need to be so complicated. There are quite simple and small versions, e.g. made for crystal ovens.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 25, 2019, 05:55:24 pm
content: +1 on using an IR filter with the CdS cells. I used to have some devices that looked like tubular light-bulbs, but with some sort of black gunk between the terminals where a filament would have been. I think they might have been Clairex. They looked nothing like CdS cells, so it possible there are other types.edit- did some more research and you might find this interesting:https://www.schematicsforfree.com/archive/file/Optical/Photoconductive%20Cells/Photoconductive%20Cell%20Application%20Design%20Handbook.pdf

author: ciccio
author_rank: Frequent Contributor
timestamp: February 27, 2019, 09:14:41 pm
content: Thanks to the fellow members that took the time to share their opinions..After testing differents LDRs, I found one model with a lower thermal effect.My initial solution had the bulb and the LDR encased in a small plastic box. The temperature inside increased too much.I tested Kleinstein's suggestion of a glass filter (a clear glass tube surrounding the bulb), and mounted it leaving space for air flow, and but LDR + bulb + glass tube ) inside a metal case (I used an aluminium cap from a discarded electrolytic) with some openings allowing air flow but no ambient light in..It is a little complicated, but I can do it (the expected production numbers are not large).My initial solution had the bulb and the LDRs encased in a small plastic box. The temperature inside increased too much.The results are acceptable, at least for a first test run.Best regards