led deterioration - Page 1
Forum section: electronics / repair

author: p.larner
author_rank: Frequent Contributor
timestamp: April 13, 2024, 01:11:53 am
content: I have a solartron 7075 dvm, It displays ok but wont display an actual value of anything measured, As far as i can see the signal path goes thru an attenuator/signal conditioning then drives an optical chopper, The chopper has 4 led's and 4 photo diodes,my uestion is do led's get dimmer as they age?.I suspect the chopper cct is iffy,does that sound a possibility?.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 13, 2024, 02:40:34 am
content: Yes, LEDs do get dimmer over time. It's called lumen depreciation.

author: coppercone2
author_rank: Super Contributor
timestamp: April 13, 2024, 08:42:00 am
content: its why I push for use of digital isolators over optical isolators

author: Gyro
author_rank: Super Contributor
timestamp: April 13, 2024, 08:45:56 am
content: I would rule out more likely causes before pulling apart an optical chopper assembly. Follow the service manual faultfinding procedures very carefully. Neon choppers are more likely to cause issues than LED ones.Edit: Choppers fading are also far more likely to cause noisy readings than no reading at all.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: April 13, 2024, 09:05:27 am
content: Quote from: Kim Christensen on April 13, 2024, 02:40:34 amYep. Experienced such with old 5 mm standard LEDs. Green and yellowbeing switched alternately for 10 s on and 10 s off over 30+ years.

author: p.larner
author_rank: Frequent Contributor
timestamp: April 13, 2024, 04:26:28 pm
content: The service manual is 200+pages but has nothing about troubleshooting as far as i can see.https://elektrotanya.com/solartron_schlumberger_7075_sm.pdf/download.html

author: p.larner
author_rank: Frequent Contributor
timestamp: April 13, 2024, 07:07:09 pm
content: As far as i can tell all the input signals go thru the chopper so was thinking its the best starting point to try and find the issue as the service manual has next to no troubleshooting info.,using the test routines does not help,the only test that displays is getting all didgits lit as in the 5th test button push,all other test button presses give a 0 reading.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 13, 2024, 10:52:38 pm
content: Quote from: p.larner on April 13, 2024, 01:11:53 amSo does the value change?ie: If measuring voltage, and you increase the input voltage, does the displayed value increase? Or does it remain unchanging.Is this the circuitry in question:

author: p.larner
author_rank: Frequent Contributor
timestamp: April 14, 2024, 12:05:36 pm
content: no if i try and measure any voltages it displays all zeros,its the same on all ranges be it volts or resistances, It just displays all zeros no matter what is hooked up to the inputs.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 14, 2024, 02:56:58 pm
content: REF: chopperLDRs.PDFIf you input a DC voltage into the meter, does something appear at the input to the main amplifier? (Across D401 & D402)If so, does the output of IC408 change with it? What about the output of IC406?

author: coppice
author_rank: Super Contributor
timestamp: April 14, 2024, 03:08:28 pm
content: Quote from: p.larner on April 13, 2024, 01:11:53 amCheck any LED data sheet and you'll find they do dim with age. Their lifetimes are generally well documented. LED makers also make higher lifetime LEDs, which are usually more expensive and less efficient, but have a longer life. They still dim with edge, but not as quickly. If you are developing for an application where a LED needs to run a large percentage of the time, and the product needs to operate for more than 10 years, you really need to check that the predicted light output of the LEDs you choose will still be adequate near the end of the product's life.