Read this print and tell me what's going on? - Page 1
Forum section: electronics / projects

author: brettboat
author_rank: Newbie
timestamp: February 07, 2017, 07:13:04 am
content: Recently had this issue at work trying to troubleshoot a broken airplane. So if you look over at the L/H side of the print, UC008 is a warning horn. There's a few inputs now shown going to those PCBs that will give it power but that's not the issue (FYI, I trouble shot the issue down to 2 faulty micro switches so nothing super important, just more curiosity).SO UC008 has a 28VDC input from NZ008 (whole system is 28VDC), when shoot voltage from 15PC058 to 7PC058 I'm reading 40VDC, I put on another techs fluke and it read the same thing. I removed PCB NZ008 and shot pin 12 to verify that I had a 28VDC input. Reinstalled NZ008 and isolated UC008 and shorted 15PC058 to 7PC058 and shot 28VDC (verifying a good output voltage at the PCB, eliminating that board from my troubleshooting).So that being said, I still don't understand WHY UC008 is reading 40VDC. It's beyond me, what could be going on with this capacitor and horn to cause me to get that reading?Thanks for any input!!!

author: ataradov
author_rank: Super Contributor
timestamp: February 07, 2017, 07:43:38 am
content: Well, it is an EM device, it will generate some back EMF, which may confuse the meter.It is hard to tell for sure without knowing the design of the horn.

author: ataradov
author_rank: Super Contributor
timestamp: February 07, 2017, 07:46:20 am
content: Also, is the horn purely passive, and the tone is actually provided by the external circuitry? Then, it is an AC/DC issue.40 V is mighty close to 28*1.41 = 39.48.

author: brettboat
author_rank: Newbie
timestamp: February 07, 2017, 05:41:41 pm
content: The horn is 100% self contained with a 28VDC input (tone produced from within unit). So I would assume there is a small inverter within the unit that will produce my AC for the speaker. So in a nut shell if the horn has 28VDC input from the PCB its screaming as there is no other input. THAT BEING SAID, there is also a capacitor in parallel with the horn (as you can see in the print) wired directly across the post. Is this suppose to prevent my backfeed of AC current?

author: ataradov
author_rank: Super Contributor
timestamp: February 07, 2017, 05:43:38 pm
content: Quote from: brettboat on February 07, 2017, 05:41:41 pmNot really. You would really have to look at the spec or design of the horn to tell what it may or may not be doing. Or look at that thing with an oscilloscope.

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2017, 06:46:58 pm
content: I agree it needs further investigation but a decoupling capacitor is a reasonable suggestion.What value is the capacitor?Is it polarised?If the capacitor is polarised then the horn is almost certainly being powered from DC and there's active circuitry in there.

author: brettboat
author_rank: Newbie
timestamp: February 07, 2017, 09:31:56 pm
content: Quote from: Hero999 on February 07, 2017, 06:46:58 pmI tried getting access to see it but I was going to have to rip half the cockpit apart to get to it so that ended up not happening lol.I called Cessna engineering at like 3am in the morning and they guy was clearly passed out at his desk and didn't really want to help me and accused me of not knowing how to use a meter. lolFigured since no one at Cessna wanted to help explain this 40VDC to me that I'd come on here because I just don't understand how it's even possible.

author: ataradov
author_rank: Super Contributor
timestamp: February 07, 2017, 09:42:30 pm
content: What meter do you have? Try AC mode and see what is the reading on this line. Some meters will reject DC component in AC mode and you will at least check that it is AC voltage that confuses the meter.

author: brettboat
author_rank: Newbie
timestamp: February 07, 2017, 10:03:49 pm
content: AC showed nothing, I was using the FLuke 177. Other tech had some snapon rebranded meter.

author: ataradov
author_rank: Super Contributor
timestamp: February 07, 2017, 10:05:16 pm
content: Without a scope or at least some documentation on the horn, it is hard to tell then.