The lonely mystery Heatsink... - Page 1
Forum section: electronics / projects

author: McBryce
author_rank: Super Contributor
timestamp: August 28, 2017, 09:59:41 am
content: I was salvaging some parts from old PCBs this morning and came across this. The PCB is from an old satelite receiver and contained both the PSU and some of the video circuitry. But then I noticed this heatsink. It was soldered to a track to nowhere, had no compomemt connected and there was no component on flyleads that might have used it. There's markings on the PCB for it and it was populated, but it's just there, doing nothing. It's within the 240V primary area of a SMPSU. Can anyone suggest any reason why this would be here?McBryce.

author: mikerj
author_rank: Super Contributor
timestamp: August 28, 2017, 10:18:43 am
content: Could it have been used as a mechanical support to prevent something touching components on the PCB, e.g. if the pcb flexes due to rough handling will that hit the chassis before something live?

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2017, 10:24:30 am
content: Possibly a bodge to prevent a straight metal object inserted in a ventilation slot touching TH100 or R112.

author: CM800
author_rank: Frequent Contributor
timestamp: August 28, 2017, 10:36:35 am
content: Quote from: Ian.M on August 28, 2017, 10:24:30 amShould have hired that guy to work on the death-star too...

author: McBryce
author_rank: Super Contributor
timestamp: August 28, 2017, 11:07:17 am
content: Nope. It was located facing a side of the device that has no holes that would allow access to anything dangerous and way too deep inside the device that the casing could ever touch anything. It's also shorter than the transformer close by, so headroom protection isn't an option either. Anyway, why would anyone use a relatively expensive heatsink to do something that a cheap plastic standoff could do.McBryce.

author: daqq
author_rank: Super Contributor
timestamp: August 28, 2017, 01:05:21 pm
content: Could be that the part using it was not on that PCB, but rather on some different one. Say, a power transistor on the logic board that needed heatsinking, but they did not want to add heat to the logic PCB, so they'd just assemble it in such a way that the transistor is located on this PCB, but has electrical connections on the other PCB.Just a possibility.

author: McBryce
author_rank: Super Contributor
timestamp: August 28, 2017, 01:54:35 pm
content: That was my first thought too, but I have the entire device and it has three seperate PCBs, but there are no other parts needing a heatsink.McBryce.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2017, 02:06:56 pm
content: Either someone was getting a kick-back from the heatsink supplieror there was another model that shared the same power/video board and used a different board next to the power/video board that needed a heatsink there as Daqq suggested.

author: tszaboo
author_rank: Super Contributor
timestamp: August 28, 2017, 02:54:30 pm
content: It is there, to cool down the air...On a more serious tone, it is probably an oversight. Like someone deleted the component, but the heatsink was a separate component in the schematic, so that stayed there. Maybe it is handled by another department, and communication was bad. Maybe they forgot to change the assembly line.I have a project with 2G or 4G modules, same PCB. 4G comes with two antenna. Last time they accidentally populated the antenna connector for the 2Gs also. I sure as hell not going to take them out.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: August 28, 2017, 03:08:23 pm
content: It was there for you...maybe the assembler put it in the wrong model and didn't notice it.or they probably have noticed it but it costed more to remove them than just keeping them there.(backlog)

author: SeanB
author_rank: Super Contributor
timestamp: August 28, 2017, 06:25:23 pm
content: Could be a relic from the design, or was used as a spacer to keep the board clear of another daughter board in some revision of the unit. Using an existing part that you already have in inventory over getting a new part number probably was a cheap way, seeing as this has never had a device screwed into the punched hole in it.

author: ThunderCat
author_rank: Contributor
timestamp: August 28, 2017, 06:40:11 pm
content: Maybe this is a 4-layer board and there are traces you can't see...

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2017, 07:04:38 pm
content: Quote from: ThunderCat on August 28, 2017, 06:40:11 pmThere's no pads for a device to mount on that heatsink, and it has a nice big bare board isolation gap round the heatsink's own pad so its really unlikely to have any hidden traves anywhere near it.

author: Zero999
author_rank: Super Contributor
timestamp: August 28, 2017, 07:34:15 pm
content: It's a single sided board. No hidden tracks.I doubt it's a relic from a previous design, otherwise I doubt it would be fitted, even if the pad and silk screen are present.I'm going with the spacer theory or perhaps it's just there to make it look more complex.

author: glarsson
author_rank: Frequent Contributor
timestamp: August 28, 2017, 08:04:15 pm
content: My first oscilloscope bought 1981 had, from new, a hot spot on the casing. Not just hot, it was burning hot.An investigation showed that the heat came from one TO-220 power transistor and one power resistor not connected to anything. The missing components were used for graticule illumination (not implemented on my model) but for some reason these two components were mounted. The only active path was from power rail through resistor and base-emitter junction. The transistor was over 100 degrees C.

author: CopperCone
author_rank: Super Contributor
timestamp: August 28, 2017, 08:07:48 pm
content: you can put a part there and attach it with wires to somethign else.

author: Jidis
author_rank: Regular Contributor
timestamp: August 28, 2017, 09:14:13 pm
content: glarsson,That's funny that people in that particular field would miss something like that. Even if they didn't catch it on paper, you'd figure someone would notice when they actually ran one. What brand was the scope (just curious)?

author: glarsson
author_rank: Frequent Contributor
timestamp: August 28, 2017, 09:29:05 pm
content: Quote from: Jidis on August 28, 2017, 09:14:13 pmTrio Kenwood

author: glarsson
author_rank: Frequent Contributor
timestamp: August 28, 2017, 09:36:18 pm
content: Quote from: Jidis on August 28, 2017, 09:14:13 pmEven built up as intended in the higher model the circuit would have been very inefficient, so perhaps nobody expected it to run anything but hot.

author: BrianHG
author_rank: Super Contributor
timestamp: August 28, 2017, 11:31:22 pm
content: Thank-you McBryce, you made my day after spending 2 weeks correcting someone else electronic schematic of wonder...

author: Fungus
author_rank: Super Contributor
timestamp: August 29, 2017, 12:42:37 am
content: It's a free gift.

author: jh15
author_rank: Frequent Contributor
timestamp: August 29, 2017, 02:25:31 am
content: When do you have to pay for a gift?

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2017, 03:07:07 am
content: Whenever they tell you its '*FREE* !!!'. 'Free' 'Gifts' fall into the same category as'Free' lunches.TANSTAAFLapplies.

author: BrianHG
author_rank: Super Contributor
timestamp: August 29, 2017, 03:08:22 am
content: (Solved) Ahhh, I see, it was originally supposed to be for the diode bridge D100, but, it's oriented wrong and it's the wrong heat sink being a bit too narrow. The the board layout guy just stuck it close to D100 instead of using a slightly wider one oriented 90 degrees to meet with D100. Or, originally D100 was a smaller unit and needed the heat sink, but, go a beefy upgrade and the heat sink was just never removed.If you didn't remove D100, it would have been easier to see......

author: McBryce
author_rank: Super Contributor
timestamp: August 31, 2017, 12:55:21 pm
content: I still think it's a mistake by the layout guy. The give away is that most other parts are labeled with "100" numbers, so the first Heatsink is HS101 (which exists on the PCB), whereas this heatsink is labeled HS1.McBryce.

author: b_force
author_rank: Super Contributor
timestamp: August 31, 2017, 01:58:03 pm
content: No matter what the reasons are.Can't you guys show some empathy?I feel sorry for the little bugger, standing there lonely and cold in the breeze, peace bro!