PTC fault?

Thread Starter

geoffers

Joined Oct 25, 2010
475
Hi all,

My cousin has dropped me a board to take a quick look at, it's all surface mount around 15-20 years old.

It controls a fertilizer spreader, control box couldn't find the spreader.

There's a buzzing/crackling noise, nearly like an arc coming from the board, it's all solid state and the only thing that jumped out at me was a PTC 0.75amp polyfuse.

Anyone experienced these failing and making a noise?

If it's not that he can have it back like it is!

Cheers Geoff
 

MrChips

Joined Oct 2, 2009
30,708
A buzzing/crackling noise could be erratic operation of a mechanical relay.
What voltages enter the board?
Post a picture of the board and let's have a look.
 

Ian0

Joined Aug 7, 2020
9,667
I once used a polyfuse on an audio amplifier, and used one rated for only the peak supply voltage, failing to realise that when there is a fault the polyfuse is exposed to twice the supply voltage.
The result was a crackling, sparking, arcing and charring effect along the bottom edge between the leads.
 

Thread Starter

geoffers

Joined Oct 25, 2010
475
Thanks for the replies, I still haven't finished the last thing me chips has been helping with!

There are 2 relays on the board and when he rang I said, it'll be a dodgy supply making a relay chatter.

It's one of those really annoying intermittent faults, work for a bit then started buzzing again.

Trouble is the sound doesn't seem to come from the relays but the area of the board with the PTC on, I'll grab a picture in the morning.

Cheers Geoff
 

Thread Starter

geoffers

Joined Oct 25, 2010
475
IMG_20230318_094504_101.jpg
IMG_20230318_094513_136.jpg
IMG_20230318_094525_382.jpg

Here it is!
The motherboard sockets into the power board.
I'm not ruling out that orange relay so think I will order one.
The power board is really covered in resin, much testing will be tricky.
Thinking about it this morning if it is the PTC what's making it do that? There is a rs232 chip bottom right corner of the mother board, that I guess is used to talk to the control box in the cab, it wouldn't take much of a short on one of those wires to draw 750mA?

Cheers Geoff
 

Thread Starter

geoffers

Joined Oct 25, 2010
475
Morning Eric,

It's yet more resin I think, will have another look at the bottom but I don't think that it came out the cap.

Cheers Geoff
 

Alec_t

Joined Sep 17, 2013
14,280
It's a job to tell from the pics, but are there any discoloured spots on the board, which could be due to excessive heating of some component?
 
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Thread Starter

geoffers

Joined Oct 25, 2010
475
Everything looks really happy! The more I look at it the more I think it could be a loom fault rather than the board.
 

Thread Starter

geoffers

Joined Oct 25, 2010
475
Update, loom seems fine.
Replaced the PTC today, the resin the board is covered in look bubbly around the PTC before I scraped it off

Anyway all back together and seems to be working ok at the moment.
Can't help thinking I'll be seeing it again though!

Thanks Geoff
 

Thread Starter

geoffers

Joined Oct 25, 2010
475
Nearly a year but I thought I might see this again, failed PTC again!

I do wonder if I cooked it a bit soldering it in, when I tried to replace it this time the replacement fell apart!

Would it be a daft idea to put a normal fuse in if it continues to play up? The data sheet on soldering the PTC says you can hand solder but not recommended ! I can't find any faults, turns out last time round the wiring loom had been crushed but that is all repaired now.

Cheers Geoff
 
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