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Thread: Conductive PCB substrate

  1. #1
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    Default Conductive PCB substrate

    Just built up a board, basic tests showed things to be fine, until I tested the MOSFET output. no matter what they always had and output - and an input. After mush pocking around with a multi meter I found that the board substrate had a voltage by comparison to the 0v side of the battery. There is a resistance between the substrate and the copper too, its high from about 0.9 to 15M ohm.

    The batteries supply 6v to the copper, which goes to the MOSFETs and 5v regulator. The board is showing about 3-4v

    I have no doubt that I will have to make another board, which will give me a chance to make some minor changes. But this is still really quite frustrating.

    Has anybody else come across something like this? It had me stumped for a couple of days.

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    Is that RBP or GRP substrate ?

    I have seen similar issues, with cheap imported RBP, was desperate for some PCB at the time. Ended up
    throwing it away & waiting for my GRP delivery from CPC-Farnell.

    Neil.
    Technician - One who does precision guesswork based on
    unreliable data provided by those with questionable knowledge

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    Hi,

    Are you sure it's not just a surface effect (moisture), it's been quite humid (foggy) here in parts of the UK recently?

    But the P in SRBP (or "Paxolin") does stand for "paper", which can absorb quite a lot of moisture. It does sound like "faulty" manufacture, but can't the FETs be "fixed" wih (say) 1 Mohm resistors between Gate and Source (generally good paractice in many applications anyway) ?

    Cheers, Alan.

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    Quote Originally Posted by AllyCat View Post
    but can't the FETs be "fixed" wih (say) 1 Mohm resistors between Gate and Source (generally good paractice in many applications anyway) ?

    Cheers, Alan.
    +1 for Alan's suggestion. Even with a "good" PCB, the slightest bit of dust in the right spot couple with a bit of moist air is possibly enough set off a FET.

    Did you "brew your own" PCB or did you have it made? Maybe the etching process failed and left tiny traces of copper in the substrate. Do you have a microscope? If so, maybe have a close look at the etched areas.

    Another thing I've come across in many designs that use FETs is PCB routing (complete removal of the PCB) between pins to further elinimate unwanted "stray" components.
    Darb

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    A shot in the dark: Does your board happen to have more than a "top" and "bottom" layer?
    - Tex
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    Its home brew board.
    The FETs have a 10K between gate and drain. they are IRF9520 (n type FET). The circuit works perfectly on bread board.
    If I remove the FETs there is still voltage on the gate and source, the tracks also become islands with them removed so should have not voltage at that point - they show about 3.5V.

    I think it has paper between some layer of GRP

    I am not sure what you mean Tex, its a single sided board if that helps

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    I am not sure what you mean Tex, its a single sided board if that helps
    That does answer my question.

    Multi-layer boards are like stacks of single layer boards. Your motherboard in your computer has many layers of traces. If you put a via through a multi-layer board, you have to be sure you clear an area on any internal layers so they won't touch the through hole.
    - Tex
    __________________________________________________ _______________________
    "Truth lies dormant in our future history." ― Tex Clodhopper LXV
    “Confidence is ignorance. If you're feeling cocky, it's because there's something you don't know.” ― Eoin Colfer, Artemis Fowl

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    Hi,

    Quote Originally Posted by oracacle View Post
    The FETs have a 10K between gate and drain. they are IRF9520 (n type FET).
    The resistor should be between the Gate and Source to hold the FET off. And that FET is a P-channel device (which is why the Source is shown at the top of the symbol in its data sheet).

    Cheers, Alan.

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    sorry, tiss. not long awake when I posted.

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    Hi,

    At 15:19? OK, I know the forum clock is a little wonky, but it sounds as if you're worse than me.

    But to be fair, I believe there are a few night-workers on the forum

    Cheers, Alan.

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