### Author Topic: How can I find a short circuit on an unknown voltage rail?  (Read 4611 times)

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#### dicky96

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #25 on: October 25, 2017, 09:33:30 am »
PS I do have NE555

#### dicky96

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #26 on: October 25, 2017, 09:38:38 am »
oOops sorry Krypton we cross posted

Is the arduino nano a direct pin-for-pin compatible for the Trinket Pro?   I'm sorry this is not my area of expertise

#### bd139

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #27 on: October 25, 2017, 11:08:47 am »
Finally found my copy of Troubleshooting Analogue Circuits. Was bathroom reading

Here's Bob Pease's solution:

(a) LM10 =  £2.50 ... (b) LM331 = £2.10 ... You can sub both of them out for (a) another opamp + current source (b) 555 with voltage controlled current source.

I might see how cheap I can design one of these.

The following users thanked this post: kripton2035, denimdragon

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #28 on: October 25, 2017, 11:11:23 am »
Finally found my copy of Troubleshooting Analogue Circuits. Was bathroom reading

Here's Bob Pease's solution:

(a) LM10 =  £2.50 ... (b) LM331 = £2.10 ... You can sub both of them out for (a) another opamp + current source (b) 555 with voltage controlled current source.

I might see how cheap I can design one of these.

if you can take out all the solder mask....

#### bd139

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #29 on: October 25, 2017, 11:20:17 am »
It's rarely if ever under the solder mask. It's either the components or around the pads.

I think I had one board with a short after repairing about 50 of the damn things with just a manky old Thandar DMM. Most were assembly stupid.

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #30 on: October 25, 2017, 11:28:51 am »
It's rarely if ever under the solder mask. It's either the components or around the pads.

I think I had one board with a short after repairing about 50 of the damn things with just a manky old Thandar DMM. Most were assembly stupid.

Obviously, but the text is kind of stupid... no such in real repair.

#### bd139

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #31 on: October 25, 2017, 11:42:19 am »
Was repairing this fecked up bodge someone did yesterday so it's not always a problem. I see a lot of boards without solder mask. I actually prefer them as they're easier to rework and last a hell of a long time (unlike the solder mask which flakes off and bubbles).

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #32 on: October 25, 2017, 12:14:15 pm »
Was repairing this fecked up bodge someone did yesterday so it's not always a problem. I see a lot of boards without solder mask. I actually prefer them as they're easier to rework and last a hell of a long time (unlike the solder mask which flakes off and bubbles).

Oh my God, I was imagining densely populated smd circuit boards like the OP HP laptop for example, where the resistances are expected to be small because they are close by that necessitate such instrument to find the shorts. Normally for electrically devices or big chunky components widely spaced, it won't be that hard to find the shorts anyway... Well those are just my views.

#### bd139

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #33 on: October 25, 2017, 12:43:54 pm »
Fair point with the HP laptop. That's a dense multi-layer board which you're usually SOL on (unless you are Louis Rossmann ).

#### kripton2035

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #34 on: October 25, 2017, 12:58:23 pm »

Is the arduino nano a direct pin-for-pin compatible for the Trinket Pro?   I'm sorry this is not my area of expertise
I used the same pins names as the trinket (as I don't have one) and it worked. then I modified the software to my needs.

#### dicky96

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #35 on: October 25, 2017, 05:46:20 pm »
Fair point with the HP laptop. That's a dense multi-layer board which you're usually SOL on (unless you are Louis Rossmann ).

Hmmm I've fixed similar faults before on, for example, a 1Tb SATA HDD which had a short circuit on the 12V rail on the controller board.  That was a SMD cap near the motor controller chip and I used the 'bench psu with current limit and see what gets hot' technique then replaced the cap and it worked.  Is the HP laptop motherboard so different?

#### eunjun

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##### Re: How can I find a short circuit on an unknown voltage rail?
« Reply #36 on: September 24, 2019, 11:08:21 am »
Good morning! I've made Shorty talking about you. However, when you close two probes, only a dull melody comes out. The parts I used are the attiny85 and mcp6041 op amps. Another is the Arduino Promini and the mcp6041 op amp. Both have the same dull melody. What op amp did you use? I want to complete this by all means. Please help.

Smf