What's dragging me down? - Page 1
Forum section: electronics / repair

author: ceecrb1
author_rank: Contributor
timestamp: January 11, 2016, 02:26:28 pm
content: So i'm trying to fault find a pcb from a moving light used in events etc.I have worked out that somewhere on the board the 5v rail is shorted to ground.What is the best way of finding where? I've open circuited all the "easy" options like caps, resistors and the LM...So is the only option left for me to de-soulder the MANY IC's? Or is there a trick?

author: matseng
author_rank: Frequent Contributor
timestamp: January 11, 2016, 02:29:55 pm
content: Easy trick? Connect a 5 volt supply with a lot of ooomph to it. Enough amps put thru the short will make it hot and/or burn and/or explode. :-)

author: grumpydoc
author_rank: Super Contributor
timestamp: January 11, 2016, 02:35:21 pm
content: A current limited power supply and a voltmeter with good resolution can be used.Hook up a supply limited to a "safe" current, let's say 100mAMeasure the voltage at the board's power terminals, say that the resistance on the power rail at that point is 0.1ohm so you'll get 10mVNow move along the power rails on the PCB, as you get "closer" to the short the voltage reading will go down, when it hits (almost) zero, you will have found the culprit.

author: N2IXK
author_rank: Frequent Contributor
timestamp: January 11, 2016, 02:39:43 pm
content: Or a 5V supply with a bit less oomph, and feel around (or use a thermal imager) and look for hot spots.A milliohmmeter can be useful for localizing a short on a board, as well. If you suspect a shorted IC, check the resistance across the 5V supply and ground pins of each one, and look for the one showing the lowest resistance.

author: ceecrb1
author_rank: Contributor
timestamp: January 11, 2016, 02:40:49 pm
content: Great thanks! Looks like I cant do this on site then. Time to take it home and get it on a proper bench!

author: Ian.M
author_rank: Super Contributor
timestamp: January 11, 2016, 02:47:12 pm
content: Measuring voltage drops from the regulator output to the chip 5V pins is good if its got a power distribution tree rather than a solid power plane, and will often help you find the area of the board that's drawing excess current. If its a power plane you are FUBARed and voltage drops probably wont tell you muchAlso, you don't always need to fully desolder ICs. Its often possible to desolder just the power pin, lifting it slightly for SOIC and TQFP, or cleaning the hole carefully for through hole, 'springing' the pin into the center of the hole and carefully heating the pin and pad alternately with flux to reflow any solder whiskers. Check with a low ohm meter if the pin has been isolated successfully. Cut power tracks if you have to to narrow it down - if done neatly, they can easily be bridged once the fault is found.

author: Robert Karl
author_rank: Contributor
timestamp: January 11, 2016, 05:10:16 pm
content: I agree with the consensus of a bench supply "with oomph".If you have a flir, you can see the location will literally glow.Good multimeter is the next best.Anecdotally, I've used a BIG capacitor to great effect too - when patience wears thin.The short (or if you are unlucky, some track on the PCB) will be vaporised.Only try this if you like to live dangerously.

author: cncjerry
author_rank: Unknown
timestamp: January 11, 2016, 05:51:29 pm
content: We used to put a light bulb in series with one leg of the mains. When the light got brighter it was time to start looking for smoke. It also limits the incoming voltage to the power supply.

author: Gyro
author_rank: Super Contributor
timestamp: January 11, 2016, 06:33:02 pm
content: Quote from: Ian.M on January 11, 2016, 02:47:12 pmI've used the constant current supply method successfully on power planed boards too. At 1A even a cheapo 3 1/2 digit DMM will give you 100u ohm resolution. It might not get you all the way but you can narrow the area down enough to eyeball it. Definitely worth trying before the more destructive methods.

author: Brumby
author_rank: Unknown
timestamp: January 11, 2016, 11:39:26 pm
content: I've been through this exercise a couple of times and I've pondered on the thermal imaging method for fault finding where heat is generated. Apply power and watch for the glow, with the possibility of identifying the problem before any magic smoke escapes.As a hobbyist, this has never been a justifiable expense, but with products like the Flir One and Seek XR hitting the market, this is attracting my interest once again. Glad there is now a board for thermal imaging.

author: N2IXK
author_rank: Frequent Contributor
timestamp: January 11, 2016, 11:56:53 pm
content: A cheap alternative to an IR imager is a thermochromic liquid crystal film like:http://www.amazon.com/American-Science-Surplus-Liquid-Crystal/dp/B002V05RWIAs long as the board is relatively uniform as far as component height, you can lay a sheet of this stuff over the PCB and look for any areas dissipating a lot of heat.I have successfully used these to find shorted tantalum bypass caps on boards full of TTL logic gates.

author: Brumby
author_rank: Unknown
timestamp: January 12, 2016, 02:16:45 am
content: Interesting idea. I hadn't thought of that.The requirement for an even board height is understandable, but not always available. I suppose you could cut into smaller panels and lay those over suspect areas - or custom cut a sheet for a specific board, if you could justify the cost.Another thing is to make sure you get a sheet with a temperature range that suits your needs.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 12, 2016, 02:43:17 am
content: Here's one of dubious nature that may work in limited situations:1. Put the board in an air-tight, dry plastic bag.2. Put it in a freezer and let it get quite cold.3. Prepare a warm room with high-humidity. Like a bathroom after a hot shower.4. Take the board out and run it for a short time before the entire board heats up.5. Bring the board as it's still cold to the high-humidity room.6. The warm spots will have condensation more slowly than the cold spots.