Current Tracer - for findinding shorts in a PCB
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From: N_Cook on 9 Apr 2010 12:04 Scrim <nospam(a)nospam.nospam> wrote in message news:I7vun.199336$ti6.7573(a)newsfe24.ams2... > <stratus46(a)yahoo.com> wrote in message > news:442f6f03-dad3-49d1-9c5b-7a240a31226c(a)q23g2000yqd.googlegroups.com... > On Apr 5, 9:49 am, "Scrim" <nos...(a)nospam.nospam> wrote: > > Is there a DIY circuit out there for a good current tracer? I have > a > > valuable multilayer pcb with a short between a power supply line > and earth I > > can't find. The basic idea is to inject a string of pulses through > shorted > > circuit and use a small solenoid type sensor to follow the current > path > > until the short is reached. > > > > Thanks, > > > > Alan > > Has it ever worked or is this a failure? Are you thinking incomplete > etch or shorted bypass cap? > > G� > > Thanks for the ideas. This is a high precision board that used to work. > Ideally I'd like to have a thermal camera, so I like the idea of trying a > digital camera in a dark room. Does it have to be a CCD? I'm not sure what > my web cam is (cmos?), but at least the infrared filter is easy to remove > and it doesn't have autofocus, so I can set it right with the lights on > first. > It's a multilayer board so tracing with an ohm meter seems like a bit of a > non-starter as I have no idea where the tracks go, and because of it's > precision I don't want to cover it in frost, although that's a great idea > for another time. > I've already repaired an area of board under an electrolytic that leaked. > The board was carbonised all the way through, but I think I've fixed that. > I've looked at all the tantalum capacitors I can see and they all seem ok. > > Alan > So as it is a populated board, how do you know it is a in-board short and not a shorted VTS , if SM can look very inniocent, or something similar? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://diverse.4mg.com/index.htm
From: Scrim on 9 Apr 2010 18:07 "N_Cook" <diverse(a)tcp.co.uk> wrote in message news:hpnj52$ego$1(a)news.eternal-september.org... > Scrim <nospam(a)nospam.nospam> wrote in message > news:I7vun.199336$ti6.7573(a)newsfe24.ams2... >> <stratus46(a)yahoo.com> wrote in message >> news:442f6f03-dad3-49d1-9c5b-7a240a31226c(a)q23g2000yqd.googlegroups.com... >> On Apr 5, 9:49 am, "Scrim" <nos...(a)nospam.nospam> wrote: >> > Is there a DIY circuit out there for a good current tracer? I have >> a >> > valuable multilayer pcb with a short between a power supply line >> and earth I >> > can't find. The basic idea is to inject a string of pulses through >> shorted >> > circuit and use a small solenoid type sensor to follow the current >> path >> > until the short is reached. >> > >> > Thanks, >> > >> > Alan >> >> Has it ever worked or is this a failure? Are you thinking incomplete >> etch or shorted bypass cap? >> >> G� >> >> Thanks for the ideas. This is a high precision board that used to work. >> Ideally I'd like to have a thermal camera, so I like the idea of trying a >> digital camera in a dark room. Does it have to be a CCD? I'm not sure >> what >> my web cam is (cmos?), but at least the infrared filter is easy to remove >> and it doesn't have autofocus, so I can set it right with the lights on >> first. >> It's a multilayer board so tracing with an ohm meter seems like a bit of >> a >> non-starter as I have no idea where the tracks go, and because of it's >> precision I don't want to cover it in frost, although that's a great idea >> for another time. >> I've already repaired an area of board under an electrolytic that leaked. >> The board was carbonised all the way through, but I think I've fixed >> that. >> I've looked at all the tantalum capacitors I can see and they all seem >> ok. >> >> Alan >> > > So as it is a populated board, how do you know it is a in-board short and > not a shorted VTS , if SM can look very inniocent, or something similar? > > > -- > Diverse Devices, Southampton, England > electronic hints and repair briefs , schematics/manuals list on > http://diverse.4mg.com/index.htm > In fact I found the shorted component was a surface mount diode connected directly across the capacitor on the reverse of the board, which I've replaced. Now to connect it all up again and see if everything's working again... Alan
From: DaveC on 10 Apr 2010 01:02 > In fact I found the shorted component was a surface mount diode connected > directly across the capacitor on the reverse of the board, which I've > replaced. Now to connect it all up again and see if everything's working > again... > > Alan And not one word about how you went about identifying the diode? Dave
From: N_Cook on 10 Apr 2010 03:14 DaveC <invalid(a)invalid.net> wrote in message news:0001HW.C7E5546000C3C3C3B01AD9AF(a)news.eternal-september.org... > > In fact I found the shorted component was a surface mount diode connected > > directly across the capacitor on the reverse of the board, which I've > > replaced. Now to connect it all up again and see if everything's working > > again... > > > > Alan > > And not one word about how you went about identifying the diode? > > Dave > So a TVS, but uni or bipolar ? and what caused the high A-t transient to knock it out ? or it will likely reoccur -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://diverse.4mg.com/index.htm
From: Scrim on 10 Apr 2010 05:22
"DaveC" <invalid(a)invalid.net> wrote in message news:0001HW.C7E5546000C3C3C3B01AD9AF(a)news.eternal-september.org... >> In fact I found the shorted component was a surface mount diode connected >> directly across the capacitor on the reverse of the board, which I've >> replaced. Now to connect it all up again and see if everything's working >> again... >> >> Alan > > And not one word about how you went about identifying the diode? > > Dave > Good point, sorry! I used an old HP 546A Logic Pulser and an HP 547A Current Tracer. Designed for use with logic circuits, but perfect for this job. You can pick them up quite cheaply on eBay sometimes. Superb bit of kit, except the labels - don't try to peel the plastic layer off. It looks like that protective peel off plastic, but it's not! Alan |