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Guest

Fri Jan 11, 2019 5:45 am   



We are seeing around 1 in 100 Tantalum caps fail shorted when initially powered up on the test fixture. There was a discussion here about such failures being an initial short from fabrication or created when reflowed on the board. An article was linked which discussed a real world app where they powered up the board with a voltage ramp and enough series resistance to prevent the cap from creating a hard short and failing. But no information on how slow is "slow" and how much resistance is needed to prevent damage while clearing the short.

On the test fixture we can add a resistance to be bypassed with a jumper. We can't control the ramp up time of that voltage rail other than adding more capacitance to slow it down. I suppose that is an option if we know a target ramp rate. I've asked the manufacturer of the regulator (CUI V7812-1000) what the ramp rate is and how it varies with capacitance. We'll see if they respond.

Any suggestions on improving this problem? It's not a big deal, but we did have one cap that failed catastrophically and actually burned the board, but not beyond repair.

Rick C.

- Get 6 months of free supercharging
- Tesla referral code - https://ts.la/richard11209

Tom Del Rosso
Guest

Fri Jan 11, 2019 7:45 am   



gnuarm.deletethisbit_at_gmail.com wrote:
Quote:
We are seeing around 1 in 100 Tantalum caps fail shorted when
initially powered up on the test fixture.


Could it be the polarity is reversed relative to the marking and the
orientation in the reel?

Boris Mohar
Guest

Fri Jan 11, 2019 8:45 am   



On Thu, 10 Jan 2019 20:28:11 -0800 (PST), gnuarm.deletethisbit_at_gmail.com
wrote:

Quote:
We are seeing around 1 in 100 Tantalum caps fail shorted when initially powered up on the test fixture. There was a discussion here about such failures being an initial short from fabrication or created when reflowed on the board. An article was linked which discussed a real world app where they powered up the board with a voltage ramp and enough series resistance to prevent the cap from creating a hard short and failing. But no information on how slow is "slow" and how much resistance is needed to prevent damage while clearing the short.

On the test fixture we can add a resistance to be bypassed with a jumper. We can't control the ramp up time of that voltage rail other than adding more capacitance to slow it down. I suppose that is an option if we know a target ramp rate. I've asked the manufacturer of the regulator (CUI V7812-1000) what the ramp rate is and how it varies with capacitance. We'll see if they respond.

Any suggestions on improving this problem? It's not a big deal, but we did have one cap that failed catastrophically and actually burned the board, but not beyond repair.

Rick C.

- Get 6 months of free supercharging
- Tesla referral code - https://ts.la/richard11209


dV/dt

https://www.avx.com/docs/techinfo/VoltageDeratingRulesforSolidTantalumandNiobiumCapacitors.pdf


Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

void _-void-_ in the obvious place



---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

Tim Williams
Guest

Fri Jan 11, 2019 8:45 am   



What rating and test voltage, out of curiosity?

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

<gnuarm.deletethisbit_at_gmail.com> wrote in message
news:8c98a491-be67-4607-8317-21085d0c223f_at_googlegroups.com...
We are seeing around 1 in 100 Tantalum caps fail shorted when initially
powered up on the test fixture. There was a discussion here about such
failures being an initial short from fabrication or created when reflowed on
the board. An article was linked which discussed a real world app where
they powered up the board with a voltage ramp and enough series resistance
to prevent the cap from creating a hard short and failing. But no
information on how slow is "slow" and how much resistance is needed to
prevent damage while clearing the short.

On the test fixture we can add a resistance to be bypassed with a jumper.
We can't control the ramp up time of that voltage rail other than adding
more capacitance to slow it down. I suppose that is an option if we know a
target ramp rate. I've asked the manufacturer of the regulator (CUI
V7812-1000) what the ramp rate is and how it varies with capacitance. We'll
see if they respond.

Any suggestions on improving this problem? It's not a big deal, but we did
have one cap that failed catastrophically and actually burned the board, but
not beyond repair.

Rick C.

- Get 6 months of free supercharging
- Tesla referral code - https://ts.la/richard11209

John Larkin
Guest

Fri Jan 11, 2019 4:45 pm   



On Fri, 11 Jan 2019 01:19:39 -0500, "Tom Del Rosso"
<fizzbintuesday_at_that-google-mail-domain.com> wrote:

Quote:
gnuarm.deletethisbit_at_gmail.com wrote:
We are seeing around 1 in 100 Tantalum caps fail shorted when
initially powered up on the test fixture.

Could it be the polarity is reversed relative to the marking and the
orientation in the reel?



Tantalum caps are bombs-by-the-reel. The tantalum pellet is the fuel
and the MnO2 is the oxidizer. It's prudent to not detonate them.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics


Guest

Fri Jan 11, 2019 4:45 pm   



On Friday, January 11, 2019 at 2:38:53 AM UTC-5, Tim Williams wrote:
Quote:
What rating and test voltage, out of curiosity?

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

gnuarm.deletethisbit_at_gmail.com> wrote in message
news:8c98a491-be67-4607-8317-21085d0c223f_at_googlegroups.com...
We are seeing around 1 in 100 Tantalum caps fail shorted when initially
powered up on the test fixture. There was a discussion here about such
failures being an initial short from fabrication or created when reflowed on
the board. An article was linked which discussed a real world app where
they powered up the board with a voltage ramp and enough series resistance
to prevent the cap from creating a hard short and failing. But no
information on how slow is "slow" and how much resistance is needed to
prevent damage while clearing the short.

On the test fixture we can add a resistance to be bypassed with a jumper.
We can't control the ramp up time of that voltage rail other than adding
more capacitance to slow it down. I suppose that is an option if we know a
target ramp rate. I've asked the manufacturer of the regulator (CUI
V7812-1000) what the ramp rate is and how it varies with capacitance. We'll
see if they respond.

Any suggestions on improving this problem? It's not a big deal, but we did
have one cap that failed catastrophically and actually burned the board, but
not beyond repair.

Rick C.

- Get 6 months of free supercharging
- Tesla referral code - https://ts.la/richard11209


It's a 20 volt cap on a 12 volt supply. We only see failures in test, not in the field. One engineer suggested the issue was moisture in the cap from improper handling, but he also is working on some invention where he used the term, "over unity" before I stopped listening.

I'd like to modify the test fixture to not have these failures if they can be prevented without too much work. As I said, I have a current sense resistor with a jumper so I can put any value there and jump across it once the circuit is powered up. It would add time to the test procedure though. Not interested in doing much rework on the thing. It's only 1 in 100 failures if that high. Might be 1 in 300.

Rick C.

+ Get 6 months of free supercharging
+ Tesla referral code - https://ts.la/richard11209

George Herold
Guest

Fri Jan 11, 2019 4:45 pm   



On Friday, January 11, 2019 at 10:00:18 AM UTC-5, gnuarm.del...@gmail.com wrote:
Quote:
On Friday, January 11, 2019 at 2:38:53 AM UTC-5, Tim Williams wrote:
What rating and test voltage, out of curiosity?

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

gnuarm.deletethisbit_at_gmail.com> wrote in message
news:8c98a491-be67-4607-8317-21085d0c223f_at_googlegroups.com...
We are seeing around 1 in 100 Tantalum caps fail shorted when initially
powered up on the test fixture. There was a discussion here about such
failures being an initial short from fabrication or created when reflowed on
the board. An article was linked which discussed a real world app where
they powered up the board with a voltage ramp and enough series resistance
to prevent the cap from creating a hard short and failing. But no
information on how slow is "slow" and how much resistance is needed to
prevent damage while clearing the short.

On the test fixture we can add a resistance to be bypassed with a jumper.
We can't control the ramp up time of that voltage rail other than adding
more capacitance to slow it down. I suppose that is an option if we know a
target ramp rate. I've asked the manufacturer of the regulator (CUI
V7812-1000) what the ramp rate is and how it varies with capacitance. We'll
see if they respond.

Any suggestions on improving this problem? It's not a big deal, but we did
have one cap that failed catastrophically and actually burned the board, but
not beyond repair.

Rick C.

- Get 6 months of free supercharging
- Tesla referral code - https://ts.la/richard11209

It's a 20 volt cap on a 12 volt supply. We only see failures in test, not in the field. One engineer suggested the issue was moisture in the cap from improper handling, but he also is working on some invention where he used the term, "over unity" before I stopped listening.

I'd like to modify the test fixture to not have these failures if they can be prevented without too much work. As I said, I have a current sense resistor with a jumper so I can put any value there and jump across it once the circuit is powered up. It would add time to the test procedure though. Not interested in doing much rework on the thing. It's only 1 in 100 failures if that high. Might be 1 in 300.


How many tant's on your board? Did you change manufacturer? Tant cap quality
seems to be continually eroding. I was fixing a ~20 year old instrument
the other day. Problem was traced down to a tant. The board had a bunch
of 16V tant's on the 15V power rails... these days I'd use a 50V one.

George H.
Quote:

Rick C.

+ Get 6 months of free supercharging
+ Tesla referral code - https://ts.la/richard11209



Guest

Fri Jan 11, 2019 6:45 pm   



On Friday, January 11, 2019 at 12:19:15 PM UTC-5, Tom Del Rosso wrote:
Quote:
John Larkin wrote:
On Fri, 11 Jan 2019 01:19:39 -0500, "Tom Del Rosso"
fizzbintuesday_at_that-google-mail-domain.com> wrote:

gnuarm.deletethisbit_at_gmail.com wrote:
We are seeing around 1 in 100 Tantalum caps fail shorted when
initially powered up on the test fixture.

Could it be the polarity is reversed relative to the marking and the
orientation in the reel?



Tantalum caps are bombs-by-the-reel. The tantalum pellet is the fuel
and the MnO2 is the oxidizer. It's prudent to not detonate them.

Yeah, I've read that they blow up if reversed. But what if the voltage
is low and derated?


When you read how they come up with the various derating factors it turns out the real issue is surge current. So rather than derate the voltage which really doesn't solve the problem necessarily, the surge current should be calculated and accounted for.

Applying a derating factor to the voltage as a way to account for surge current is lazy engineering.

Rick C.

-+ Get 6 months of free supercharging
-+ Tesla referral code - https://ts.la/richard11209

Tom Del Rosso
Guest

Fri Jan 11, 2019 6:45 pm   



John Larkin wrote:
Quote:
On Fri, 11 Jan 2019 01:19:39 -0500, "Tom Del Rosso"
fizzbintuesday_at_that-google-mail-domain.com> wrote:

gnuarm.deletethisbit_at_gmail.com wrote:
We are seeing around 1 in 100 Tantalum caps fail shorted when
initially powered up on the test fixture.

Could it be the polarity is reversed relative to the marking and the
orientation in the reel?



Tantalum caps are bombs-by-the-reel. The tantalum pellet is the fuel
and the MnO2 is the oxidizer. It's prudent to not detonate them.


Yeah, I've read that they blow up if reversed. But what if the voltage
is low and derated?

John Larkin
Guest

Fri Jan 11, 2019 6:45 pm   



On Fri, 11 Jan 2019 07:33:37 -0800 (PST), George Herold
<gherold_at_teachspin.com> wrote:

Quote:
On Friday, January 11, 2019 at 10:00:18 AM UTC-5, gnuarm.del...@gmail.com wrote:
On Friday, January 11, 2019 at 2:38:53 AM UTC-5, Tim Williams wrote:
What rating and test voltage, out of curiosity?

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

gnuarm.deletethisbit_at_gmail.com> wrote in message
news:8c98a491-be67-4607-8317-21085d0c223f_at_googlegroups.com...
We are seeing around 1 in 100 Tantalum caps fail shorted when initially
powered up on the test fixture. There was a discussion here about such
failures being an initial short from fabrication or created when reflowed on
the board. An article was linked which discussed a real world app where
they powered up the board with a voltage ramp and enough series resistance
to prevent the cap from creating a hard short and failing. But no
information on how slow is "slow" and how much resistance is needed to
prevent damage while clearing the short.

On the test fixture we can add a resistance to be bypassed with a jumper.
We can't control the ramp up time of that voltage rail other than adding
more capacitance to slow it down. I suppose that is an option if we know a
target ramp rate. I've asked the manufacturer of the regulator (CUI
V7812-1000) what the ramp rate is and how it varies with capacitance. We'll
see if they respond.

Any suggestions on improving this problem? It's not a big deal, but we did
have one cap that failed catastrophically and actually burned the board, but
not beyond repair.

Rick C.

- Get 6 months of free supercharging
- Tesla referral code - https://ts.la/richard11209

It's a 20 volt cap on a 12 volt supply. We only see failures in test, not in the field. One engineer suggested the issue was moisture in the cap from improper handling, but he also is working on some invention where he used the term, "over unity" before I stopped listening.

I'd like to modify the test fixture to not have these failures if they can be prevented without too much work. As I said, I have a current sense resistor with a jumper so I can put any value there and jump across it once the circuit is powered up. It would add time to the test procedure though. Not interested in doing much rework on the thing. It's only 1 in 100 failures if that high. Might be 1 in 300.

How many tant's on your board? Did you change manufacturer? Tant cap quality
seems to be continually eroding.


More like it's always been continually erratic. You can switch
manufacturer and be satisfied that you have solved your reliability
problem. Then after a while the good ones turn bad.

It's really a design problem: don't detonate them and they will be
very reliable.


Quote:
I was fixing a ~20 year old instrument
the other day. Problem was traced down to a tant. The board had a bunch
of 16V tant's on the 15V power rails... these days I'd use a 50V one.


Right. Tants on a supply rail should be avoided (which is usually
where you want them) or at least derated 3:1 on voltage. Keep dv/dt
under control too.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics


Guest

Fri Jan 11, 2019 6:45 pm   



On Friday, January 11, 2019 at 10:33:41 AM UTC-5, George Herold wrote:
Quote:
On Friday, January 11, 2019 at 10:00:18 AM UTC-5, gnuarm.del...@gmail.com wrote:
On Friday, January 11, 2019 at 2:38:53 AM UTC-5, Tim Williams wrote:
What rating and test voltage, out of curiosity?

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

gnuarm.deletethisbit_at_gmail.com> wrote in message
news:8c98a491-be67-4607-8317-21085d0c223f_at_googlegroups.com...
We are seeing around 1 in 100 Tantalum caps fail shorted when initially
powered up on the test fixture. There was a discussion here about such
failures being an initial short from fabrication or created when reflowed on
the board. An article was linked which discussed a real world app where
they powered up the board with a voltage ramp and enough series resistance
to prevent the cap from creating a hard short and failing. But no
information on how slow is "slow" and how much resistance is needed to
prevent damage while clearing the short.

On the test fixture we can add a resistance to be bypassed with a jumper.
We can't control the ramp up time of that voltage rail other than adding
more capacitance to slow it down. I suppose that is an option if we know a
target ramp rate. I've asked the manufacturer of the regulator (CUI
V7812-1000) what the ramp rate is and how it varies with capacitance. We'll
see if they respond.

Any suggestions on improving this problem? It's not a big deal, but we did
have one cap that failed catastrophically and actually burned the board, but
not beyond repair.

Rick C.

- Get 6 months of free supercharging
- Tesla referral code - https://ts.la/richard11209

It's a 20 volt cap on a 12 volt supply. We only see failures in test, not in the field. One engineer suggested the issue was moisture in the cap from improper handling, but he also is working on some invention where he used the term, "over unity" before I stopped listening.

I'd like to modify the test fixture to not have these failures if they can be prevented without too much work. As I said, I have a current sense resistor with a jumper so I can put any value there and jump across it once the circuit is powered up. It would add time to the test procedure though.. Not interested in doing much rework on the thing. It's only 1 in 100 failures if that high. Might be 1 in 300.

How many tant's on your board? Did you change manufacturer? Tant cap quality
seems to be continually eroding. I was fixing a ~20 year old instrument
the other day. Problem was traced down to a tant. The board had a bunch
of 16V tant's on the 15V power rails... these days I'd use a 50V one.


Just the one tantalum. It may well be rather overkill but it's too late now. The board has been in production for 10 years and I don't want to make any changes that might require new EMI testing. No change to the manufacturer.

I was making these units at a facility which did everything including shipping. They were not reporting failure information it seems. Once when I was there someone mentioned that when they had any problems the first thing they did was replace this part. I thought that was an odd thing to do. They didn't mention that they saw some number of shorts on the power rail. That is one of the things the test procedure tests for, shorted supplies.


Rick C.

-- Get 6 months of free supercharging
-- Tesla referral code - https://ts.la/richard11209

John Larkin
Guest

Fri Jan 11, 2019 7:45 pm   



On Fri, 11 Jan 2019 12:16:40 -0500, "Tom Del Rosso"
<fizzbintuesday_at_that-google-mail-domain.com> wrote:

Quote:
John Larkin wrote:
On Fri, 11 Jan 2019 01:19:39 -0500, "Tom Del Rosso"
fizzbintuesday_at_that-google-mail-domain.com> wrote:

gnuarm.deletethisbit_at_gmail.com wrote:
We are seeing around 1 in 100 Tantalum caps fail shorted when
initially powered up on the test fixture.

Could it be the polarity is reversed relative to the marking and the
orientation in the reel?



Tantalum caps are bombs-by-the-reel. The tantalum pellet is the fuel
and the MnO2 is the oxidizer. It's prudent to not detonate them.

Yeah, I've read that they blow up if reversed. But what if the voltage
is low and derated?



Bypassing a power rail, voltage derating by 3:1 seems safe, but we
never use them on a big input power rail... usually on a regulator
output. I prefer aluminum polymer caps, but some voltage regulators
like the ESR of tantalum caps.

We don't deliberately reverse bias tantalums; I'm not sure what's safe
there. I've tested some aluminum polymers for bipolar use, and
selected some that seem reliable at serious reverse voltage.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

John Larkin
Guest

Fri Jan 11, 2019 9:45 pm   



On Fri, 11 Jan 2019 02:16:14 -0500, Boris Mohar
<borism_void__at_sympatico.ca> wrote:

Quote:
On Thu, 10 Jan 2019 20:28:11 -0800 (PST), gnuarm.deletethisbit_at_gmail.com
wrote:

We are seeing around 1 in 100 Tantalum caps fail shorted when initially powered up on the test fixture. There was a discussion here about such failures being an initial short from fabrication or created when reflowed on the board. An article was linked which discussed a real world app where they powered up the board with a voltage ramp and enough series resistance to prevent the cap from creating a hard short and failing. But no information on how slow is "slow" and how much resistance is needed to prevent damage while clearing the short.

On the test fixture we can add a resistance to be bypassed with a jumper. We can't control the ramp up time of that voltage rail other than adding more capacitance to slow it down. I suppose that is an option if we know a target ramp rate. I've asked the manufacturer of the regulator (CUI V7812-1000) what the ramp rate is and how it varies with capacitance. We'll see if they respond.

Any suggestions on improving this problem? It's not a big deal, but we did have one cap that failed catastrophically and actually burned the board, but not beyond repair.

Rick C.

- Get 6 months of free supercharging
- Tesla referral code - https://ts.la/richard11209

dV/dt

https://www.avx.com/docs/techinfo/VoltageDeratingRulesforSolidTantalumandNiobiumCapacitors.pdf



Fig 4 is interesting, especially for "one or more current surges."

Fig 8 is crazy. Add all that stuff to every cap?

Their math is pretty flakey.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

piglet
Guest

Sat Jan 12, 2019 3:45 pm   



On 11/01/2019 4:52 PM, John Larkin wrote:
Quote:
On Fri, 11 Jan 2019 07:33:37 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Friday, January 11, 2019 at 10:00:18 AM UTC-5, gnuarm.del...@gmail.com wrote:
On Friday, January 11, 2019 at 2:38:53 AM UTC-5, Tim Williams wrote:
What rating and test voltage, out of curiosity?

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

gnuarm.deletethisbit_at_gmail.com> wrote in message
news:8c98a491-be67-4607-8317-21085d0c223f_at_googlegroups.com...
We are seeing around 1 in 100 Tantalum caps fail shorted when initially
powered up on the test fixture. There was a discussion here about such
failures being an initial short from fabrication or created when reflowed on
the board. An article was linked which discussed a real world app where
they powered up the board with a voltage ramp and enough series resistance
to prevent the cap from creating a hard short and failing. But no
information on how slow is "slow" and how much resistance is needed to
prevent damage while clearing the short.

On the test fixture we can add a resistance to be bypassed with a jumper.
We can't control the ramp up time of that voltage rail other than adding
more capacitance to slow it down. I suppose that is an option if we know a
target ramp rate. I've asked the manufacturer of the regulator (CUI
V7812-1000) what the ramp rate is and how it varies with capacitance. We'll
see if they respond.

Any suggestions on improving this problem? It's not a big deal, but we did
have one cap that failed catastrophically and actually burned the board, but
not beyond repair.

Rick C.

- Get 6 months of free supercharging
- Tesla referral code - https://ts.la/richard11209

It's a 20 volt cap on a 12 volt supply. We only see failures in test, not in the field. One engineer suggested the issue was moisture in the cap from improper handling, but he also is working on some invention where he used the term, "over unity" before I stopped listening.

I'd like to modify the test fixture to not have these failures if they can be prevented without too much work. As I said, I have a current sense resistor with a jumper so I can put any value there and jump across it once the circuit is powered up. It would add time to the test procedure though. Not interested in doing much rework on the thing. It's only 1 in 100 failures if that high. Might be 1 in 300.

How many tant's on your board? Did you change manufacturer? Tant cap quality
seems to be continually eroding.

More like it's always been continually erratic. You can switch
manufacturer and be satisfied that you have solved your reliability
problem. Then after a while the good ones turn bad.

It's really a design problem: don't detonate them and they will be
very reliable.


I was fixing a ~20 year old instrument
the other day. Problem was traced down to a tant. The board had a bunch
of 16V tant's on the 15V power rails... these days I'd use a 50V one.

Right. Tants on a supply rail should be avoided (which is usually
where you want them) or at least derated 3:1 on voltage. Keep dv/dt
under control too.



Yes, dv/dt is the key. I recall 1990s datasheet reccomending series R of
1 ohm per applied volt - in other words tantalum capacitor surge current
must be limited to one ampere max. Rather limits application as bypass
on beefy power rails!

piglet

George Herold
Guest

Sat Jan 12, 2019 5:45 pm   



On Saturday, January 12, 2019 at 9:27:54 AM UTC-5, piglet wrote:
Quote:
On 11/01/2019 4:52 PM, John Larkin wrote:
On Fri, 11 Jan 2019 07:33:37 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Friday, January 11, 2019 at 10:00:18 AM UTC-5, gnuarm.del...@gmail.com wrote:
On Friday, January 11, 2019 at 2:38:53 AM UTC-5, Tim Williams wrote:
What rating and test voltage, out of curiosity?

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

gnuarm.deletethisbit_at_gmail.com> wrote in message
news:8c98a491-be67-4607-8317-21085d0c223f_at_googlegroups.com...
We are seeing around 1 in 100 Tantalum caps fail shorted when initially
powered up on the test fixture. There was a discussion here about such
failures being an initial short from fabrication or created when reflowed on
the board. An article was linked which discussed a real world app where
they powered up the board with a voltage ramp and enough series resistance
to prevent the cap from creating a hard short and failing. But no
information on how slow is "slow" and how much resistance is needed to
prevent damage while clearing the short.

On the test fixture we can add a resistance to be bypassed with a jumper.
We can't control the ramp up time of that voltage rail other than adding
more capacitance to slow it down. I suppose that is an option if we know a
target ramp rate. I've asked the manufacturer of the regulator (CUI
V7812-1000) what the ramp rate is and how it varies with capacitance.. We'll
see if they respond.

Any suggestions on improving this problem? It's not a big deal, but we did
have one cap that failed catastrophically and actually burned the board, but
not beyond repair.

Rick C.

- Get 6 months of free supercharging
- Tesla referral code - https://ts.la/richard11209

It's a 20 volt cap on a 12 volt supply. We only see failures in test, not in the field. One engineer suggested the issue was moisture in the cap from improper handling, but he also is working on some invention where he used the term, "over unity" before I stopped listening.

I'd like to modify the test fixture to not have these failures if they can be prevented without too much work. As I said, I have a current sense resistor with a jumper so I can put any value there and jump across it once the circuit is powered up. It would add time to the test procedure though. Not interested in doing much rework on the thing. It's only 1 in 100 failures if that high. Might be 1 in 300.

How many tant's on your board? Did you change manufacturer? Tant cap quality
seems to be continually eroding.

More like it's always been continually erratic. You can switch
manufacturer and be satisfied that you have solved your reliability
problem. Then after a while the good ones turn bad.

It's really a design problem: don't detonate them and they will be
very reliable.


I was fixing a ~20 year old instrument
the other day. Problem was traced down to a tant. The board had a bunch
of 16V tant's on the 15V power rails... these days I'd use a 50V one.

Right. Tants on a supply rail should be avoided (which is usually
where you want them) or at least derated 3:1 on voltage. Keep dv/dt
under control too.



Yes, dv/dt is the key. I recall 1990s datasheet reccomending series R of
1 ohm per applied volt - in other words tantalum capacitor surge current
must be limited to one ampere max. Rather limits application as bypass
on beefy power rails!

piglet


So what do you use on power rails? Al electrolytics? I guess
I should 'scope the power rail turn on of all my circuits.

I use a series inductor/ multi-turn ferrite bead sometimes.

George H.

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