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A question for you LM324 experts on failure problems.

Started by Jamie October 24, 2011
  We have some devices that have been in use for many years (not made
by us). These are analog PID boards that employ a 324N chip.

  The circuit works very well and is very flexible in what it can do.

  THis is the problem. Lately been getting a few random failures of
the op-amp. Today, one failed simply by turning one of the pots on the
board that is in the feed back loop as the GAIN control of the PID on
the final output op-amp.

   There are a couple of large BP caps in the circuit, one for LAG(I) and
one for LEAD (D). The only thing I can think of at this time is when the
gain value was changed a large load swing on the power rails may have 
taken place, due to the CAPS hitting the inputs and causing the circuit 
to wobble before it stabilized, this is common to happen when changing 
the POT settings while in operation.

   The reason I am thinking towards this way is due to the fact that this
circuit power rails are dual +/-15.6 volts or there abouts and the specs
on the chip is +-16 volts MAX.

   With all of this data at hand, is it possible the filter caps on the 
dual rail supply are weak and the short unloaded condition that may take
place when the circuit is not stable could be causing voltage peaks to
exceed the 16 volts at times and there by causing failures to the chip?

  I don't know how critical the limits are on these 324s but we are 
starting to see random failures here and there all about the same type 
of results and this is when connected to various types of drives etc.

  2 things normally happen when it fails, the chip may start to operate
HOT or, the output will refuse to swing in both directions when driving
the (-) input with a +/- signal.

   With today's example, driving the (-) input with a (-) signal was 
actually causing a (-) output and never developed any (+) output at all.
  Now neglected to notice in which direction the output was moving to 
tell if we were getting a shorted output from the other side or simply 
no output at all from the (+) side.

  I will be doing more debugging tomorrow on this problem with the 
scope, it was at the end of the day so I didn't have any time. But I 
suspect
power supply rails are dancing above 16 volts at time due to the caps 
not holding.

   What do you think? Are the 324's sensitive to this kind of rail 
problem? I suppose we could be getting line noise in there and the caps
just isn't subbing it and maybe exceeding the max voltage.


  Jamie


Jamie wrote:
> We have some devices that have been in use for many years (not made > by us). These are analog PID boards that employ a 324N chip. > > The circuit works very well and is very flexible in what it can do. > > THis is the problem. Lately been getting a few random failures of > the op-amp. Today, one failed simply by turning one of the pots on the > board that is in the feed back loop as the GAIN control of the PID on > the final output op-amp. > > There are a couple of large BP caps in the circuit, one for LAG(I) and > one for LEAD (D). The only thing I can think of at this time is when the > gain value was changed a large load swing on the power rails may have > taken place, due to the CAPS hitting the inputs and causing the circuit > to wobble before it stabilized, this is common to happen when changing > the POT settings while in operation. > > The reason I am thinking towards this way is due to the fact that this > circuit power rails are dual +/-15.6 volts or there abouts and the specs > on the chip is +-16 volts MAX. > > With all of this data at hand, is it possible the filter caps on the > dual rail supply are weak and the short unloaded condition that may take > place when the circuit is not stable could be causing voltage peaks to > exceed the 16 volts at times and there by causing failures to the chip? > > I don't know how critical the limits are on these 324s but we are > starting to see random failures here and there all about the same type > of results and this is when connected to various types of drives etc. > > 2 things normally happen when it fails, the chip may start to operate > HOT or, the output will refuse to swing in both directions when driving > the (-) input with a +/- signal. > > With today's example, driving the (-) input with a (-) signal was > actually causing a (-) output and never developed any (+) output at all. > Now neglected to notice in which direction the output was moving to > tell if we were getting a shorted output from the other side or simply > no output at all from the (+) side. > > I will be doing more debugging tomorrow on this problem with the scope, > it was at the end of the day so I didn't have any time. But I suspect > power supply rails are dancing above 16 volts at time due to the caps > not holding. > > What do you think? Are the 324's sensitive to this kind of rail > problem? I suppose we could be getting line noise in there and the caps > just isn't subbing it and maybe exceeding the max voltage. >
How long are the lines/traces/wires from power supply to the rail near those LM324? How much in local bypass capacitance? Turning on a system, can cause some ringing there, easy to check with a DSO. Whack the unit, do some wild power cycling, and so on. Running them at 31.2V total while abs max is 32V is like Russian roulette. Did you consider replacing them with an opamp that can take more? There are opamp with abs max 44V and such. The MC33174 comes to mind, not sure if it'll fit the bill in your case: http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000477.pdf An old Motorola design, I have used them a lot in the last century. -- Regards, Joerg http://www.analogconsultants.com/
Joerg wrote:

> Jamie wrote: > >> We have some devices that have been in use for many years (not made >>by us). These are analog PID boards that employ a 324N chip. >> >> The circuit works very well and is very flexible in what it can do. >> >> THis is the problem. Lately been getting a few random failures of >>the op-amp. Today, one failed simply by turning one of the pots on the >>board that is in the feed back loop as the GAIN control of the PID on >>the final output op-amp. >> >> There are a couple of large BP caps in the circuit, one for LAG(I) and >>one for LEAD (D). The only thing I can think of at this time is when the >>gain value was changed a large load swing on the power rails may have >>taken place, due to the CAPS hitting the inputs and causing the circuit >>to wobble before it stabilized, this is common to happen when changing >>the POT settings while in operation. >> >> The reason I am thinking towards this way is due to the fact that this >>circuit power rails are dual +/-15.6 volts or there abouts and the specs >>on the chip is +-16 volts MAX. >> >> With all of this data at hand, is it possible the filter caps on the >>dual rail supply are weak and the short unloaded condition that may take >>place when the circuit is not stable could be causing voltage peaks to >>exceed the 16 volts at times and there by causing failures to the chip? >> >> I don't know how critical the limits are on these 324s but we are >>starting to see random failures here and there all about the same type >>of results and this is when connected to various types of drives etc. >> >> 2 things normally happen when it fails, the chip may start to operate >>HOT or, the output will refuse to swing in both directions when driving >>the (-) input with a +/- signal. >> >> With today's example, driving the (-) input with a (-) signal was >>actually causing a (-) output and never developed any (+) output at all. >> Now neglected to notice in which direction the output was moving to >>tell if we were getting a shorted output from the other side or simply >>no output at all from the (+) side. >> >> I will be doing more debugging tomorrow on this problem with the scope, >>it was at the end of the day so I didn't have any time. But I suspect >>power supply rails are dancing above 16 volts at time due to the caps >>not holding. >> >> What do you think? Are the 324's sensitive to this kind of rail >>problem? I suppose we could be getting line noise in there and the caps >>just isn't subbing it and maybe exceeding the max voltage. >> > > > How long are the lines/traces/wires from power supply to the rail near > those LM324? How much in local bypass capacitance? Turning on a system, > can cause some ringing there, easy to check with a DSO. Whack the unit, > do some wild power cycling, and so on. > > Running them at 31.2V total while abs max is 32V is like Russian > roulette. Did you consider replacing them with an opamp that can take > more? There are opamp with abs max 44V and such. The MC33174 comes to > mind, not sure if it'll fit the bill in your case: > > http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000477.pdf > > An old Motorola design, I have used them a lot in the last century. >
This is the manual which has the schematic towards the end. http://www.contrexinc.com/PDF/Reflex/tm_12m03_104.pdf This is old stuff but it works very well and alot of it in service in various parts of our multi-plants. This problem is starting to get more announced now that these units are getting older. So after closer inspection I think what is happening is that maybe the supply caps are weak and the load shift may be causing some extra higher voltages to reach the less loaded side of the rail when instability takes place. P.S. Even though that schematic states 15V +/-, I get almost 16 volts with the DMM. Jamie
On 10/25/2011 9:01 AM, Jamie wrote:
> Joerg wrote: > >> Jamie wrote: >> >>> We have some devices that have been in use for many years (not made >>> by us). These are analog PID boards that employ a 324N chip. >>> >>> The circuit works very well and is very flexible in what it can do. >>> >>> THis is the problem. Lately been getting a few random failures of >>> the op-amp. Today, one failed simply by turning one of the pots on the >>> board that is in the feed back loop as the GAIN control of the PID on >>> the final output op-amp. >>> >>> There are a couple of large BP caps in the circuit, one for LAG(I) and >>> one for LEAD (D). The only thing I can think of at this time is when the >>> gain value was changed a large load swing on the power rails may have >>> taken place, due to the CAPS hitting the inputs and causing the circuit >>> to wobble before it stabilized, this is common to happen when changing >>> the POT settings while in operation. >>> >>> The reason I am thinking towards this way is due to the fact that this >>> circuit power rails are dual +/-15.6 volts or there abouts and the specs >>> on the chip is +-16 volts MAX. >>> >>> With all of this data at hand, is it possible the filter caps on the >>> dual rail supply are weak and the short unloaded condition that may take >>> place when the circuit is not stable could be causing voltage peaks to >>> exceed the 16 volts at times and there by causing failures to the chip? >>> >>> I don't know how critical the limits are on these 324s but we are >>> starting to see random failures here and there all about the same type >>> of results and this is when connected to various types of drives etc. >>> >>> 2 things normally happen when it fails, the chip may start to operate >>> HOT or, the output will refuse to swing in both directions when driving >>> the (-) input with a +/- signal. >>> >>> With today's example, driving the (-) input with a (-) signal was >>> actually causing a (-) output and never developed any (+) output at all. >>> Now neglected to notice in which direction the output was moving to >>> tell if we were getting a shorted output from the other side or simply >>> no output at all from the (+) side. >>> >>> I will be doing more debugging tomorrow on this problem with the scope, >>> it was at the end of the day so I didn't have any time. But I suspect >>> power supply rails are dancing above 16 volts at time due to the caps >>> not holding. >>> >>> What do you think? Are the 324's sensitive to this kind of rail >>> problem? I suppose we could be getting line noise in there and the caps >>> just isn't subbing it and maybe exceeding the max voltage. >>> >> >> >> How long are the lines/traces/wires from power supply to the rail near >> those LM324? How much in local bypass capacitance? Turning on a system, >> can cause some ringing there, easy to check with a DSO. Whack the unit, >> do some wild power cycling, and so on. >> >> Running them at 31.2V total while abs max is 32V is like Russian >> roulette. Did you consider replacing them with an opamp that can take >> more? There are opamp with abs max 44V and such. The MC33174 comes to >> mind, not sure if it'll fit the bill in your case: >> >> http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000477.pdf >> >> >> An old Motorola design, I have used them a lot in the last century. >> > This is the manual which has the schematic towards the end. > > http://www.contrexinc.com/PDF/Reflex/tm_12m03_104.pdf > > This is old stuff but it works very well and alot of it in service in > various parts of our multi-plants. This problem is starting to get more > announced now that these units are getting older. So after closer > inspection I think what is happening is that maybe the supply caps are > weak and the load shift may be causing some extra higher voltages to > reach the less loaded side of the rail when instability takes place. > > P.S. > Even though that schematic states 15V +/-, I get almost 16 volts with > the DMM. > > Jamie > >
Well I ain't the brightest bulb around here :-) but for old gear my rule #1 is replace all the electro's, seems to sort a lot of strange happenings. Cheers ....... Rheilly
On Mon, 24 Oct 2011 20:21:12 -0400, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

> We have some devices that have been in use for many years (not made >by us). These are analog PID boards that employ a 324N chip. > > The circuit works very well and is very flexible in what it can do. > > THis is the problem. Lately been getting a few random failures of >the op-amp. Today, one failed simply by turning one of the pots on the >board that is in the feed back loop as the GAIN control of the PID on >the final output op-amp. > > There are a couple of large BP caps in the circuit, one for LAG(I) and >one for LEAD (D). The only thing I can think of at this time is when the >gain value was changed a large load swing on the power rails may have >taken place, due to the CAPS hitting the inputs and causing the circuit >to wobble before it stabilized, this is common to happen when changing >the POT settings while in operation. > > The reason I am thinking towards this way is due to the fact that this >circuit power rails are dual +/-15.6 volts or there abouts and the specs >on the chip is +-16 volts MAX. > > With all of this data at hand, is it possible the filter caps on the >dual rail supply are weak and the short unloaded condition that may take >place when the circuit is not stable could be causing voltage peaks to >exceed the 16 volts at times and there by causing failures to the chip? > > I don't know how critical the limits are on these 324s but we are >starting to see random failures here and there all about the same type >of results and this is when connected to various types of drives etc. > > 2 things normally happen when it fails, the chip may start to operate >HOT or, the output will refuse to swing in both directions when driving >the (-) input with a +/- signal. > > With today's example, driving the (-) input with a (-) signal was >actually causing a (-) output and never developed any (+) output at all. > Now neglected to notice in which direction the output was moving to >tell if we were getting a shorted output from the other side or simply >no output at all from the (+) side. > > I will be doing more debugging tomorrow on this problem with the >scope, it was at the end of the day so I didn't have any time. But I >suspect >power supply rails are dancing above 16 volts at time due to the caps >not holding. > > What do you think? Are the 324's sensitive to this kind of rail >problem? I suppose we could be getting line noise in there and the caps >just isn't subbing it and maybe exceeding the max voltage. > > > Jamie >
The supplies are unregulated. They could spike really high in some circumstances. John
On Oct 24, 6:20=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Mon, 24 Oct 2011 20:21:12 -0400, Jamie > > > > > > > > > > <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote: > > =A0We have some devices that have been in use for many years (not made > >by us). These are analog PID boards that employ a 324N chip. > > > =A0The circuit works very well and is very flexible in what it can do. > > > =A0THis is the problem. Lately been getting a few random failures of > >the op-amp. Today, one failed simply by turning one of the pots on the > >board that is in the feed back loop as the GAIN control of the PID on > >the final output op-amp. > > > =A0 There are a couple of large BP caps in the circuit, one for LAG(I) =
and
> >one for LEAD (D). The only thing I can think of at this time is when the > >gain value was changed a large load swing on the power rails may have > >taken place, due to the CAPS hitting the inputs and causing the circuit > >to wobble before it stabilized, this is common to happen when changing > >the POT settings while in operation. > > > =A0 The reason I am thinking towards this way is due to the fact that t=
his
> >circuit power rails are dual +/-15.6 volts or there abouts and the specs > >on the chip is +-16 volts MAX. > > > =A0 With all of this data at hand, is it possible the filter caps on th=
e
> >dual rail supply are weak and the short unloaded condition that may take > >place when the circuit is not stable could be causing voltage peaks to > >exceed the 16 volts at times and there by causing failures to the chip? > > > =A0I don't know how critical the limits are on these 324s but we are > >starting to see random failures here and there all about the same type > >of results and this is when connected to various types of drives etc. > > > =A02 things normally happen when it fails, the chip may start to operat=
e
> >HOT or, the output will refuse to swing in both directions when driving > >the (-) input with a +/- signal. > > > =A0 With today's example, driving the (-) input with a (-) signal was > >actually causing a (-) output and never developed any (+) output at all. > > =A0Now neglected to notice in which direction the output was moving to > >tell if we were getting a shorted output from the other side or simply > >no output at all from the (+) side. > > > =A0I will be doing more debugging tomorrow on this problem with the > >scope, it was at the end of the day so I didn't have any time. But I > >suspect > >power supply rails are dancing above 16 volts at time due to the caps > >not holding. > > > =A0 What do you think? Are the 324's sensitive to this kind of rail > >problem? I suppose we could be getting line noise in there and the caps > >just isn't subbing it and maybe exceeding the max voltage. > > > =A0Jamie > > The supplies are unregulated. They could spike really high in some > circumstances. > > John
Not to mention that power line voltages have been creeping upwards over the last several years, in some parts of the US at least. Hard to find concrete documentation of this phenomenon, but anecdotes abound. -- john
On a sunny day (Tue, 25 Oct 2011 09:06:17 +0800) it happened Rheilly Phoull
<rheilly@bigslong.com> wrote in
<nMOdnabYSOIwljvTnZ2dnUVZ_gqdnZ2d@westnet.com.au>:

>On 10/25/2011 9:01 AM, Jamie wrote: >> Joerg wrote: >> >>> Jamie wrote: >>> >>>> We have some devices that have been in use for many years (not made >>>> by us). These are analog PID boards that employ a 324N chip. >>>> >>>> The circuit works very well and is very flexible in what it can do. >>>> >>>> THis is the problem. Lately been getting a few random failures of >>>> the op-amp. Today, one failed simply by turning one of the pots on the >>>> board that is in the feed back loop as the GAIN control of the PID on >>>> the final output op-amp. >>>> >>>> There are a couple of large BP caps in the circuit, one for LAG(I) and >>>> one for LEAD (D). The only thing I can think of at this time is when the >>>> gain value was changed a large load swing on the power rails may have >>>> taken place, due to the CAPS hitting the inputs and causing the circuit >>>> to wobble before it stabilized, this is common to happen when changing >>>> the POT settings while in operation. >>>> >>>> The reason I am thinking towards this way is due to the fact that this >>>> circuit power rails are dual +/-15.6 volts or there abouts and the specs >>>> on the chip is +-16 volts MAX. >>>> >>>> With all of this data at hand, is it possible the filter caps on the >>>> dual rail supply are weak and the short unloaded condition that may take >>>> place when the circuit is not stable could be causing voltage peaks to >>>> exceed the 16 volts at times and there by causing failures to the chip? >>>> >>>> I don't know how critical the limits are on these 324s but we are >>>> starting to see random failures here and there all about the same type >>>> of results and this is when connected to various types of drives etc. >>>> >>>> 2 things normally happen when it fails, the chip may start to operate >>>> HOT or, the output will refuse to swing in both directions when driving >>>> the (-) input with a +/- signal. >>>> >>>> With today's example, driving the (-) input with a (-) signal was >>>> actually causing a (-) output and never developed any (+) output at all. >>>> Now neglected to notice in which direction the output was moving to >>>> tell if we were getting a shorted output from the other side or simply >>>> no output at all from the (+) side. >>>> >>>> I will be doing more debugging tomorrow on this problem with the scope, >>>> it was at the end of the day so I didn't have any time. But I suspect >>>> power supply rails are dancing above 16 volts at time due to the caps >>>> not holding. >>>> >>>> What do you think? Are the 324's sensitive to this kind of rail >>>> problem? I suppose we could be getting line noise in there and the caps >>>> just isn't subbing it and maybe exceeding the max voltage. >>>> >>> >>> >>> How long are the lines/traces/wires from power supply to the rail near >>> those LM324? How much in local bypass capacitance? Turning on a system, >>> can cause some ringing there, easy to check with a DSO. Whack the unit, >>> do some wild power cycling, and so on. >>> >>> Running them at 31.2V total while abs max is 32V is like Russian >>> roulette. Did you consider replacing them with an opamp that can take >>> more? There are opamp with abs max 44V and such. The MC33174 comes to >>> mind, not sure if it'll fit the bill in your case: >>> >>> http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000477.pdf >>> >>> >>> An old Motorola design, I have used them a lot in the last century. >>> >> This is the manual which has the schematic towards the end. >> >> http://www.contrexinc.com/PDF/Reflex/tm_12m03_104.pdf >> >> This is old stuff but it works very well and alot of it in service in >> various parts of our multi-plants. This problem is starting to get more >> announced now that these units are getting older. So after closer >> inspection I think what is happening is that maybe the supply caps are >> weak and the load shift may be causing some extra higher voltages to >> reach the less loaded side of the rail when instability takes place. >> >> P.S. >> Even though that schematic states 15V +/-, I get almost 16 volts with >> the DMM. >> >> Jamie >> >> > > >Well I ain't the brightest bulb around here :-) but for old gear my rule >#1 is replace all the electro's, seems to sort a lot of strange >happenings. > >Cheers ....... Rheilly
Right, replace by tantalums ;-) LOL No seriously ;-) But I think Joerg is right, out of spec for the supply voltage and needs a different chip too.
On Mon, 24 Oct 2011 21:01:55 -0400, the renowned Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

>Joerg wrote: > >> Jamie wrote: >> >>> We have some devices that have been in use for many years (not made >>>by us). These are analog PID boards that employ a 324N chip. >>> >>> The circuit works very well and is very flexible in what it can do. >>> >>> THis is the problem. Lately been getting a few random failures of >>>the op-amp. Today, one failed simply by turning one of the pots on the >>>board that is in the feed back loop as the GAIN control of the PID on >>>the final output op-amp. >>> >>> There are a couple of large BP caps in the circuit, one for LAG(I) and >>>one for LEAD (D). The only thing I can think of at this time is when the >>>gain value was changed a large load swing on the power rails may have >>>taken place, due to the CAPS hitting the inputs and causing the circuit >>>to wobble before it stabilized, this is common to happen when changing >>>the POT settings while in operation. >>> >>> The reason I am thinking towards this way is due to the fact that this >>>circuit power rails are dual +/-15.6 volts or there abouts and the specs >>>on the chip is +-16 volts MAX. >>> >>> With all of this data at hand, is it possible the filter caps on the >>>dual rail supply are weak and the short unloaded condition that may take >>>place when the circuit is not stable could be causing voltage peaks to >>>exceed the 16 volts at times and there by causing failures to the chip? >>> >>> I don't know how critical the limits are on these 324s but we are >>>starting to see random failures here and there all about the same type >>>of results and this is when connected to various types of drives etc. >>> >>> 2 things normally happen when it fails, the chip may start to operate >>>HOT or, the output will refuse to swing in both directions when driving >>>the (-) input with a +/- signal. >>> >>> With today's example, driving the (-) input with a (-) signal was >>>actually causing a (-) output and never developed any (+) output at all. >>> Now neglected to notice in which direction the output was moving to >>>tell if we were getting a shorted output from the other side or simply >>>no output at all from the (+) side. >>> >>> I will be doing more debugging tomorrow on this problem with the scope, >>>it was at the end of the day so I didn't have any time. But I suspect >>>power supply rails are dancing above 16 volts at time due to the caps >>>not holding. >>> >>> What do you think? Are the 324's sensitive to this kind of rail >>>problem? I suppose we could be getting line noise in there and the caps >>>just isn't subbing it and maybe exceeding the max voltage. >>> >> >> >> How long are the lines/traces/wires from power supply to the rail near >> those LM324? How much in local bypass capacitance? Turning on a system, >> can cause some ringing there, easy to check with a DSO. Whack the unit, >> do some wild power cycling, and so on. >> >> Running them at 31.2V total while abs max is 32V is like Russian >> roulette. Did you consider replacing them with an opamp that can take >> more? There are opamp with abs max 44V and such. The MC33174 comes to >> mind, not sure if it'll fit the bill in your case: >> >> http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000477.pdf >> >> An old Motorola design, I have used them a lot in the last century. >> >This is the manual which has the schematic towards the end. > >http://www.contrexinc.com/PDF/Reflex/tm_12m03_104.pdf > >This is old stuff but it works very well and alot of it in service in >various parts of our multi-plants. This problem is starting to get more >announced now that these units are getting older. So after closer >inspection I think what is happening is that maybe the supply caps are >weak and the load shift may be causing some extra higher voltages to >reach the less loaded side of the rail when instability takes place. > > P.S. > Even though that schematic states 15V +/-, I get almost 16 volts with >the DMM. > >Jamie >
That's a really dodgy circuit with unregulated supply rails right up at the abs max supply voltage. Best to just redesign it, or at least sub a higher voltage quad if you're not selling many of them. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Jamie wrote:
> Joerg wrote: > >> Jamie wrote: >> >>> We have some devices that have been in use for many years (not made >>> by us). These are analog PID boards that employ a 324N chip. >>> >>> The circuit works very well and is very flexible in what it can do. >>> >>> THis is the problem. Lately been getting a few random failures of >>> the op-amp. Today, one failed simply by turning one of the pots on the >>> board that is in the feed back loop as the GAIN control of the PID on >>> the final output op-amp. >>> >>> There are a couple of large BP caps in the circuit, one for LAG(I) and >>> one for LEAD (D). The only thing I can think of at this time is when the >>> gain value was changed a large load swing on the power rails may have >>> taken place, due to the CAPS hitting the inputs and causing the circuit >>> to wobble before it stabilized, this is common to happen when changing >>> the POT settings while in operation. >>> >>> The reason I am thinking towards this way is due to the fact that this >>> circuit power rails are dual +/-15.6 volts or there abouts and the specs >>> on the chip is +-16 volts MAX. >>> >>> With all of this data at hand, is it possible the filter caps on the >>> dual rail supply are weak and the short unloaded condition that may take >>> place when the circuit is not stable could be causing voltage peaks to >>> exceed the 16 volts at times and there by causing failures to the chip? >>> >>> I don't know how critical the limits are on these 324s but we are >>> starting to see random failures here and there all about the same type >>> of results and this is when connected to various types of drives etc. >>> >>> 2 things normally happen when it fails, the chip may start to operate >>> HOT or, the output will refuse to swing in both directions when driving >>> the (-) input with a +/- signal. >>> >>> With today's example, driving the (-) input with a (-) signal was >>> actually causing a (-) output and never developed any (+) output at all. >>> Now neglected to notice in which direction the output was moving to >>> tell if we were getting a shorted output from the other side or simply >>> no output at all from the (+) side. >>> >>> I will be doing more debugging tomorrow on this problem with the scope, >>> it was at the end of the day so I didn't have any time. But I suspect >>> power supply rails are dancing above 16 volts at time due to the caps >>> not holding. >>> >>> What do you think? Are the 324's sensitive to this kind of rail >>> problem? I suppose we could be getting line noise in there and the caps >>> just isn't subbing it and maybe exceeding the max voltage. >>> >> >> >> How long are the lines/traces/wires from power supply to the rail near >> those LM324? How much in local bypass capacitance? Turning on a system, >> can cause some ringing there, easy to check with a DSO. Whack the unit, >> do some wild power cycling, and so on. >> >> Running them at 31.2V total while abs max is 32V is like Russian >> roulette. Did you consider replacing them with an opamp that can take >> more? There are opamp with abs max 44V and such. The MC33174 comes to >> mind, not sure if it'll fit the bill in your case: >> >> http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000477.pdf >> >> >> An old Motorola design, I have used them a lot in the last century. >> > This is the manual which has the schematic towards the end. > > http://www.contrexinc.com/PDF/Reflex/tm_12m03_104.pdf > > This is old stuff but it works very well and alot of it in service in > various parts of our multi-plants. This problem is starting to get more > announced now that these units are getting older. So after closer > inspection I think what is happening is that maybe the supply caps are > weak and the load shift may be causing some extra higher voltages to > reach the less loaded side of the rail when instability takes place. > > P.S. > Even though that schematic states 15V +/-, I get almost 16 volts with > the DMM. >
Unregulated 15V assuming stable mains ... oh-oh. That does require a high voltage amp and also some voltage clamping. Clamping might not be too easy because TVS have large tolerance. Also, the dancer remote connection at pin 10 of the LM324 is totally unprotected against EMI and ESD. So is the gate of Q1, or 1Q as they call it. Looks like this board was designed by a rookie :-) There is a chance that a lot of these units are now out there and still work, sort of, but where the LM324 chips in there have bruises and aren't 100% anymore. -- Regards, Joerg http://www.analogconsultants.com/
John Miles, KE5FX wrote:
> On Oct 24, 6:20 pm, John Larkin > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> On Mon, 24 Oct 2011 20:21:12 -0400, Jamie >> >> >> >> >> >> >> >> >> >> <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote: >>> We have some devices that have been in use for many years (not made >>> by us). These are analog PID boards that employ a 324N chip. >>> The circuit works very well and is very flexible in what it can do. >>> THis is the problem. Lately been getting a few random failures of >>> the op-amp. Today, one failed simply by turning one of the pots on the >>> board that is in the feed back loop as the GAIN control of the PID on >>> the final output op-amp. >>> There are a couple of large BP caps in the circuit, one for LAG(I) and >>> one for LEAD (D). The only thing I can think of at this time is when the >>> gain value was changed a large load swing on the power rails may have >>> taken place, due to the CAPS hitting the inputs and causing the circuit >>> to wobble before it stabilized, this is common to happen when changing >>> the POT settings while in operation. >>> The reason I am thinking towards this way is due to the fact that this >>> circuit power rails are dual +/-15.6 volts or there abouts and the specs >>> on the chip is +-16 volts MAX. >>> With all of this data at hand, is it possible the filter caps on the >>> dual rail supply are weak and the short unloaded condition that may take >>> place when the circuit is not stable could be causing voltage peaks to >>> exceed the 16 volts at times and there by causing failures to the chip? >>> I don't know how critical the limits are on these 324s but we are >>> starting to see random failures here and there all about the same type >>> of results and this is when connected to various types of drives etc. >>> 2 things normally happen when it fails, the chip may start to operate >>> HOT or, the output will refuse to swing in both directions when driving >>> the (-) input with a +/- signal. >>> With today's example, driving the (-) input with a (-) signal was >>> actually causing a (-) output and never developed any (+) output at all. >>> Now neglected to notice in which direction the output was moving to >>> tell if we were getting a shorted output from the other side or simply >>> no output at all from the (+) side. >>> I will be doing more debugging tomorrow on this problem with the >>> scope, it was at the end of the day so I didn't have any time. But I >>> suspect >>> power supply rails are dancing above 16 volts at time due to the caps >>> not holding. >>> What do you think? Are the 324's sensitive to this kind of rail >>> problem? I suppose we could be getting line noise in there and the caps >>> just isn't subbing it and maybe exceeding the max voltage. >>> Jamie >> The supplies are unregulated. They could spike really high in some >> circumstances. >> >> John > > Not to mention that power line voltages have been creeping upwards > over the last several years, in some parts of the US at least. Hard > to find concrete documentation of this phenomenon, but anecdotes > abound. >
Sometimes I had the impression this was done because it increases revenue. I had to complain hard when light bulbs started blowing. It had crept up to 127V early in the night. So finally they lowered it to around 121.5V. -- Regards, Joerg http://www.analogconsultants.com/