Hello all.
I have recently experienced failure of one of my electrolitics in my homemade linear PSU.
It is a 85ºC 3300uF/63V one, and the largest voltage I have seen on it is around 64V.
The thing is that the supply was working ok but after one hour or so of operation, it shown clear signs of overheating and almost exploded (like if It had been boiling inside), spreading the acid around.
The capacitor was not exposed to a high temperature.
The thing is... can a voltage rating slightly exceeded produce this kind of failure, or what is the typical failure mode for electrolytics?
I know, I must reduce the voltage as thermal drift has produced this nasty result, but I am a bit concerned about such a catastrophic consequence with such a slightly exceeded voltage.
Thanks!
I have recently experienced failure of one of my electrolitics in my homemade linear PSU.
It is a 85ºC 3300uF/63V one, and the largest voltage I have seen on it is around 64V.
The thing is that the supply was working ok but after one hour or so of operation, it shown clear signs of overheating and almost exploded (like if It had been boiling inside), spreading the acid around.
The capacitor was not exposed to a high temperature.
The thing is... can a voltage rating slightly exceeded produce this kind of failure, or what is the typical failure mode for electrolytics?
I know, I must reduce the voltage as thermal drift has produced this nasty result, but I am a bit concerned about such a catastrophic consequence with such a slightly exceeded voltage.
Thanks!
I wouldn't recommend approaching / exceeding the ratings. Last linear psu i built was in an amplifier described in silicon chip. It had +/- 60v rails and the electolytics used were 80v.
Basically i come to the conclusion that once all mains variations and off load voltages are taken into account the maximum voltage the cap will see is maybe 10v below its rating in a well designed supply perhaps?
I'd say replace the electros with 75v or 80v types and they should never blow up.
Basically i come to the conclusion that once all mains variations and off load voltages are taken into account the maximum voltage the cap will see is maybe 10v below its rating in a well designed supply perhaps?
I'd say replace the electros with 75v or 80v types and they should never blow up.
Hi Pierre,
It is recommended to stay 10 – 20 % below rated voltage for electrolytic caps, also depends on operating temperature. But more important is to keep in mind the current rating of the caps. Max. RMS current can vary a lot among different makes.
If you overstress them by ripple current they can become really hot and finally vent or even explode.
Cheers
It is recommended to stay 10 – 20 % below rated voltage for electrolytic caps, also depends on operating temperature. But more important is to keep in mind the current rating of the caps. Max. RMS current can vary a lot among different makes.
If you overstress them by ripple current they can become really hot and finally vent or even explode.
Cheers
As said above, ripple current is a big factor. If you just put DC on the cap, it would probably be fine quite a bit beyond the rating, but that's not how you use it. Also, if you have a diode breaking down, that will cause exactly the problem described. Look at the cap data sheet and factor in high line conditions, ripple current, temperature derating, and a safety factor! IMO, you should try to measure the ripple current if possible.
You will also find from data sheets that the ripple rating of a capacitor does not double with doubling of capacitance. So two 10,000uF caps will have ( total ) higher ripple capability than one 22,000uF cap.
From one data sheet :
63 volt 10,000 uF ( per unit ) .....ripple current 9.1 A at 85 deg C 100Hz ( ESR = 0.036 ohms )
63 volt 22,000 uF.......................ripple current 11.2 A at 85 deg C 100 Hz (ESR = 0.017 ohms )
From one data sheet :
63 volt 10,000 uF ( per unit ) .....ripple current 9.1 A at 85 deg C 100Hz ( ESR = 0.036 ohms )
63 volt 22,000 uF.......................ripple current 11.2 A at 85 deg C 100 Hz (ESR = 0.017 ohms )
Hi,
I don't know how you measured the 64Vdc maximum, but most voltmeters read the average voltage.
One is told by the capacitor manufacturers that the critical voltage is the average plus the ripple voltage. I would have expected the peak voltage be Vdc + half Vripple, but Vpk is not what the manufacturer says.
What was the ripple voltage at the time of failure?
I don't know how you measured the 64Vdc maximum, but most voltmeters read the average voltage.
One is told by the capacitor manufacturers that the critical voltage is the average plus the ripple voltage. I would have expected the peak voltage be Vdc + half Vripple, but Vpk is not what the manufacturer says.
What was the ripple voltage at the time of failure?
Hi,
if the cap di-electric has deteriorated whilst in storage, would the di-electric require to be reformed before being put back into full voltage duty?
What happens if the safe working voltage is exceeded (but still below the rated voltage) due to degraded di-electric? Is the leakage simply higher causing some internal heating, or does it cause a local hot spot that gradually deteriorates to the point of catastophic failure?
if the cap di-electric has deteriorated whilst in storage, would the di-electric require to be reformed before being put back into full voltage duty?
What happens if the safe working voltage is exceeded (but still below the rated voltage) due to degraded di-electric? Is the leakage simply higher causing some internal heating, or does it cause a local hot spot that gradually deteriorates to the point of catastophic failure?
AndrewT said:
Pretty much yes to all of the above.
Generally it is a very good idea to derate the cap to 80% based Vdc+Vpk ripple and keep ripple currents to 80% or less of rating. Voltage ratings higher than this usually buy you little increase in reliability, but using high temp caps will and keeping them cool are big wins. (Every 10 degree C reduction in internal cap temperature results in roughly double the mtbf.)
Best way IMO is to measure the unloaded transformer voltage. Multiply it by 1.41 to get the peak value, Then add the margin for high line voltage and an extra 10% safety margin. Then you have the minimum rated voltage for your buffer cap. At least that is the way I do it most time for a PSU smoothing buffer cap
Cheers
Cheers
Pjotr said:Best way IMO is to measure the unloaded transformer voltage. Multiply it by 1.41 to get the peak value, Then add the margin for high line voltage and an extra 10% safety margin. Then you have the minimum rated voltage for your buffer cap. At least that is the way I do it most time for a PSU smoothing buffer cap
Cheers
This is absolutely the right way, and in line with good commercial practice.
For reference
For reference, Underwriters Laboratories pretty much requires that you do not exceed 80% of the maximum voltage on electrolytic capacitors in a number of their safety testing standards.
Otherwise you have to run worst case scenario testing where they will shortout or open the capacitors.
It's a lot easier to just spec in higher voltage caps than deal with those tests.
For reference, Underwriters Laboratories pretty much requires that you do not exceed 80% of the maximum voltage on electrolytic capacitors in a number of their safety testing standards.
Otherwise you have to run worst case scenario testing where they will shortout or open the capacitors.
It's a lot easier to just spec in higher voltage caps than deal with those tests.
Hi
I have just last week been updating a musical fidelity P270 mk2 power amplifier. The amp was working but due to its age I decided to replace the electrolytic capacitors amd make a change to the bias which is set to low in the mk 2 version.
The rail voltages in the amplifier are + 62 volts and - 62 volts. However the power supply capacitors were 50 volt 10,000 uf. This amplifier has been working fine for 3 years. I believe these are the original capacitors fitted when the amp was built as there is no sign of change.
I do not recommend fitting caps with a lower voltage rating than the rail - infact I recommend adding 15 percent to the rating - but this example does show that capacitors can last a few years wnen overloaded.
Don
I have just last week been updating a musical fidelity P270 mk2 power amplifier. The amp was working but due to its age I decided to replace the electrolytic capacitors amd make a change to the bias which is set to low in the mk 2 version.
The rail voltages in the amplifier are + 62 volts and - 62 volts. However the power supply capacitors were 50 volt 10,000 uf. This amplifier has been working fine for 3 years. I believe these are the original capacitors fitted when the amp was built as there is no sign of change.
I do not recommend fitting caps with a lower voltage rating than the rail - infact I recommend adding 15 percent to the rating - but this example does show that capacitors can last a few years wnen overloaded.
Don
Afaik,
the P270 had 22.000uf/63V/85C ELNA caps, 4 lengthy black cans with the familiar/traditional ELNA "black neg" markings.
Could be my recollection of the P270 is the MK-I model, but the MK-I should look like THIS
A trifle awkward to imagine that Mr Michaelson would have changed the caps to 50V models in the MK-II edition
the P270 had 22.000uf/63V/85C ELNA caps, 4 lengthy black cans with the familiar/traditional ELNA "black neg" markings.
Could be my recollection of the P270 is the MK-I model, but the MK-I should look like THIS
A trifle awkward to imagine that Mr Michaelson would have changed the caps to 50V models in the MK-II edition
Jacco
Thanks for your comments.
I have a P270 mk1 and a p270 mk2 also a a370 mk1 and an a370 mk2 and other Musical Fidelity solid state amps - which I like the sound of.
The p270 mk1 has 4 elne 63v 10,000 uf long caps as you mention ( The A370 Mk1 and Mk2 has 6 of these. The P270 mk2 has 4 elna 50 v 10,000 uf short caps. The metal support that carries the caps has been placed differently in the mk2 and there is only room for short caps. The support is in its original position and there is no sign of any resoldering ( this does not mean there has been no soldering but there is no sign of any ) on the caps which are held in place by a continuous length of copper wire wound the cap terminals on the back side of the capacitor support plate. So the mk 2 has 4 short caps against 4 long caps in the mk 1. I wondered at first if MF had fitted a lower voltage transformer in the MK 2 but it is the same 60v as that in the mk 1. So the capacitor voltage rating is a mystery.
I do not comment on Musical fidelity policy or standards. I am sure they are a fine company. I do like their solid state amps and own a lot of them. My comment was not to make any allegations against Musical Fidelity bit merely to point out that this particular amplifier has ben operating for at least a few years with 50 volt caps on 60 volt rails. Again I point out that I do not recommend this practice. It was just a comment for the information of forum members.
Hope this clears up your concerns Jacco.
Don
Thanks for your comments.
I have a P270 mk1 and a p270 mk2 also a a370 mk1 and an a370 mk2 and other Musical Fidelity solid state amps - which I like the sound of.
The p270 mk1 has 4 elne 63v 10,000 uf long caps as you mention ( The A370 Mk1 and Mk2 has 6 of these. The P270 mk2 has 4 elna 50 v 10,000 uf short caps. The metal support that carries the caps has been placed differently in the mk2 and there is only room for short caps. The support is in its original position and there is no sign of any resoldering ( this does not mean there has been no soldering but there is no sign of any ) on the caps which are held in place by a continuous length of copper wire wound the cap terminals on the back side of the capacitor support plate. So the mk 2 has 4 short caps against 4 long caps in the mk 1. I wondered at first if MF had fitted a lower voltage transformer in the MK 2 but it is the same 60v as that in the mk 1. So the capacitor voltage rating is a mystery.
I do not comment on Musical fidelity policy or standards. I am sure they are a fine company. I do like their solid state amps and own a lot of them. My comment was not to make any allegations against Musical Fidelity bit merely to point out that this particular amplifier has ben operating for at least a few years with 50 volt caps on 60 volt rails. Again I point out that I do not recommend this practice. It was just a comment for the information of forum members.
Hope this clears up your concerns Jacco.
Don
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