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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Hi,
I've noticed a high failure rate in appliances built-in mains filters. These are the filters that are typically found at the point the power lead enters the appliance. They're silver coloured things about the size of a matchbox and they tend to fail short and blow the fuse. The ones I've seen are usually rated for 1A since the only stuff I do is low power. However, some of that low power stuff is test equipment with SMPSs that initially look like a short circuit at power-on. I think this is what blows those filters. Early SMPS designs didn't seem to pay much attention to surge prevention so I'm wondering if it would pay to retrospectively fit MOVs at the beginning of the SMPS section to prolong filter and power supply life (hopefully). Any thoughts on the advisability of doing this and which MOVs are best suited to this purpose? Thanks. CD. |
#2
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On Fri, 22 Apr 2016 08:11:21 -0000 (UTC), Cursitor Doom
wrote: I've noticed a high failure rate in appliances built-in mains filters. These are the filters that are typically found at the point the power lead enters the appliance. They're silver coloured things about the size of a matchbox and they tend to fail short and blow the fuse. The ones I've seen are usually rated for 1A since the only stuff I do is low power. However, some of that low power stuff is test equipment with SMPSs that initially look like a short circuit at power-on. I think this is what blows those filters. Early SMPS designs didn't seem to pay much attention to surge prevention so I'm wondering if it would pay to retrospectively fit MOVs at the beginning of the SMPS section to prolong filter and power supply life (hopefully). Any thoughts on the advisability of doing this and which MOVs are best suited to this purpose? Thanks. I think you're mixing up MOV and NTC Thermistor (surgistor or inrush current limiter). The MOV is an overvoltage limiter and goes ACROSS the power line. The NTC thermistor goes in SERIES with the AC power and limits the initial inrush current. You'll need to trace out the schematic of the power line filter to see which one (or both) you're dealing with. In normal use, I've never seen MOV's or surgistors blow. I've seen pleny of them blow up when the AC line conditions are all wrong, such as lightning hits, unbalanced phases causing the voltage to increase, power supply overload causing the surgistor to draw too much current, and such. I've also seen a few design errors, where the surgistor specs were inproperly selected. I can guess my way through your problem, but I'm busy/lazy and would prefer you to do the dirty work. Do some measurements such as a scope picture of the inrush current, normal operation, extreme operation, etc with a line current sensor. Get the numbers off the MOV and surgistor and I'll see if they were properly selected. Read some design articles on selecting inrush current limiters. http://powerelectronics.com/community/how-do-you-choose-right-type-ntc-thermistor-limit-inrush-current-capacitive-applications Incidentally, most (not all) of the devices I've seen that have inrush current problems also use slow blow fuses. Good luck. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#3
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Thanks for that correction, Jeff. Yes, in my haste I got the wrong term
for the component in question. Must be my advancing age. |
#4
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#5
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Cursitor Doom wrote:
I've noticed a high failure rate in appliances built-in mains filters. These are the filters that are typically found at the point the power lead enters the appliance. They're silver coloured things about the size of a matchbox and they tend to fail short and blow the fuse. The ones I've seen are usually rated for 1A since the only stuff I do is low power. ** Going short circuit is likely the result of one of the capacitors across the AC line failing - generally these are "class X" types which have a limited life expectancy. The other possibility is the "common mode" choke but that is very unlikely. However, some of that low power stuff is test equipment with SMPSs that initially look like a short circuit at power-on. I think this is what blows those filters. ** Very unlikely too. The surge current may be quite large but also very brief, so not enough energy to damage copper winding wire which in any case would blow open. Early SMPS designs didn't seem to pay much attention to surge prevention ** So the equipment IS rather old ? Like 20 years + so I'm wondering if it would pay to retrospectively fit MOVs at the beginning of the SMPS section to prolong filter and power supply life (hopefully). ** Small SMPSs can use a resistor in series with the supply to limit surges - say 5 or 10 ohms, wire-wound. Otherwise it is normal to use an NTC thermistor with similar cold resistance values. The resistor or NTC limits the surge current to a value that is tolerated indefinitely by fuses, diodes and switches used in the PSU - ie something like 20amps peak for a couple of milliseconds. PSU makers also have to consider the situation where multiple supplies are on the same AC circuit and all switched together creating a very large surge that may trip the breaker. ..... Phil Any thoughts on the advisability of doing this and which MOVs are best suited to this purpose? Thanks. CD. |
#6
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On Fri, 22 Apr 2016 18:44:11 -0700, Phil Allison wrote:
** Going short circuit is likely the result of one of the capacitors across the AC line failing - generally these are "class X" types which have a limited life expectancy. The other possibility is the "common mode" choke but that is very unlikely. Yes, most like a cap. ** So the equipment IS rather old ? Like 20 years + Almost always. Typically 70s & 80s. I can't deal with SMDs anyway. ** Small SMPSs can use a resistor in series with the supply to limit surges - say 5 or 10 ohms, wire-wound. Otherwise it is normal to use an NTC thermistor with similar cold resistance values. Noted, thank you. |
#7
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The resistor or NTC limits the surge current to a value that is tolerated indefinitely by fuses, diodes and switches used in the PSU - ie something like 20amps peak for a couple of milliseconds. I think that since the surge current occurs during the peak of the 120V 60Hz power waveform, I think the timing is more like several milliseconds or a few 10's of milliseconds at most, but not just a couple of milliseconds. |
#8
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wrote: " The resistor or NTC limits the surge current to a value that is tolerated indefinitely by fuses, diodes and switches used in the PSU - ie something like 20amps peak for a couple of milliseconds. " I think that since the surge current occurs during the peak of the 120V 60Hz power waveform, I think the timing is more like several milliseconds or a few 10's of milliseconds at most, but not just a couple of milliseconds. ** The surge occurs soon as the PSU is switched on - the magnitude depends on the timing relative to the AC wave. My comments referred to the worst case surge, when the switch on is at or near an AC voltage peak. When this happens and if the total series resistance is say 5 ohms and the main electro is 220uF, the time constant is 1mS. Remember, the context was all about *SMALL* SMPSs. ..... Phil |
#9
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I suppose the advantage of the resistor solution - apart from the fact that I already have lots of suitable ones in my junk box - is that the PSU remains protected even when the power supply is temporarily interrupted, say when someone turns it off and on again over a short interval. |
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