Identifying AC mains smd devices (possibly PTCs) - diyAudio
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Old 8th April 2014, 06:35 AM   #1
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Default Identifying AC mains smd devices (possibly PTCs)

AC mains input to an ICEpower50ASX2 class-d amplifier module goes initially through two series smd size 1812 parts, with the active then passing through a fuse and a NTC, and then the mains passes through CM choke to the diode bridge.

It appears that a ceramic smd cap across one of the bridge diodes failed with arcing. Sufficient time and current have blown the fuse and thermally cracked the casing of one of the unidentified devices, as well as melt the solder pads to those two devices. The other (still intact) device measures 5 ohm, same as the leaded NTC part.

I guess the devices are most likely ptc devices for over-current protection, in addition to the T2.5A fuse. Can anyone confirm the type of part?

A photo of the pcb is in post #37 in the class D thread ICEpower 50asx2 modules. The two devices of interest to me are beside the P100 connector pins in lower left of photo. No screenprint identification on the pcb, and no schematic on-line that I can find.

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Old 8th April 2014, 10:22 AM   #2
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If these 5ohm devices are NTCs, then that means they have used three NTCs where most use only one.

That would be a waste of money.
They must be something else.
Could they be inductors?
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Old 8th April 2014, 12:50 PM   #3
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They appears to me to be ferrite beads, but 5 ohm for them is too much resistance. And, I usually work with SMD devices and never I had seen PTC or NTC except for thermal sensing If they are in series in the line or neutral, the must support and discipate instantaneous high power, and a SMD device difficultly can support it.
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Old 8th April 2014, 12:57 PM   #4
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Nah - not inductors imho. And no other sensing or non-standard circuit configuration up to the diode bridge.

I can't rationalise them to be NTC's, because of the smd operating temperature issue - and there is a 5 ohm NTC in circuit.

I can only think they are PTCs - but I haven't come across PTCs used in that manner, but then again I don't look at too many new smps designs. If they are PTC's, then the operating resistance can't be very high due to 1812 size thermals.

The supply is header selectable for 115/230V, so the fuse and NTC, and those unidentified parts must be designed to cope with 115V current levels - the max ac rms current is about 2A for 115V as it's a 170Wrms output rated class D.

It has got me intrigued to know what the design rationale is for using them. The measured 5 ohm for one of those parts is probably a damaged device value.

I mean I could just short out the pads for a quick fix, and replace the fuse, because its highly likely the fault was just the diode emi cap, and hasn't caused any other collateral damage.

PS. I just emailed the Denmark manufacturer - so here's hoping.

Last edited by trobbins; 8th April 2014 at 01:13 PM. Reason: PS.
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Old 8th April 2014, 12:59 PM   #5
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Can't you take a more detailed photo?
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Old 8th April 2014, 09:28 PM   #6
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I don't have a good macro at the moment, but found this additional view on the web:
DSC09099.jpg Photo by yoke75 | Photobucket

Parts in question are on RHS towards top of pcb.
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Old 9th April 2014, 11:43 AM   #7
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Pehaps they may be a Transorb, or a Transil, say, protection diodes. Are they in series, or in parallel to the mains?
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Old 9th April 2014, 12:25 PM   #8
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Devices are in series with active and neutral lines - so no they aren't over-voltage protection devices.
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Old 9th April 2014, 12:52 PM   #9
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I exhaust the options. They are in series, so line current pass though them, then low resistance and AC devices. No inductors (Did you try to remove one of then and place a magnet near it to eliminate FB option?). No diodes, no temp dependent resistors (PTC in series to the main is of no sense).

I have no other idea :-(
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Last edited by Osvaldo de Banfield; 9th April 2014 at 12:55 PM.
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Old 9th April 2014, 09:18 PM   #10
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I removed one of those two unknown devices - the one that had blown a bit of its encapsulation away - it certainly isn't an inductor. I'm >90% sure its a PTC device, but haven't come across their use in a smps primary situation.

A PTC is technically plausible, as it may provide some additional form of protection than a a fuse if there is a scenario where the PTC resistance would rise enough to suppress an overcurrent situation, but not damage itself from overheating - and then reset due to the fault condition removing itself somehow.
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