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| Can you get PTC fuse style devices but designed for continuous operation ? |
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| Psi:
I'm in need of a PTC fuse style device, but it will be tripped more often than a periodic fault situation. The PTC fuse datasheets all seem to say they're not intended for repetitive use. Can you get PTC style devices intended for continuous operation? Or does anyone have any tips for derating a PTC fuse so it will be fine in a continuous operation environment? Anyone know what mechanism causes a PTC to fail from overuse? |
| tom66:
I'm interested in the answer here too. I've generally shyed away from these types of devices for continuous fault protection having seen a few fail catastrophically. |
| Gyro:
The only type of PTC that I've seen that definitely handled regular and repeated tripping and long use in tripped mode were the ones used in series with CRT degaussing coils. These were fairly large bodied devices (mains rated and high initial current) and held in a small enclosure with spring contacts to the element. Probably not a lot of use to you, but it shows that such devices are possible. |
| Ian.M:
Degaussing posistors were quite interesting but not what I'd call reliable. They commonly had two disc elements sandwiched together with three leads, mains Line to the common, and one element in series with the degaussing coil with the other higher resistance one across the mains supply. The extra element had a characteristic that let it continue to pass significant current at higher temperatures than the series one effectively cut off at so the series element could be maintained at temperature with negligible current flow in the degaussing coil. A common failure mode was for the extra element to crack and flash over resulting in its destruction and a short across the mains. A useful rule of thumb was if it rattles its failed! Replacement of the posistor and mains fuse was a quick and easy repair with very little risk of a warranty 'bounce'. Later premium quality sets tended towards a single element posistor in series with a SPST relay under control of the MCU to provide degaussing on demand, or every N powerups. |
| Gyro:
Yes, I always thought that they were dual element - another variation was a PTC followed by (or combined with) an NTC to shunt any residual current. I do have a single element TDK part however, that wasn't bypassed. Irrc, I pulled it from an old Hitachi 'quick start' TV where one of the gun heaters had predictably failed (and I hadn't filled out the Tube extended warranty card!). It's just occurred to me that this would be an ideal candidate for building Joe Smith's current clamp degausser (sorry, I digress). |
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