Can you get PTC fuse style devices but designed for continuous operation ? - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: April 17, 2022, 10:08:00 am
content: 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?

author: tom66
author_rank: Super Contributor
timestamp: April 17, 2022, 10:19:57 am
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: April 17, 2022, 10:25:49 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: April 17, 2022, 11:07:18 am
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: April 17, 2022, 12:00:18 pm
content: 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).

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2022, 05:16:38 pm
content: Daft question: why not use a circuit breaker? There are types which automatically reset, which no doubt consist of a bimetal strip, next to a current shunt.https://www.mouser.co.uk/datasheet/2/645/D_1610_ENG-1109200.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 17, 2022, 06:28:09 pm
content: Ceramic PTCs may be more reliable? They don't have as abrupt a range though. Upside: they're available in higher voltages, including mains operation.Otherwise, something electronic?Tim

author: tom66
author_rank: Super Contributor
timestamp: April 17, 2022, 06:56:51 pm
content: In days when you can buy them, the TI eFuse devices were rather good -- current limit set using usually a single resistor, and just a few caps plus the IC required. Many of them implement a mode where brief transients are permitted (or where the device will linearly regulate) before cutting off.

author: floobydust
author_rank: Super Contributor
timestamp: April 17, 2022, 07:24:02 pm
content: PTC fuses are very difficult to design in. Overall, I find they are usable only for gross overloads- many times their rated holding current.High ambient temperature aggravates their trip point dropping lower, but also it's highest just off the reel and lowers after the first 1-3 operations:"Upon cooling, the polymer reforms to its semi-crystallized state and the conductive pathways are reestablished. However, when the polymer recrystallizes it does not return immediately to the same base resistance. It does not compact as tightly as when it was pre-tripped, and thereforethe post trip resistance in most cases is typically 50% higherthen the initial resistance. Note the post trip resistance increase is not a cumulative effect; additional tripping will not cause increases in resistance in excess of the first trip event. Further resistance recovery is possible if the device is conditioned by actively current cycling the device or passively heating the device below 85°C."Bourns Technical Paper - Soldering Polymer PTC Resettable FusesSo Bourns is saying its resistance increases, which is basically the amount of heat for a given current, so they'll trip at a lower threshold than the datasheets. This is a trap and always best to get samples and evaluate the parts in the lab.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: April 17, 2022, 11:30:13 pm
content: In general terms what are the:1. Voltage and current requirements?2. AC or DC?3. How important is efficiency? Is this battery-operated?4. Power budget? Cost budget? Space budget?5. Applicable standards?

author: nctnico
author_rank: Super Contributor
timestamp: April 17, 2022, 11:48:24 pm
content: Quote from: Psi on April 17, 2022, 10:08:00 amPrimary rule: A fuse is intended to prevent against fire, not to cover up circuit malfunction or act as a current limiter.So what you need is a current limiter. For low voltages (up to 5V) there are plenty of chips out there (look for load switches) that have overcurrent and overtemperature protection.For higher voltages you could use a programmable current source like the LT3092.

author: Psi
author_rank: Super Contributor
timestamp: April 18, 2022, 02:18:52 am
content: Quote from: Zero999 on April 17, 2022, 05:16:38 pmThe use case was quite unusual.I wanted it to detect the heat from the component next to it and reduce current if it got too hot.I was in need of a really simple circuit that anyone with some simple instructions could soldered together deadbug on the back of a solar panel in the field to add simple max voltage cutoff to prevent battery overcharge (18V 5W panel and 12V SLA). So I was thinking of just a 5W 14V zener across the panel. However the space available is tiny and burning off 18-14V= 4V @ 350mA as heat in the space available in high ambient heat was going to warp/melt the plastic case.So I was thinking if I put a PTC in the circuit it could just disconnect the panel if the zener started to get hot. Which would only happen once the battery was full. etc..However things have changed since yesterday and i'm now looking at other options. About to start a new thread.Thanks to everyone who responded.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 18, 2022, 02:46:04 am
content: Just put a LM317 in series with the panel output? Or LM1117 or LowerDO if you need every volt. Solar panels don't care if they're being loaded, they can be open-circuited too.Tim

author: Psi
author_rank: Super Contributor
timestamp: April 18, 2022, 04:08:30 am
content: Quote from: T3sl4co1l on April 18, 2022, 02:46:04 amThe issue with a typical linear reg is when the panel is producing full power (18V at 350mA) the reg will burn of the excess as heat and that is too much power for this tiny location and ambient temps.I created a new thread for a prototype circuit to disconnect the panel once the SLA is fullhttps://www.eevblog.com/forum/chat/simple-voltage-cutoff-schematic-for-solar-charger-any-tips/msg4124638/#msg4124638