PTC heater longevity - Page 1
Forum section: electronics / projects

author: jz79
author_rank: Contributor
timestamp: December 19, 2024, 12:00:41 pm
content: first - this isn't an AD for particular heater, I'm simply looking for real world, long term usage experienceparticular heaters look like this, that aluminum casing measures around 50mm width and ~75mm in lengthThese are 80-120W, 220V, decribed as max 180C, a pair of these in parallel are heating a block of aluminum with a mass of about 0.5kg to 95 degrees C and keep it there for 3-4 hours, daily use, initial power draw doesn't exceed 240W, once reached temperature PID is taking over and pulsing the heater to maintain stable temperature.The original heater used in the device was copper track pcb, with ~190ohm resistance, but they degrade with time and fail, manufacturer doesn't provide a replacement part, so I opted to use couple of these PTC heaters, using M4 screws and thermal paste to mount them to the aluminum block, there is an overheating switch there for safety, and the power draw with those 2 PTC heaters is a bit less than it was with original pcb heater, control circuit is using a thyristor to apply mains power to the heater, everything is grounded etc.My concern was about what sort of longevity can be expected from these PTC heaters, and what are common signs of them starting to fail, I can add some additional monitoring to the device, like current measurement etc to log how they perform over time, just looking for some real world feedback about these.

author: psydaddy
author_rank: Contributor
timestamp: December 19, 2024, 11:57:59 pm
content: You can monitor the current and add a fuse to avoid short circuiting when they fail.Ceramic Infrared Heaters might be a good solution for you?

author: thm_w
author_rank: Super Contributor
timestamp: December 20, 2024, 01:25:43 am
content: I've never seen one fail, only seen the control circuitry fail (bi-metal thermostat). If its from a quality brand I would expect 10+ years of use.If its not a quality brand, no idea.Why did the copper track PCB heaters degrade? Was it over the temperature of FR4 and caused delamination etc, or something else.

author: jz79
author_rank: Contributor
timestamp: December 20, 2024, 03:49:37 pm
content: PCB heaters on all of these units (we have probably 20 of them in total) always fail where track goes to via, just plain oxidation is my guess, and if repaired at that failure point, couple months later it will burn in another spot, 5+ years and they start to fail, just the usual "modern design" practice to make something like that and then not offer spare partsthere isn't much space to put something else in there, thickness needs to be below 10mm, this ceramic element was the perfect fit, and there is a fuse of course, I was just wondering if there are any particular signs when/how these start to fail, maybe I'll put a current sensor in one of them with esp32 to log the data, see how they behave over time

author: Siwastaja
author_rank: Super Contributor
timestamp: December 20, 2024, 05:00:36 pm
content: Quote from: jz79 on December 20, 2024, 03:49:37 pmProbably too much current density in vias and localized overheating. Plated metal in drill wall can be pretty thin. Especially if it's done by a low cost PCB manufacturer there could be quality variation. High power traces requirea lotof paralleled vias.

author: CosteC
author_rank: Regular Contributor
timestamp: December 20, 2024, 06:14:58 pm
content: In one on 'my' products other PTC heater design is used for over a decade. It has more aluminium for convection heating. No issues I know in thousands pieces.If you are concerned about your:- Cool it and apply maximum voltage. Thermal camera may show hotspots, but as it is self-regulating it shall be particularly resistant to overloads.- Check uniformity of heating. The more uniform, the more stable it will be.- Asses cables quality and try not to mechanically load them in final application.In any case, it has far higher maximum working temperature than PCB, where degradation is fast at 180 degrees, at least for "normal PCB material" like FR4.Fuse is always a good idea.

author: thm_w
author_rank: Super Contributor
timestamp: December 20, 2024, 11:14:34 pm
content: Quote from: jz79 on December 20, 2024, 03:49:37 pmIf you're going to be repairing a lot of them, it could make some sense to redesign the PCB heater to be more robust. Use a high Tg board and lots of capped vias. But just 20pc might not be worth it.QuoteHere they noted cracking and increased resistance:https://koreascience.or.kr/article/JAKO201501255363750.pdfFrom cars seems to be a mix ofopen circuitor short circuit.Some PTCs are fail safe design, though probably not the common ones.The problem with monitoring current is it jumps around with ambient temperature, so maybe you'd just look at max current seen (assuming these devices hard switch on the heater).You could also contact the manufacturer and ask what they claim.

author: wraper
author_rank: Unknown
timestamp: December 20, 2024, 11:18:44 pm
content: Quote from: thm_w on December 20, 2024, 11:14:34 pmIf you're going to design your own, IMHO Aluminum PCB would be more appropriate. It's the most popular solution for 3D printer heated bed, and works just fine.

author: thm_w
author_rank: Super Contributor
timestamp: December 20, 2024, 11:25:55 pm
content: Yeah use aluminum PCB, forgot about that.They still have FR4 in them, so you don't want to get anywhere near ~130C if possible, unless you verify its higher grade.https://www.eevblog.com/forum/chat/aluminum-pcb-safe-temperature-range-without-risk-of-delamination/

author: wraper
author_rank: Unknown
timestamp: December 20, 2024, 11:26:48 pm
content: Quote from: jz79 on December 20, 2024, 03:49:37 pmYou need some heavy via stitching if you're going to jump layers. Don't forget that vias have much thinner copper than traces and will be heating within already hot PCB on top of that.

author: jz79
author_rank: Contributor
timestamp: December 21, 2024, 12:40:12 pm
content: I'm not looking for any pcb type heater for this, IMHO it is a terrible design choice considering the temperatures involved and mains power, I'm also not considering any sort of major redesign or production, just maintaining the units we have in working condition

author: Siwastaja
author_rank: Super Contributor
timestamp: December 21, 2024, 01:23:34 pm
content: Quote from: jz79 on December 21, 2024, 12:40:12 pmYeah, FR4 PCB copper track heaters work when designed for highest hotspot temperature of maybe 140-150degC. Which means average track temperature little over 100degC. Which, after thermal coupling to the heated system, means you ca use it to heat things to maybe 70-80degC continuously and any more is pushing it.