double insulated radiator - Page 1
Forum section: electronics / projects

author: KarolNi
author_rank: Newbie
timestamp: June 27, 2015, 10:16:23 am
content: Hi everybody!I'am thinking about building a power controller for various loads - heaters, lights and universal motors. I'am on 230V mains and need to control about 2.2kW electric oven or about 1.8kW vacuum cleaner without build in regulation (not at the same time). It's for personal use so no need for CE or FCC or something, but it will be used by my family. Most of power outlets in my home have no PE connector.I was thinking about phase controlled triac. It will need to dissipate up to about 12W of heat.Do you think that it is safe to use metal, floating (no PE) case as radiator? It will use BTA25 triac (built in isolation) and proper* silpad.*https://www.infineon.com/dgdl/Infineon+-+Application+Note+-+PowerMOSFETs+-+CoolMOS%E2%84%A2+-+Electrical+Safety+and+Isolation+in+high+voltage+Applications.pdf?fileId=db3a30433d1d0bbe013d20e0cbf017fe

author: Ian.M
author_rank: Super Contributor
timestamp: June 27, 2015, 10:55:32 am
content: I've seen failed isolated TO220 devices that have split in half so the answer is probably not. As you don't have a reliable PE, a metal case is a really bad idea. Cant you use an internal heatsink in a plastic fan cooled box?

author: dom0
author_rank: Super Contributor
timestamp: June 27, 2015, 12:56:57 pm
content: Many triacs in TO-220 with metal tabs have insulated dice  to meet appliance class II protection requirements. So, theoretically, they are safe, but I would only trust reputable IC makers there... or rather PE'd the tab.Also, I'd recommend using a complete SSR, you can get high-quality ones as NOS on eBay for next to nothing.For inductive motors (like the vacuum cleaner) you likely need an additional snubber (regardless whether you use a COTS SSR or a triac). I'd generally recommend full/half wave control, because it's efficient, low-noise (phase-cutting on e.g. a heater can really make a lot of noise, acoustically and electrically) and generally just works well. SSRs with built-in ZC lift the requirement of precisely detecting ZC of mains.OTOH full/half wave control doesn't work overly well with motors (very audible "ripple").If you decide for phase-cutting, don't forget a fat inductor in series to limit di/dt. They get huge, heavy, expensive and noisy very quickly...

author: Ian.M
author_rank: Super Contributor
timestamp: June 27, 2015, 01:12:10 pm
content: If anything comes loose from your circuit it could short to the case, It may be possible to avoid that by lining the entire case with Micarta and only using insulating mounting hardware for all internal parts but designing insulation that will be effective after a fault has flash-plated a layer of copper and carbon over the inside of the box is *DIFFICULT*.For safe one-off projects, all external metalwork that can be made contact to from the inside of the case *MUST* be earthed. With no PE, that effectively means an insulating case with no external metalwork that penetrates it.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 28, 2015, 04:50:18 pm
content: I agree with others that anything that can be inadvertently touched has to be protected. I would leave a little room for creative thinking though. If the "case" was in a locked closet or other external case with appropriate warning labels (think of the "No user serviceable parts inside") it might well be adequate. It is your family, and you know how much is good enough for you. If you go this route you should probably remove this system if you ever get old enough to lose control of service or if you sell the property. These last two conditions may convince you that the conventional solutions to the problem are a better way to go.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 28, 2015, 05:14:52 pm
content: Any reason to not use an off the shelf heavy duty dimmer?

author: KarolNi
author_rank: Newbie
timestamp: June 28, 2015, 08:21:22 pm
content: Thanks fellows for replies.I was thinking about using it as actuator driver part of closed loop control system via some RF link (Bluetooth?, for convenience and isolation), so of the shelf systems are rather out of question (maybe some heavy duty DMX512 dimmer, I may look once more).I'll use microcontroler, so I'll do two modes - one half/full/no wave for heaters, and phase-cutting for motor.Back to topic: I'm convinced - I'll use plastic box and reserve place for fan in case it will be needed.