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Power resistors for heating/UL concerns

Power resistors for heating/UL concerns

(OP)
Hi,
So I had an idea recently while working on a client's project that was inspired by a plug in air freshener. (See here). I've worked on some thermal heating projects before and used PTC's, Tube heaters, Cartridge heaters, and foil heaters. The thing about this project is that we only need a few degrees of warming heat. 25 watts would be about right. Heating approximately 6oz of fluid inside an aluminum reservoir. Shooting for about 10-20 deg F over room temperature in 5 minutes or so, and none of those numbers are particularly critical at this point. I thought I might take a page from the SC Johnson playbook and use a nice cheap 4k power resistor but now I'm wondering what UL will think of that. In the freshener application its the only component- goes right to line voltage and just warms the scented oil. I'm all for keeping it simple, but can I get away with that too? Plan B is use of a PTC but its hard to find one with that low of a power rating. Of course it will add to the expense but the self-regulating feature and UL Cert would likely be worth the cost. So if I can find one that tops out at 30-35c I'll probably be happy.
Just wanted to see if anyone else has gone down this route and what their experience has been. The other thing is if it turns out it IS easy to integrate a power resistor, I could use two of different values and then have a cheap/simple Low/Med/High circuit.
Thanks for any help!
-k

RE: Power resistors for heating/UL concerns

Either of those methods would be applicable and the switch scheme is fine too.

p.s. I can't stand any perfumes of any type.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Power resistors for heating/UL concerns

(OP)
Well, Keith- you'll be delighted to hear that my product isn't an air freshener product! :) Still, my question is more regarding UL/Costs. The less I have to deal with UL the better, so I want to make sure this will be as painless as possible if I use a power resistor(s) for heating.

RE: Power resistors for heating/UL concerns

Your PTC or resistor solutions will likely cost the same in the UL arena. Of course you can make your life greatly easier by selecting either of the devices from the field of already listed devices.

If your device can be thermally insulated inadvertently by the user, be they morons, clueless, or even doing it deliberately, and the result would be the device/product reaching ignition or burned-hand temperature then you should probably go with a PTC type solution. Alternatively you can go with a resistor in the 'insulated' case described above and an industry standard thermal fuse which are all UL listed and solve a world of approval issues.

Keith Cress
kcress - http://www.flaminsystems.com

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