OK, I need to supply 1.5W of power through a resistive heating element for 30 seconds, then have it auto shutoff. I need a battery, a fuse, and whatever amount of nichrome wire it takes for the right resistance. I've never used PPTC fuses before, so I'm not sure if this is feasible. Also, this must be VERY compact (1.25" x 1.25").

I've got a 3.7V lithium polymer on hand. Here are the specs:

Kokam High Discharge LiPo Battery
Part Number: KOK145T
Kokam 145 mAh High Discharge (5C) Lithium Polymer Single Cell with a Bare PCB Attached.
Size: 27.5mmH x 20.5mmW x 4.5mmT
Weight: 3.5 grams
Ratings: 5C Max. Cont. Discharge
Outputs: 3.7V Nominal, 145 mAh

At 3.7V I need 9.1ohms @ 400mA to get 1.5W power dissipation.

So basic question, is there a PPTC/Polyswitch solution for this? I don't know how sharp the knee in the curve is for them when they cutoff. Also, this will be in close proximity to the heater, which could (and hopefully will) reach 90 degree C.

Actually, it would be real nice to put this right next to the heating element, so it would kick back on as soon as it fell below some threshold value (70 or 80 degree C say) and then have it heat back up to 90 degree.

I just don't know if a polyswitch is a feasible part to use for this. It would make this a lot simpler and cheaper than using a PIC12 and a FET and a thermistor and a switching regulator for the PIC and all that, which would kill it as far as the space I have to cram this in.

Thanks for any help!

 

hi,
You can buy ready made bi-metal temperature control switches for this tempr range, 70C thru 90C.

How accurate do you want the temp control to be?

I can't see why you are considering a self resetting fuse link???

For ref:
http://www.physics.ubc.ca/~scho/P309/pptc.pdf

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Temp switches are a little pricey, at least the ones I see at digikey are. What's a typical price for one just in case my search criteria is sucky?

 

Those thermal switches sure look nice though. Hmm. If I weren't building a bunch of these that would be an easy pick. They're like $6ea at digikey.

 

hi,
If you want to build yourself, you could consider using a thermistor, with say a schmitt and relay?
But you will require a psu to drive it, by the time you have finished I would expect the price to exceed $6

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Mkay, after considering all my options it looks like I'm going with a PIC12F w/ internal osc, a very cheap FET or BJT for a switch, and a thermistor into an ADC on the pic. Nothing like the brute force approach eh?

 

Hi,
Welcome to dark side, hope it works out, post your project, others maybe interested.
Regards.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Given the use of 0402's, an SOT23, and an SOIC8, I dunno how many people will want to solder that up I whipped up the schematic and board layout today. Managed 1.20x1.25" single sided including the battery. Pretty tight layout. I might get around to building it next week. Dunno when I'll have the coding done.