The heating pad element has been cut into 12, 16", sections. One inch of plenum removed from each end and ~ 1/2" of insulation was burned off using a lighter. This exposed the wire with was twisted around some sort of a multi-stranded fiber.

I guess I should clean the exposed wire ends with flux? None on hand, so it might be a little while before I can continue.

Ran into a non-sticky problem--'super glue' didn't adhere to either the plastic insole or the wire Twist ties, sewing thread, or fishing line might work to tie them down.

 

I was looking at the commercial toe-insole warmer which is basically a copper trace on some plastic material with what feels like a neoprene backing. Why not simplify the physical design and get make a long trace on similar material or a aluminum cad PCB?

This PCB Trace Resistance Calculator would make the design calculation simple.

Selecting a 5 oz/ft\(^2\) copper backing a 1 mil wide 1 foot long trace would yield a 1.22 \(\Omega\) at 100°F with thermal resistance of 1.71e+5 (0.971 \(\Omega\) @ 0°F and 1.35 \(\Omega\) @ 150°F.) This would be an example for a typical 1 watt heater element using the NiMh D cell for an up to 10hr run time.

Setting the temperature of the heating element might be as simple as deriving its resistance from its power consumption.

Looks like a 'good' electronics project--were do I start?

Selection of PCB prototype manufacturers, design software for Linux, and board materials (flex/rigid.)

I'll need a PWM circuit with feedback from the derived heater element temperature.

Lots of technical details to bang out, like what impact the thermal resistance of the PCB with its 'protective' layer will have (temperature drop) from the heater element to the surface of the insole (I doubt that will actually be a big problem as it's doubtful that the 'user' selected temperature control will be calibrated to an absolute temperature--just a pot to spin.)

Does this idea make more sense than attempting to work with the heating pad wire cable?

[Edit] I'm reinventing the wheel--It's out there, but for higher voltages--time for more searching...
[More] Picture of glove heater element here.

 

Calculated that I need a 1400mm x 5mm trace at 1 oz/ft\(^2\) to yeild a 0.143\(\Omega\) resistance for ~10W using a NiMH ~1.2V D cell consuming ~8.4A and that I can expect a 100°F degree temperature rise from 0°F (assuming a straight trace?).

This ANSI PCB Trace Width Calculator makes me think that I can pass 15A @ 2V with a 100°F temperature rise from 0°F on 1 oz/ft\(^2\) Cu thickness copper clad for 1400mm providing the trace width is at least 4.56mm and I maintain a 'Track Clearance' of 23.4mm (the space in between traces on the open air side?). Basically it looks like the trace won't burn up.

So, that's ~40,000mm\(^2\) surface area--too large for my insole. I figure I have about 10,000mm\(^2\), so I'm expecting a 2.5mm space between traces. Depending on which PCB I end up using it sounds like I'll need a heat sink/spreader and a temperature sensor. I not really sure if there is enough delta resistance to detect the average temperature of the heating element. Thoughts on if that's really possible?

Any horror stories I should know about using a PCB prototype service?

Do you think the 5mm trace width with only a 2.5mm space will be a problem for 'standard' prototypes? Since I don't have my PCB design done yet, I haven't tried any on-line quotes, but one site made me think that 5mm was the smallest spacing that could be used. Strange though, 'cause I thought 0.25mm was a common trace width. Please tell me I mis-read the requirements? Thanks!

An interesting tidbit...the average 'foot' temperature is 83°F.

 

Permission to shoot myself--I found replacement heater elements w/soles for $40! I ordered them. The whole unit sells for over $150 with the battery, charger, and mounting system, but that's not what I need/want.

Anyway, I'll wait for the heating elements to arrive, test their resistance, and start working on my temperature control and battery pack--sometime next week.

Although it would have been fun to attempt to assemble the bulky heater wire and/or design a PCB, this is a more cost effective solution. Till next week--thanks

 

It's not good to make quick decisions in the middle of the night. In my excitement at finding the heater module, I forgot they're most likely not in the voltage and power range I actually desire. I'll still get them anyways since I broke the other two.

Meanwhile I'm installing Spice under Ubuntu Linux, so I can start on the controller circuit and I'm back to mechanically attaching the heater wires to an insole (just one for now until I decide it works.)

 

The 6 pair heater wire design failed to reach the desired power output

The NiMh voltage dropped from 1.285 to below 0.9, the wires were 'luke warm (no temperature measurement--broke my thermocouple), the 10 awg power feeds got warm, and the battery got hot (so did my fingers since I didn't have a battery holder rated at 10 amps.)

Looks like to obtain the desired higher power I'll need to increase the voltage--bye bye single D cell.

 

...replacement heater elements w/soles...

Got'em. They have an IC which prevented a direct resistance measurement, but with both elements under load they appear to be ~4 ohms. Burnt out the right foot IC--looks like three NiMH cells were one too many Disassembled it and was unable to remove enough of the IC to access either end of the elements, so I attempted to examine a cross section (destructive test), but I guess I'd need a microscope. Anyway it's a sturdy assembly--I can see why it's a popular product.

 

Sorry here...doctor giving a non electronics reply. At least half the battle of staying warm is moisture control. Taking things off before you sweat. If that is not possible there is an antiperspirant called DrySol by prescription that works extremely well on feet and hands believe it or not.

 

...there is an antiperspirant called DrySol by prescription that works extremely well on feet and hands believe it or not.

That's very interesting. Do marathon runners use this? Keeping my feet dry, and therefore less prone to blisters, could be really useful in long runs. My feet get soaked even skiing.

 

Over the counter in Canada. Worth asking the doctor at the next physical.