Installing heating elements into ski boots

This is how I install the heating element in the ski boots.

 

 

 

This is the Cozy Battery Powered Foot Warmer. It's available on EBay for about $13 a pair. I got 5 pairs for about $70. The product was designed to operate with 2 AA batteries but the heat is very weak and Alkaline batteries will fail in cold weather. Rechargeable NiMH AA batteries does better in cold, but with 2.4volt, the heat is weak and last less then 2 hours. I tested them in my freezer.

 

One cell rechargeable lithium battery provides 3.7v and provide a very good and comfortable heat. Without socks, it does get too hot after a minute. But with the heat troller to regulate the heat, it would be perfect. 

This is the liner for the ski boots.

The insole was removed.

The wire on the foot warmer was cut to length.

A hole was made on the insole to run the wire to the bottom of the insole.

The self adhesive on the back of the Cozy Foot Warmer

On the bottom of the insole, a electrical tape was used to secure the wire.

Strapping tape is better because stronger adhesive, but I ran out of those.

A hole was made on the back of the boot liner to run the wire out.

Now the insole was placed back into the foot liner with the wire running out thru the hole made on the boot liner

A JST plug soldered onto the wire. You can use any type of plug you choose.

I soldered a small LED with 300ohm resister near the plug.

The wire should be secured on the back of the liner with strapping tape.

 

The liner was placed back in the ski boot.

With battery connected, the LED lit up.

The lithium cell I used is 18650 size lithium ion battery with protection. I ordered a bunch of them online from China. They are rated about 2300mAh.

Under voltage protection is a must, if the lithium cell drain to 0 volt, the cell is permanently damaged.

This size of the lithium cell is commonly used in portable cylindrical USB charger or bicycle headlamp. Portable USB charger has a voltage regulator to step up to 5volt.

I don't need 5 volt, plus it's inefficient to run the 1-2 amp of current thru a voltage regulator.

 

 

The battery holder is actually a small flashlight holder. They are pretty cheap ordering online from China.

Details of the circuit

For those who like to know more detail, the 555 timer is operating in Astable mode.

This is the typical circuit for 555 timer operating in astable mode. This means the output will be turning on and off. When it's on, it will supply the same voltage as the supply voltage to power the 555 timer chip.

Duration of output on (sec) = 0.693* (R1 +R2) * C
Duration of output off (sec) =0.693* R2 * C
Frequency = 1.44/ ((R1+R2+R2) * C)
Duty Cycle = Duration of output on / (Duration of output on + Duration of output off)

The most important thing in my application is the duty cycle. As the duty cycle increases, my heater will be hotter.  As you can see, the capacity value get cancelled out when calculating duty cycle. so the size of the capacity will not affect the duty cycle, only the frequency.

One has to vary the resistance of both R1 and R2 to get the widest range of duty cycle, from 0 to 100%. and the easiest way to do this is replacing both R1 and R2 with a single variable resistor.

Heat Troller Circuit

 

Ok, I'm not an engineer and have a limited knowledge of electronic circuit diagram. I draw the 555 timer as it is with all the pins in its physical location. Same as the MOSFET picture. This help me wiring up the circuit board.

 

You can use 50k ohm variable resistor in place of 100k ohm variable resistor. It would just affect the frequency of on and off.

 

 

I found it works without the 0.1uF capacitor on the diagram. Most of the 555 circuit I see on the websites have this capacitor in place. I have some laying around, so I put it on. Don't know why it's required.

 

 

The other polar capacitor connect pin 1 and 2. make sure the negative end connect to pin 1.

 

power is supply to pin 8 and ground to pin 1 

 

Heat troller Parts

Inside the heat troller

 

Finished product

 

MOSFET to act as a gate to switch the power to heating element on and off.

There are many choices, as long as the current rating and voltage rating are high enough.

My source is Sayal Electronic store. sayal.com. They are not the cheapest but is more convenience for me.

 

 

One capacitor required. The Farad rating will affect the frequency of the 555 timer.

I use 47uF.

 

 

This is a typical 555 timer from Sayal Electronics.

You will also need a variable resistor. I use a 5k ohm variable resistor.

Heat Troller

Ok, I have a purpose for blogging now. I'll share my design of heat troller to anyone who's interested in building one from scratch.
The project started as a way to keep my wife's feet warm while skiing. I have thought of this few years back when snowmobiling and making something to keep the rider warm using electrical heat powered by snowmobile's 12v battery. After spending hundreds of dollar buying heating clothes and socks and heat troller, the whole setup is still tie to the snowmobile's 12v battery.
In order for a portable setup, I need to use lithium battery to power the heated clothes and a way to adjust the temperature to prolong the battery life.
The biggest limitation is that average heated socks draw about 4-5amp of current at 12 volt. This will drain a portable battery very fast.
In order to save battery life and also have ability to control the temperature, I need a control to regulate the current. The most efficient way is to turn the power on and off frequently.
Changing voltage with a voltage regulator or reduce the current with resistors will waste a lot of battery power. The most common way of switching power on and off is using a 555 timer chip to open and close the gate on a transistor or MOSFET.
Having a portable 12 volt battery is also not convenient. Each lithium cell is 3.7volt. This will require a 3 cell lithium battery pack.  Ideally one cell is the best. More cells means that there's a higher potential that the cells will be out of balance and shorten the battery pack life. Almost all the cellphones run on one cell.
The draw back is that a lot of power transistor and MOSFET requiring higher voltage to operate better, because most are designed for the automobile industry. Smaller transistors can not tolerate higher current required in the heat troller.
My initial design is based on 2 cell lithium pack which is 7.4 volt. I'll get to 3.7volt design later after I found a low gate voltage MOSFET.




... to be continue...