Heated 35x50 mm surface.

[Matienzo]

Hi there,

I'm trying to create an electric heater for an insect repellent mat.

The only constraint is that I must use a 18650 battery.
The mats are about 35x50 mm and they need to be heated at about 130F.

In the chat, Mike_2545 suggested a power resistor like this one

and also pointed out these foot warmers

What approach do you think will give me more operation time? How would you go about making a resistance like the ones on the foot warmer?
Thank you for any guidance. I'll be closely following this thread today and tomorrow and I'll try to answer ASAP to any question.

Paulo

 

[Pommie]

According to this calculator, you will need about 20W to heat your plate. A 0.65 Ohm resistor will provide that on 3.6V. However, your battery will only last about 15 mins.

Mike.

 

[Matienzo]

According to this calculator..."

Hi Mike, I changed 2 parameters, Length and Area to 35mm and 17.15cm^2 and it seems I need about 3W. Is this correct or I'm missing something?

 

[Pommie]

Looks like you are correct. I would insulate the underneath very well.

Mike.
Edit, hit wrong button on calculator.

 

[spec]

Hi there,

I'm trying to create an electric heater for an insect repellent mat.

The only constraint is that I must use a 18650 battery.
The mats are about 35x50 mm and they need to be heated at about 130F.

In the chat, Mike_2545 suggested a power resistor like this one

and also pointed out these foot warmers

What approach do you think will give me more operation time? How would you go about making a resistance like the ones on the foot warmer?
Thank you for any guidance. I'll be closely following this thread today and tomorrow and I'll try to answer ASAP to any question.

Hi Matienzo,

Just a bit of information: 130 Deg F = 54.4 Deg C

To give you a reasonably accurate answer we would need to know what the ambient temperature is, and how much air flow (wind) is expected across the surface of the mat, or are the mats to be used in still air (in a room).

Assuming that the back and sides of the mat are thermally insulated, around 10% of the heat would be lost by radiation and the remainder would be lost by thermal convection. To minimize thermal radiation a white smooth surface would be best.

One approach you can use is to have the surface of your mats insulated, but with a grid of holes exposing the thermally conductive material underneath. Of course, the dimensions of the matrix of holes would have to cater for the smallest insect that you needed to repel.

At a guess you may be able to make your mats from copper-clad fibre glass with tracks etched to make up the heating element that will be required. The grid of exposed holes could then be formed in a solder resist coating. You could even have the exposed holes gold plated and your logo written on the solder resist. The whole thing could be designed in an ECAD like EAGLE (free).

spec

 

[Les Jones]

Just out of interest I measured the resistance of a mains voltage version of one of these devices. The resistance is 9.44K which is 6.1 watts at 240 volts or 5.1 watts at 220 volts. the heating plate is 22mm x 36mm The same size tablets for these devices are sold in Greece and Turkey.

Les.

 

[spec]

Just out of interest I measured the resistance of a mains voltage version of one of these devices. The resistance is 9.44K which is 6.1 watts at 240 volts or 5.1 watts at 220 volts. the heating plate is 22mm x 36mm The same size tablets for these devices are sold in Greese and Turkey.

Practical as ever Les.

spec

 

[Matienzo]

these devices are sold in Greece and Turkey

In Argentina are also very popular and I grew up with one or two of these at home. Based on what you said is it safe to assume that I should be able to do the same with a ≈6 W system? Thank you for taking the measurement.

Hi Matienzo,

Just a bit of information: 130 Deg F = 54.4 Deg C

To give you a reasonably accurate answer we would need to know what the ambient temperature is, and how much air flow (wind) is expected across the surface of the mat, or are the mats to be used in still air (in a room).

Assuming that the back and sides of the mat are thermally insulated, around 10% of the heat would be lost by radiation and the remainder would be lost by thermal convection. To minimize thermal radiation a white smooth surface would be best.

One approach you can use is to have the surface of your mats insulated, but with a grid of holes exposing the thermally conductive material underneath. Of course, the dimensions of the matrix of holes would have to cater for the smallest insect that you needed to repel.

At a guess you may be able to make your mats from copper-clad fibre glass with tracks etched to make up the heating element that will be required. The grid of exposed holes could then be formed in a solder resist coating. You could even have the exposed holes gold plated and your logo written on the solder resist. The whole thing could be designed in an ECAD like EAGLE (free).

spec

Hi spec, glad you chime in in another of my projects.

• Room temperature is about 65F
• Wind is minimal because it is enclosed and the vapor is channeled out.

I saw a lot of heating elements and from the pictures I couldn't figure what they where made of. I like the idea of the heating element. How do I know the thickness and length of the tracks? I assume there is an Ohm/mm^2 value I could find.

I would insulate on the bottom and put the white smooth surface on the top. What do you think is a good material for this surface. Ceramic?

Out of curiosity, why a white smooth surface?

 

[spec]

Hi spec, glad you chime in in another of my projects.

Very kind of you Matienzo.

• Room temperature is about 65F
• Wind is minimal because it is enclosed and the vapor is channeled out.

• 65F = 18.3 Deg C. The temperature difference between the mat surface temperature is thus, 54.4 - 18.3 = 36.1 Deg C. The mat heat loss is highly dependent on this temperature difference
  
• Minimal wind means lower heat loss which means less power required to maintain the required surface temperature.

I saw a lot of heating elements and from the pictures I couldn't figure what they where made of. I like the idea of the heating element. How do I know the thickness and length of the tracks? I assume there is an Ohm/mm^2 value I could find.

Yes, there are standard tables for working out the track dimensions. There are some PCB experts on ETO. Perhaps one of them can point you in the right direction.

The important thing is to make the heating element reliable.

I would insulate on the bottom and put the white smooth surface on the top.

Sounds good.

What do you think is a good material for this surface. Ceramic?

Ceramic should be hard-wearing, easy to clean, and have a pleasing appearance. But make sure it is a thermally conductive ceramic and as thin as possible. There may be an issue with cracking due to differential expansion between the printed circuit and the ceramic. This can be investigated and perhaps a PCB manufacturer can advise.

Out of curiosity, why a white smooth surface?

A white material has minimum radiation, but this is not significant.

A smooth surface has a minimum surface area and thus minimum radiation (not significant) and minimum convection heat loss.

As Les has found that similarly sized mats use 6W, I suggest you go with that power.

This will mean that a good quality, 3AH, 18650 LiIon battery (battery voltage = 3.6V) will last for, (3.6V * 3AH)/6 = 1.8 hours.

The current would be 6W/3.6V = 1.67A.

The resistance would be 3.6V/1.67A = 2.16 Ohms.

spec

 

[Matienzo]

Spec, I followed your advice and etched a heating element and it works like a charm!! (pictures soon). The only thing is that I'm not sure how much resistance the HE has. Still need to find some of those reference tables to design it properly.

 

[spec]

Spec, I followed your advice and etched a heating element and it works like a charm!! (pictures soon). The only thing is that I'm not sure how much resistance the HE has. Still need to find some of those reference tables to design it properly.

Good news Matienzo- can't you just measure the current or resistance?

Below is a link to a PCB trace resistance calculator which should be a help with your mat design. I would be inclined to use as thick a copper cladding as you can get and go for as long a trace as you can practically fit on the mat. That means that the traces will be bigger and thus stronger and will give a long life.

Of course, best use high quality laminate, one again, for long life.

One last bit of lecturing, if these mats are to come into contact with food or drink of any kind, make dead sure there are no heath hazards with any materials you use.

spec

LINK
https://circuitcalculator.com/wordpress/2006/01/24/trace-resistance-calculator/

 

[Matienzo]

Hi spec, thanks a lot for that calculator, I'll start working with it.

Here are two of the heating elements, they where done before any calculation considering the resistance of the surface. Straight one measures about 0.3Ω and the rounded one 0.8Ω. Rounded one gives me around 25 min in the best case scenario and too much heat. I'm working on a new one with thicker lines now.

 

[kubeek]

Why thicker lines? If you want lower power, thus lower resistance, you need longer or thinner traces.

 

[spec]

Hi spec, thanks a lot for that calculator, I'll start working with it.

Here are two of the heating elements, they where done before any calculation considering the resistance of the surface. Straight one measures about 0.3Ω and the rounded one 0.8Ω. Rounded one gives me around 25 min in the best case scenario and too much heat. I'm working on a new one with thicker lines now.

Hi Matienzo,

looking good.

spec

 

[Les Jones]

You could try etching the item that has a resistance of 0.8 ohms to reduce the thickness of the copper. You would need to reduce the thickness to about one third so as to increase the resistance by 3 times (0.8 x 3 = 2.4 ohms) In practice you would only need need a bit more than a third of the thickness left as the etching would reduce the width of the tracks. I would suggest putting some etch resist over the end of the track where the connections will be soldered. It will take some trial and error of the etching time to reduce the thickness by the right amount.

Les.