Layout heating element

[ewinek]

Hi all,

Did you ever created a resistor via traces in the layout design? I need to create a heating element (100 mW) and I wonder if this is doable.

Thanks!

[Circlotron]

What voltage?
What current?
What area?

[Gyro]

It's certainly doable, there are several 3D printers that use this method for their heated beds. 100mW dissipation seems like a very low bar, which will only be achievable with very low voltage and current.

[ajb]

If you know the required power dissipation, divide the square of the voltage you'd like to operate it at by the power, and that will tell you the required resistance for the heater.  Then you can play with a PCB trace resistance calculator to figure out what length/width of track will be required -- you might want to adjust your design up or down in resistance from there to account for tolerances, depending on whether it's more important to hit a minimum heating power or keep below a maximum power consumption.  Then it's a question of whether you can fit that much length of track in the required area. 

100mW is going to be tricky, though.  Even operating at 5V, you might find that the required trace length is impractically long (roughly 80 meters at 6mil & 1oz ).  You could run at a much lower voltage/higher current using a constant-current buck converter, but.....might be easier to find another method. 

If you just want the form factor of a PCB, you can buy film heaters in various sizes/shapes/materials, and I'm sure custom solutions are possible if you have the volume.

[KerimF]

You likely have a good reason/application to create a 100mW heating track.
This implies you know in advance the dimensions of this heating element (that let it achieve its function properly).
By knowing the limit of its dimensions/shape and the available current to heat it, it will help to figure out the possible layouts for it.

[ejeffrey]

Hi all,

Did you ever created a resistor via traces in the layout design? I need to create a heating element (100 mW) and I wonder if this is doable.

Thanks!

100 mW is very low power.  A single small SMD resistor can dissipate that much.  Unless you specifically need the heater to be a trace, that's going to be a lot easier to do.

You can use any of the online resistance calculators.  For instance, a 6 mil / 150 micron trace of 1/2 oz copper that is 2 meters long (and presumably spiraled or meanders back and forth) will have a resistance of about 13 ohms.  One volt applied across that trace will dissipate a little under 100 mW.  However, slight manufacturing variations in the trace width will change the resistnace significantly.

[ebastler]

Assuming that you have a microcontroller in your design anyway, consider using a PWM output. That lets you adjust the heating power in software, so you can cope with tolerances of the resistance.

[ewinek]

Thanks all for the contributions.

Maybe I should explain more in detail the purpose of the heater. I need to measure capacitance of a plastic container and heat it at the same time. Therefore I cannot place a resistor directly because the container is flat and it wouldn't make contact with the tracks that are used to measure the capacitance of it, or I would have an additional dielectric (air in between) which I don't want to have...

So I see the film heater might be a good option but I still see them too big. Regarding the traces in the PCB there is no way to do it since I needed in a sub 10 mm space in the PCB.

Maybe I can make two PCBs and stack them. The one on top with a slot and the one in the bottom with the SMD resistor going through the slot to have at the end a flat surface with the heater element in it.

[eutectique]

Therefore I cannot place a resistor directly because the container is flat and it wouldn't make contact with the tracks that are used to measure the capacitance of it, or I would have an additional dielectric (air in between) which I don't want to have...

Place resistor(s) on the other side of PCB.

[ebastler]

Regarding the traces in the PCB there is no way to do it since I needed in a sub 10 mm space in the PCB.

If you use PWM, you can also route a heater trace with a much lower resistance (i.e. higher heating power than needed), then drive it with a low duty cycle.

[ewinek]

Therefore I cannot place a resistor directly because the container is flat and it wouldn't make contact with the tracks that are used to measure the capacitance of it, or I would have an additional dielectric (air in between) which I don't want to have...

Place resistor(s) on the other side of PCB.

I guess in that way the heat will spread over a much bigger area than needed.

[ewinek]

Regarding the traces in the PCB there is no way to do it since I needed in a sub 10 mm space in the PCB.

If you use PWM, you can also route a heater trace with a much lower resistance (i.e. higher heating power than needed), then drive it with a low duty cycle.

I will give it a try and share results, thanks

[ejeffrey]

Therefore I cannot place a resistor directly because the container is flat and it wouldn't make contact with the tracks that are used to measure the capacitance of it, or I would have an additional dielectric (air in between) which I don't want to have...

Place resistor(s) on the other side of PCB.

I guess in that way the heat will spread over a much bigger area than needed.

You can control how the heat spreads with copper polygons and insulators.  Having a 10mm copper patch on the top connected with vias to a resistor on the bottom side and it will probably do what you want.

PWM with a higher current source should also work.

[Siwastaja]

With low enough voltage available, like 3.3V for the MCU, even lower would be even better, it's certainly doable with PWM.

You don't want extremely short and large current pulses at near-zero duty cycle, and you can't do arbitrarily thin traces (0.12mm or so is minimum at cheap fabs, and there is quite some variation in realized trace width), therefore get voltage down as small as possible first to get the power within right order of magnitude (or maybe max 1 order of magnitude too much), then PWM it down to suitable level.

[Randy222]

Are the containers footprint about same size as the heater?

How much temp rise is needed on the container? This is an engineering factor that is a must have.
100mW is gonna sink out everywhere, so it's questionable as to how much temp rise you'll get on the container, or at least the sink profile the plastic has. There's gonna be a temp gradient as the bottom heats while the rest sinks it out, the further away from heat source the cooler it's gonna be.

Sub 10mm space? What does that mean? Are you trying to make a 100mW heater in a 10x10mm area on PCB? 13x13 is 0.5x0.5in, very small area. Not sure how you could run enough trace on such small PCB. You might be better off laminating such small area with a reistance fluid that drys hard (or the like), then you apply a voltage across that pad to create heat.