the way of calculating the value of resistor

Can anyone help please?

Hi

I need to design a heater(resistor) for heating a plastic surface from 25C to 37C

How to calculate the value of resistor when I know the the following parameters:

1) Time to heat surface 45 sec ( temperature should reach 37C)

2) Voltage apply to heater is 3.3V DC

3) Plastic surface size is 1.5" by 1.5 " and 1mm thick.(ABS)

I need to know the heater power rated in W and max current that will flow into it

Thanks

Bobi

Re: Can anyone help please?

Hey i can't suggest you how to create it technically but i can suggest you a book on Electricity which can help you to resolve your problem.

Re: Heater assembly - Can anyone help please?

This is a complex problem to calculate. You must know the thermal resistance (°C/W) of the heater assembly to the environment, to calculate the required power. You must also know the thermal capacity of the heater assembly to calculate the heat-up time.

I suggest you build a breadboard and do some experiments. This is probably the quickest way. I guess <0.5 W is required.

Can anyone help please?

at first you must use M*C*delta(teta)=Q TO FIND energy
M is mass of plastic
C is spicial parametr for each material that u can find it in google or wikipedia
teta is your temperature
Q is your needed energy

now u can use Q=P*T=(V^2/R)T
v is voltage
r is resistor value
and T is time

your parameter and must be metric

be successful

Re: Can anyone help please?

In that case, heat formula Q = mC(dt) is our main equation to be used.

where Q = heat energy, also expressed as Power*time
m = mass
C = specific heat value of the material to be heated
dt = change of temperature

from the standard values, C for plastics varies with respect to their nature of material:

C = 1.0 kJ/kg-K (light plastics)
C = 0.9 kJ/kg-K (sand plastics)
C = 1.3 kJ/kg-K (foam plastics)
C = 1.67 kJ/kg-K (solid plastics)

In your problem, let's say it is a solid plastic... So,

Power*time=mC(dt)

but m = density*Volume
NOTE: (density = 850 - 1400 kg/m^3) for polypropylene, PETE/PVC

Let's say we use 850 kg/m^3 as our density. Therefore,

Volume (computed based on data)= 2.177415x10^-6 cu. meter

Power*(45 sec) = (850 kg/m^3)(2.177415 x 10^-6 m^3)(1.67 kJ/kg-K)(12+273)K

Power = 19.57532375 watts

Let's say Power = 19.58 watts

Therefore, Resistance = (3.3^2)/19.58

Resistance = 0.5562 ohm (ANSWER)

Imax = 19.58/3.3

Imax = 5.933 Amps (ANSWER)

As an engineer, you'll be the one to decide what rating to be used based on the theoretical values obtained...

Good luck!!!

Added after 2 minutes:

Please be guided to the values I used as constants for specific heat of plastic, and its density...

Just alter them based on your values used...

I just showed the solution flow..