I want to use a capacitor to slow down the heat dissipation of the mat.
I need to improve the on/off ratio and improve its efficiency.
If possible what type and size capacitor should I use.
Is there a simple way to increase the speed of the heating process.?

 

... Using a capacitor for that purpose would not be a good idea. Try using a second heat mat over the top of the first.

 

... Using a capacitor for that purpose would not be a good idea. Try using a second heat mat over the top of the first.

The heater mat is 1mtrx1mtr I have already cut it in half and placed both halves on either side of a 1.5mm alum sheet.
e.g. The set temperature is 55 deg C and the reset temperature is 54 deg C.
It takes 1.30 mins to rise from 54 deg to 55 deg.
It takes 55 seconds to fall from 55 to 54.
I want to balance the two.

 

Have you considered using phase angle control ? You could build one of buy a ready made light dimmer rated for 500 watts. You can also buy PID temperature controllers (For example on ebay.) that use burst firing to control the heat input. They would work better than simple on/off control.

Edit. Ignore the phase control suggestion. I was thinking that your plan was to put the capacitor (Which would need to be a large one.) in series with the heating element to slow down the heating but after re reading your post I see you want to speed up the heating. You may be able to achieve what you want by switching the power to a lower level during the cooling phase rather than reducing it to zero

Les.

 

Is your objective to have a constant temperature rather than one that is transient? Can you elaborate on what you describe as balanced?

 

You need better insulation for your oven, to prevent heat loss.

 

I am striving to get 50% duty cycle to ensure the most economical use of the mat.
The target temperature is 55 degrees C.
I am controlling the temperature with a digital thermostat.
When the target temperature is reached the power to the mat switches off.
The mat is allowed to cool down to 54 deg C and then the power switches on.

If I used 2 capacitors in series I could double the power to the mat for a few seconds to speed up reheating.
The mat may not accept the excess power and this may also reduce it's life span.

I thought that a storage capacitor would slow down the cooling time and give me my 50% duty cycle.
That would mean the mat would operate like a constantly on 250 watt heater.

 

Typically, capacitors are used for correcting the power factor of a device that has an inductive component. The flat, 2 dimensional design of your heating pad seems to be primarily resistive, and probably has little if any inductive reactance. Consequently, you will not achieve any advantage by using a capacitor. ... Look up power factor correction.
If you want to smooth out thermal transient temperatures, try using a metallic mass ... a thick plate of aluminum or copper might work. Then adjust the heating element pwm on a heuristic basis.

 

Typically, capacitors are used for correcting the power factor of a device that has an inductive component. The flat, 2 dimensional design of your heating pad seems to be primarily resistive, and probably has little if any inductive reactance. Consequently, you will not achieve any advantage by using a capacitor. ... Look up power factor correction.
If you want to smooth out thermal transient temperatures, try using a metallic mass ... a thick plate of aluminum or copper might work. Then adjust the heating element pwm on a heuristic basis.

Thank you. I had in mind a circuit I saw on YouTube. The circuit was first shown without a capacitor and when the power was switched off the result was "instant off". Then with a capacitor in the circuit the "off" was gradual dissipation.

 

Have you considered using phase angle control ? You could build one of buy a ready made light dimmer rated for 500 watts. You can also buy PID temperature controllers (For example on ebay.) that use burst firing to control the heat input. They would work better than simple on/off control.

Edit. Ignore the phase control suggestion. I was thinking that your plan was to put the capacitor (Which would need to be a large one.) in series with the heating element to slow down the heating but after re reading your post I see you want to speed up the heating. You may be able to achieve what you want by switching the power to a lower level during the cooling phase rather than reducing it to zero

Les.

Hi Les, That's something like what I had in mind. I have an idea how to do but what would you suggest.?

 

Possibly the video was describing the behavior of a capacitor in a direct current circuit, such as that supplied by a battery. Your 500 watt heating pad is powered by alternating current, which is of a completely different nature and follows different rules ... the primary one being 'don't touch' because it can be lethal.

 

Re post #10. You say "That's something like what I had in mind." but you do not say which of the suggestions that I made that you are referring to. Can you explain exactly what you mean by "most economical use of the mat." ? If that mean to minimise the total power consumption then you can't change that. From your figures in post #2 you have the heater on for 90 seconds and off for 55 seconds so the average power consumption will be 500 x 90/(90 + 55) = 45000/145 = 310 watts. So if your aim is to have 50% on time then the heater would have to be 620 watts. Assuming the rate of loss of heat does not change the AVERAGE power will always be 310 watts. this can be 310 watts with a 100% duty cycle, 610 watts with a 50% duty cycle, 500 watts with your existing 62% or any other duty cycle. I agree with the others that you do not understand how capacitors behave. Can you tell us exactly what your goal is ? (For example most accurate temperature control or 50 % duty cycle for some reason that you have not explained.)

Les.

 

If you are trying to increase efficiency, you cannot. The efficiency of a resitive heater is 100%.
The reqired duty cycle to achieve a constant temperature is a function of the power of the heater and the heat loss in the system.

Bob

 

Thank you all for your time

 

If you are trying to increase efficiency, you cannot. The efficiency of a resitive heater is 100%.
The reqired duty cycle to achieve a constant temperature is a function of the power of the heater and the heat loss in the system.

Bob

I am sure I could slow the heat dissipation by using a 3mm thick alum plate with the mat and come close to 50% duty cycle.

 

I am sure I could slow the heat dissipation by using a 3mm thick alum plate with the mat and come close to 50% duty cycle.

How would you do that? There are 2 different things in play here. Conversion of electromotive force into heat. The thermodynamics of how the heat generated is used. So you need to learn about both electronics and thermodynamics.

 

You can put a foot thick layer of styrofoam around the whole thing and lower the duty cycle to a few percent. The point is that ypu cannot change the efficiency electrically.

Bob

 

You can put a foot thick layer of styrofoam around the whole thing and lower the duty cycle to a few percent. The point is that ypu cannot change the efficiency electrically.

Bob

Very smart. Thank you.