I am trying to control a small pizza oven using a BTA12-600TW triac.

I can do the microprocessor control but need help with the mains switching side!

The oven is rated at 1,500 Watts which would need the triac to switch 6A.

The optoisolator I’m using is a MOC3163-M zero crossing one.

I have calculated the probable heating power of the triac as ~7Watts.

I am wanting to put the mains part of the control in an ABS box and force air through it with a 30 mm or 40 mm 5V fan.

I can’t quite get my head round how to calculate if this set up would be safe and sufficient!

South

 

3.413 x P = 1.08 CFM dT

 

3.413 x P = 1.08 CFM dT

Hi #12,

OK I'm totally confused!

Do you think you could elaborate a little on your equation?

Kind Regards

South

 

3.413 x 7 watts = 1.08 (cubic feet per minute) x (change in air temperature)
7 watts isn't a lot of heat so a little bit of air should do the job.
22.12 = CFM x dT
You only need 1 CFM to get the air to only heat up by 22.12 degrees F

It's enough to make you think you don't need a fan...just a chunk of aluminum outside the box to radiate heat.

 

3.413 x 7 watts = 1.08 (cubic feet per minute) x (change in air temperature)
7 watts isn't a lot of heat so a little bit of air should do the job.
22.12 = CFM x dT
You only need 1 CFM to get the air to only heat up by 22.12 degrees F

It's enough to make you think you don't need a fan...just a chunk of aluminum outside the box to radiate heat.

Hi #12,

Thank you for the explanation.

I need to get an overview of heating effects and wattages, heatsinks and fans!

I suppose, though that, this comes of experience and familliarity?

I think I'll take a trip to the library and see if they have "Power management for dummies" or something of that ilk?

South

 

Here is a calculator for heatsinks...

http://celsiainc.com/heat-sink-size-calculator/

 

Here is a calculator for heatsinks...

http://celsiainc.com/heat-sink-size-calculator/

Thanks Dave that reference was very useful

I've also been to the library and borrowed a kind of general book on practical power electronics!

Or at least I've borrowed an electronis book which I hope has a readable section on heatsink practicalities?

South

 

I am trying to control a small pizza oven using a BTA12-600TW triac.

I can do the microprocessor control but need help with the mains switching side!

The oven is rated at 1,500 Watts which would need the triac to switch 6A.

The optoisolator I’m using is a MOC3163-M zero crossing one.

I have calculated the probable heating power of the triac as ~7Watts.

I am wanting to put the mains part of the control in an ABS box and force air through it with a 30 mm or 40 mm 5V fan.

I can’t quite get my head round how to calculate if this set up would be safe and sufficient!

South

Hi,

A book wont be quite as good as simply looking at some heat sinks for sale. That connects you immediately with the data as well as the heat sink itself so you get what you need right off the bat. A book may delve into theory which will take time to process, while the heat sink data sheet will have data you need right away. Some data sheets even give curves on thermal resistance with forced air for a range of air flow.

The most important factor is the surface area of the heat sink, not the volume of material as some sources quote. The surface area is the total surface area so more fins means more surface area. This is because the temperature rise is based on an exponential ratio of power to surface area with surface area in the denominator and power in the numerator. Of course the heat sink has to be designed for optimal conduction, but we assume it is good enough even if not optimal based on the specs we find in the data sheet.

Forced air is more old school though because a fan has moving parts and it's a shame to have to take a purely electrical design that has no moving parts and purposely add a moving part just to keep it cool. Yes sometimes there's no other way, but also sometimes the design can be improved so it does not need a fan with a little more thought into the design. So if you can, avoid using a fan whenever possible.

One place were you need a fan is a computer CPU where there is a high power density and the only way to get it out is with a heatsink and fan. Some modern CPU's even require a water cooled heat sink. There's no way out of that until the new technology rolls in. With your design there should be a way to do it without a fan because there are heat sinks that can handle 10 watts well enough to not need a fan. It's only if your design needs to be as small as possible that you might need a fan or there is some air flow restriction that does not allow for proper natural convection cooling.

 

It looks like your 6 amp triac is marginal. At 240 volts 6 amps is only 1440 watts. The heater is likely to require a lot more current when it is cold. I wouldn't use less than a 15 amp triac -- and that might not be enough depending on the cold resistance of the heater.

 

It looks like your 6 amp triac is marginal. At 240 volts 6 amps is only 1440 watts. The heater is likely to require a lot more current when it is cold. I wouldn't use less than a 15 amp triac -- and that might not be enough depending on the cold resistance of the heater.

thanks for your reply

I was actually going to use a 12 Amp triac but after reading this I may well substitute a 16 Amp one?

I did play fast and loose with the value of the power calculation as it is a lot easier to do the sum with 250 than 240!

Definitely my bad!

Holding my hands up to that!

Ok at the moment I'm looking at two contendors for free air cooled heat sink one with a rating of 3.3 degC per Watt and the other 7.2 deg C per watt.

Will the smaller one do the job if I have it pertruding out of the case?

South

 

Ok at the moment I'm looking at two contendors for free air cooled heat sink one with a rating of 3.3 degC per Watt and the other 7.2 deg C per watt.

Will the smaller one do the job if I have it pertruding out of the case?

I don't like the heat sink extending out of the box for safety reasons. If the insulation between the triac and the heat sink fails the heat sink will have 240 volts on it.

Keep the heat sink in the box and use vents to get the proper air flow. This will probably require a larger heat sink because of the restricted air flow.

 

I don't like the heat sink extending out of the box for safety reasons. If the insulation between the triac and the heat sink fails the heat sink will have 240 volts on it.

Keep the heat sink in the box and use vents to get the proper air flow. This will probably require a larger heat sink because of the restricted air flow.

Hi Richardo,

ok that's were I was a while ago but from as different perspective!

I would prefer all to be within an ABS box but, not being experienced with heatsinks, I am unfamilliar with the relative importance of the various perameters, I will need you to bear with me.

For an BTA16600SW triac dissapating ~7-10 Watts, with zero crossing optoisolator, of sufficient rating, and enclosed with vents would the heat sink need to be a large, (100mm x 5mm x 2.5mm), 3.3 deg C per Watt, extruded, finned beast or will the little one, 7.2 deg C per Watt, do the job?

 

I would prefer all to be within an ABS box but, not being experienced with heatsinks, I am unfamilliar with the relative importance of the various perameters, I will need you to bear with me.

For an BTA16600SW triac dissapating ~7-10 Watts, with zero crossing optoisolator, of sufficient rating, and enclosed with vents would the heat sink need to be a large, (100mm x 5mm x 2.5mm), 3.3 deg C per Watt, extruded, finned beast or will the little one, 7.2 deg C per Watt, do the job?

Be warned that ABS gets soft and melts at fairly low temperatures. You should look up these numbers to be safe.

Let's assume 7.2 deg C per watt and 10 watts. That would be a 72 deg C temperature rise in still air. If the air temperature is 40 deg C -- that is, a hot day -- then you will have 112 deg C on the heat sink. The triac will be even hotter because there is a thermal resistance from its case to the heat sink of a deg C or so.

The inside of the box will be hotter than the outside air temperature so the triac will be even hotter.

 

Be warned that ABS gets soft and melts at fairly low temperatures. You should look up these numbers to be safe.

Let's assume 7.2 deg C per watt and 10 watts. That would be a 72 deg C temperature rise in still air. If the air temperature is 40 deg C -- that is, a hot day -- then you will have 112 deg C on the heat sink. The triac will be even hotter because there is a thermal resistance from its case to the heat sink of a deg C or so.

The inside of the box will be hotter than the outside air temperature so the triac will be even hotter.

ok Richardo

then if I use the 3.3 deg C per watt one I have 33 deg C above ambient, in this case within the case , so if ambient is 40 deg C I have at least 73 deg C.

I have it on trust from Wikipedia that ABS is certainly useable from -20 to 80 deg C so that should give a little bit of head room?

I think I'd still like to add a fan as well!

South