Hello all,
I am trying to find an dc to ac relay for running a 6 amp heating element with PWM.
I have found this:
Teledyne STH24D12
http://www.alliedelec.com/Images/Products/Datasheets/BM/TELEDYNE_RELAYS/601-0047.PDF
It is rated for 12 amps. I could not find in the data sheet what the highest amperage drops to without a heat-sink (I am trying to avoid the heat-sink because it costs more that the relay itself).
Do you think that this relay could handle 6 amps without a huge heat sink?
TIA
Well the spec sheet says the device must be kept at 80C or less. At 6amps if may or may not require a heat sink but I can't calculate that for you from the datasheet. Keep in mind that a heat sink does not have to be expensive, a sheet of 1/8" alumina plate of the proper size can be effective if needed.
Lefty
I would not consider this an application for PWM in the sense that most Arduino folks would understand.
Relays will not normally switch faster than about 50 to 100ms. Switching a significant current is something you want to do a few times a minute at most, or else the contacts will go in short order.
You will find lots of examples of how to connect a relay to an Arduino. This is a regular topic and searching "Arduino relay" will find lots of good stuff.
Switching household mains current is not a beginner project, so I would recommend studying what others have done carefully.
I would like to use this relay in a similar case to the PWM control on this page:
http://www.arduino.cc/playground/Main/BarebonesPIDForEspresso
I think calling it "PWM" is misleading as it runs 500x slower than the built-in analogWrite() funtion.
Only possibly misleading in context with the Arduino. There are a kazzion industrial standalone PID temperature controllers used in industry that use a simple (slow) digital output wired to standard (or larger) SSR relays. Temperature process control is a comparatively slow process as the typical thermomass is slow to heat and cool, so having pwm even 500x slower is still faster then the process can change and works very effectively.
As a side note, temperature control loops are among the hardest to tune the PID parameters effectively because the process being controlled is so slow it's very time consuming to see the effects of tuning changes.
Want to talk real slow temperature control? Think how slow the thermostat cycles on and off for a typical home central heating system, which is basically a form of PWM, just slowwwwww pwm. 
Lefty
Want to talk real slow temperature control? Think how slow the thermostat cycles on and off for a typical home central heating system, which is basically a form of PWM, just slowwwwww pwm. Wink
Tell me about it, it took me months to get it somewhere near on my arduino controlled system. I've got it keeping it +/- 0.2C now which I consider exceptional in the light of experience. Uses a combination of best guessing the required temperature of the water in the radiators and room temperature trending. Early efforts usually involved a 1 degree overrun. There's usually at least a 10 minute gap between turning the heating on and the room temperature starting to move and it has to turn it off early on the way up to stop it over-running, how early depends on how cold it is outside.....
Only possibly misleading in context with the Arduino.
Right. But this is an Arduino forum, after all. Readers are entirely likely to interpret "PWM" as meaning "I will connect this to Pin 3 and do an AnalogWrite(3, my_value) to it."
There's usually at least a 10 minute gap between turning the heating on and the room temperature starting to move
That is known as the thermal time constant. The converted mill where I used to work (Salts Mill Saltair - World Heritage site) had a thermal time constant of three days. Therefore temperature control was a little tricky.
Right. But this is an Arduino forum, after all. Readers are entirely likely to interpret "PWM" as meaning "I will connect this to Pin 3 and do an AnalogWrite(3, my_value) to it."
I'm certain you are correct about that. However I'm also sure that some Arduino readers have or want to build a Arduino controlled thermostat for their central heating someday and they will have to come to terms with what pwm control is in theory also, not just as the standard Arduino library implements it. Heck we still deal with beginners overwhelming thinking that hobby servos should utilize Arduino pwm commands rather then ppm control as implemented in the servo library.
There is one thing I really like about this forum: Ask one question and you get expert answers to questions you did not even know you needed to ask. 
Thank you all for your help!
Searching around other manufacturers datasheets, it seems that a heatsink is required for any current more than 25% of the label rating, and is a special necessity for high-speed switching, which (according to IDEC) generates a lot of heat. I have some high-quality aluminium available, so I think I will make my own heatsink and keep tabs on the temp of the SSR to make sure it is sufficient.
I realise now, based on your valuable feedback, that the sample page I quoted is not using the built-in Arduino PWM feature but is rather implementing its own switching. Although it is not apparent to me where in the code the 60Hz (as mentioned in the code's comments) switching is set.
I need this heat control for a 12x12x12-inch heat cabinet that needs to maintain a temperature between 175 and 179°F. I think my "thermal time constant" will be a few seconds at most.
The thermal time constant may be a bit longer than a few seconds depending on many factors including the thermal mass,
About 10oz. holt melt glue and several pounds of steel.
how powerful (and efficient) the heating element is,
750w finned heat strip, 6" x 2".
and how well the box is insulated from the ambient
R14-R20 foil backed foam board.
(as well as how much different is the ambient temp?)
50°F in Winter and up to 100°F in Summer.
If the switching only needs to be once every 1 or 2 seconds, is there any reason not to use a mechanical relay?