I have an electric heater that I would like to control with the following SCR power regulator:
http://www.maxwell-fa.com/products.asp?Id=1174
There are several options for controlling its output:
Voltage input signal: 0-5VDC,0-10VDC,1-5VDC,2-10VDC
Current input signal: 4-20mA,0-20mA,0-10mA
Can you suggest which one of those controlling signals is easiest to implement with Particle and how to do it.
Well, the Photon has two DAC pins. You can set those programmatically to 0-3.3VDC. If you really needed to get all of the way up to 5V, you can build simple amplifier.
You probably DON’T want to use current to control it.
The core and photon both have 3.3 volt outputs. That voltage, should trigger a 0 - 5 volt input well.
Are you wanting the core to turn the heater on and off, or do you want the core to tell the SCR how much to turn the heater on and off?
Are we to assume you have a thermostat attached to the core, so you can determine when to turn the heater on and off?
I would like to control SCR power from 0 to 100%.
Yes, I will use a thermostat.
@besimple, the voltage control approach is the most compatible with the Photon. Given that the DAC output is 0-3.3v, you will need a (buffering) amplifier with gain to get the 0-5vdc output you need. An non-inverting op-amp powered via the 5v rail with a gain of 1.5 will do the job and also provide isolation between the Photon and the SCR controller.
With 4095 steps and a gain of 1.5, each DAC bit will produce 1.2mv to the SCR giving you more than enough control. 
Thanx for the info.
Do you have a drawing for the circuit? Or if you can at leaste suggest a op-amp model.
@besimple, here is a quick tutorial on non-inverting amplifiers:
http://www.electronics-tutorials.ws/opamp/opamp_3.html
The gain is easily set with two resistors where R(F) is half of R(2) for a total gain of 1.5.
An LM358 is a great op-amp to work with and can be powered with 5V.