Okay guys, I've been tasked with making a light-dimming control system for a 240V heat lamp dissipating around 250W. I need to adjust the heat output on the lamp by control from a microprocessor.
I'm developing on an 8051-based SoC with RF circuitry and some sensors and actuators. Basically it's a node in a wireless sensor network. I've left all other components out.
I'm mostly a software guy, so I might need some help here. Pardon my lingo and if I raise fundamental questions, I don't have a lot of experience with this.
I am thinking of a design like this for the dimmer circuit:
.. and I have some questions :) I've marked red boxes with numbers matching the questions:
- Is this a sensible way to drive a triac ? The resistor on pin 6 should have a higher power rating. Like 5W or something I would guess.
- The GPIO leg can supply up to 500mA, so it should be plenty to drive the gate for the transistor right?
- And mostly any transistor will do I take it.
I don't know how inductive the heat lamp is, or how its resistance changes with temperature. I want to do an estimate of how the triac should be spec'd.
- 240VAC(rms) would be ~340VAC(peak2peak), so 400V max isolation voltage should suffice, no?
- A max surge-current rating of 4A would leave some overhead I would assume, but I'm not sure how much I need. I am assuming a switch-on from cold could draw a lot of current - should I instrument to be sure? I don't have any data on the heat lamp other than its rated for 250W max.
- Am I understanding correctly that this is the snubber?
I've found a couple of topics that helped me, but I'd appreciate a comment. I read these:
- Trouble with triac driven dimmer circuit
- Sanity-check on snubber design
- Why Snub an Optoisolator?
- Is a protection diode needed on a TRIAC optocoupler and what for?
TL;DR: Am I way off with the schematic? Some help on guestimating the rating of the components would be nice as well :)