Phase angle fired (PAP) AC is used in the industry. If operating into a transformer, it must have current limiting.
I used variable DC (1.5 KW) for a low voltage tantalum heating element system. It limited dv/dt or the change the voltage could make over time (5 sec) amoung other things and we got EXTREMELY long heater life compared to PAP. I did use PWM on a car fan motor and it made all sorts of high frequency noises.
The applications were not for toaster oven or PC board making. Bang-Bang (on/off) control is bad. Causes car set heaters, for instance, to break to soon.
--- On Mon, 5/18/09, Daniel Howard Bryant <coloradobryant@...> wrote:
From: Daniel Howard Bryant <coloradobryant@...>
Subject: [Homebrew_PCBs] Discreet versus Variable control
To: Homebrew_PCBs@yahoogroups.com
Date: Monday, May 18, 2009, 12:48 AM
I just recently got a small toaster oven from a thrift store for $3 (it was half off day). I have already removed the timer, the thermal switch, rewired and tested the unit(can reach reflow temperatures with just one heating element, but plan on using both). I have uploaded some photos of the progress(I need to get a new camera). The oven has an internal working area of 5.5" by 9.25", which is perfect for me because I usually cook SMD PCB's whose area is 4" by 6" or less. The heating elements are basically resistive and both have a measured resistance of 60 Ohms each and are operated in parallel.
In the past I made a reflow oven whose heater elements were driven using discreet control with a SPST relay, which was controlled by a microcontroller that measured the oven temperature via a glass bead thermistor. That setup worked well, but I am thinking of taking a different direction with this model.
I am thinking about rectifying the incoming AC to DC, then driving the heater elements with pulse width modulation using a high-voltage high-current HEXFET(IRFP250) . I am thinking along the lines of a PID control loop, with all of the gain tuning handled in firmware.
My question is: "Is it more or less beneficial to use a variable output controller over a discreet output controller?"
I have seen some very affordable discreet output oven controllers on ebay that come with a thermocouple probe and they are PID controllers.
I like the idea of the variable controller because it would make no noise compared to the relay of a discreet model, but it would also cost more to construct a variable vesus a discreet model.
I'm just trying to get an idea of what other people are doing and what suggestions they may have for me.
Sorry for the long post, and thank you in advance.
Daniel
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