HomeRoast Digest


Topic: SCR control of roaster heat (68 lines)
1) From: Kevin DuPre
Your controller needs to detect when the line voltage
crosses through zero volts and then fire a single-shot
timer whose time is no longer than about 8
milliseconds (for 60Hz line). This timer would be
variable, practically from a minimum of 1-2
milliseconds (for maximum RMS voltage) to 8
milliseconds (near zero RMS voltage). As this timer
times out, it fires another pulse used to trigger the
gate of the SCR for the remainder of the AC line's
half cycle before it shuts off due to reverse bias.
Once an SCR is triggered it will conduct until it
reverse biases and shuts off waiting for another
trigger pulse. The line must be going in the right
polarity for it to conduct at all, though. Every line
cycle, or 60 times a second, the cycle repeats. 
With an SCR, though you only get a max of line voltage
for half of the AC line cycle, effectively cutting the
power density applied to the heater by 2.  A triac is
like 2 back-to-back SCRs which allow the power to be
controlled in a similar manner for the full line
cycle.
A dimmer is convenient because it already contains the
detection, timer, and gate firing circuitry and
control knob.
Long ago in my career I used to design
capacitor-discharge magnetizers and demagnetizers. 
There are some very complex issues to controlling SCRs
much of which has to do with synchronizing the trigger
pulse with some function of the line frequency.
Our currents were insane compared to those involved in
home coffee roasters.  It was not uncommon to charge
3-6 banks of capacitors, each of which used 16
capacitors the size of large beer cans, to 600 volts
DC, and then through an SCR, discharge them into a
coil of #2 solid copper through 4/0 welding cables. 
Currents were in excess of 150,000 amps for about
15-30 milliseconds, long enough to create an intense
pulse of magnetic energy and permanently magnetize
things like motor or speaker magnets or other
permanent magnet material. We also did some impromptu
experiments after learning about the military's
mag-rail gun. As it turns out, if you put a scratch
awl slightly off center in a large solenoid and then
pulse it with one of these magnetizers, the
acceleration is enough to put the business end through
3/4" of plywood.
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Kevin DuPre
obxwindsurfhttp://profiles.yahoo.com/obxwindsurf"The real voyage of discovery consists not in seeking new landscapes but in having new eyes -- Marcel Proust"
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