[Homeroast] variac basics

Allon Stern allon at radioactive.org
Mon Nov 15 11:04:08 CST 2010


On Nov 14, 2010, at 7:59 AM, Edward Bourgeois wrote:

> I like using a variable transformer/variac over going PID. Been using
> one with my roaster for 5 years. From all I've heard over the years
> there have been many failures with the models out of China. Unless
> there have been recent improvements in these I highly recommend
> looking for a a US made model Staco, Superior etc., in good condition
> used. Many are priced high on ebay but if your patient they can be
> found for $50.-$100.

Just curious, what are your reasons for going with a variac versus a PID?
They do different things - a variable transformer gives you control over the heat applied, but where you set it is entirely up to you.
A PID runs a feedback loop with input from a thermocouple to achieve the same end - variable control of the heat, but instead of giving you control of the heat applied, it gives you control over the temperature that the heat applied gives you.

A variac is like holding the throttle on a car in the same position regardless of speed; it gives you the same power applied, but you'll go faster downhill than uphill. A PID is more like a cruise control (in fact, cruise controls incorporate PID algorithms) - you set the speed you want, and it asks the motor for more power when climbing hills and less power when going downhill, to keep the speed the same.

Roasting is anything but flat open road; there are sivitz bumps at various stages, and the roast builds inertia; knowledge of these bumps and other aspects will let you adjust your variac to attempt to manage them for smooth acceleration, but it's a battle.

Also, many PIDs will let you do more than fix a setpoint; you can program a profile to run, and that programming is real control over the roast.

I'd rather adjust a PID profile and watch it run, maybe be able to tweak it on the fly, than have to babysit and tweak a variac and watch a thermometer and try to hit specific points at particular times.
-
allon




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