This schematic has been modified (Aug 3, 2003) to address concerns that the original proposal did not completely remove power from the Kiln when the Fuji controller was off. While true, the only safety issue was to provide additional protection for those who stick their fingers inside of appliances without unplugging them. Never a good idea. The modification adds a second Solid State Relay to switch Line2 in addition to Line1. Note that if you build this circuit, it is unlikely that the additional parts can be fit inside the original box.
There was never a safety issue for kiln operation with the old schematic.
There has been a suggestion that the Solid State Relays could be replaced with a mechanical relay to save a few dollars. A single DPST relay with a 120v coil could be used to replace both of the SSRs. I cannot support this modification for safety reasons.
Why? The issue is the life of the power contacts within the relay, and the the consequences of contact failure. A typical failure of relay contacts is to weld closed, meaning that your kiln controller will fail in the full power-on condition. Not a pretty sight. Your artwork will be destroyed, as well as the floor of your kiln, not to mention the creation of a significant fire hazard from an overheated, unsupervised kiln.
What is the expected lifetime of relay contacts? Well, if you grab a relay from a surplus store, you don't know. If you buy a new relay from some suppliers, they may provide the design life. Here is an example: http://www.worldproducts.com/CH.htm
The stated electrical design life by this manufacturer is 100,000 cycles. An apparently big number. What's the problem?
The Fuji is set up to cycle every 2 seconds. This was chosen to protect the heating elements from thermal fatigue failure. At this cycling frequency, 100,000 cycles will be reached in only 7 days operating at 8 hours per day. A design life of seven days! Not a good idea.
William Tobler, Ph.D.
WilliamTobler@CrittersWoods.org (please don't flame Wendy)