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Temperature Control System Wiring Diagrams
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And now that I've got the all-important weasel paragraph out of the way, here are some wiring diagrams that illustrate how I have wired various annealers and
electric glass furnaces over the years. With the exception of a molybdenum disilicide heated glass furnace, the control system for an electric glass furnace
is essentially the same as for an annealer. The only real difference is the current carrying capacity of the power components.
Any relays on power control systems like those on this page MUST be the normally open type for safety. This way the relay will automatically open and cut power to the heating elements in case power to the controller or relay coil fails for any reason. Although this may seem counter-intuitive to someone inexperienced, it is the ONLY correct way to do it.
120Vac, single pole, normally open electro-mechanical relay system
|The diagram above is the way I
usually wire an all-in-one control box
for a small 120Vac annealer. (Although I'll often use a two pole relay
instead of the single pole relay shown.)
For higher powered applications I will also usually separate the power
feeding the controller from the power going to the heating element,
which can't easily be done in an all-in-one, plug-and-play type control
The MOV attached to the relay coil is to help prevent large electrical spikes caused by the inductance of the coil. An RC snubber circuit might also be used across the relay contacts for a similar purpose. It is best to check the manufacturer's literature to see what they might recommend, as some equipment already has built-in snubber circuitry. If unfamiliar with RC snubbers or metal oxide varistors (MOV), there is plenty of info on the internet. There are also a couple of decent videos on YouTube. This one in particular nicely demonstrates the value of electrical noise suppression. The actual demo starts at about 7:30 in the video - https://www.youtube.com/watch?v=c6I7Ycbv8B8
is essentially the same system as shown in the prior diagram, but wired
for 240Vac instead of 120Vac. This diagram also assumes a controller
that can use 240Vac for power and a normally open relay with a 240Vac
coil. (220Vac will typically also work for 240Vac components.)
A two pole 120Vac system would be wired the same way as this, with the neutral line in place of Line 1 and a relay with the appropriate 120Vac coil. A fuse on the neutral line wouldn't be necessary in that case.
|In places where
mercury relays are still legal, this is a decent compromise between
cost and reliability. Mercury relays are normally rated for about 10
times the expected life of an electro-mechanical relay and can operate
at shorter cycle times, thereby enabling more precise temperature
control. Their primary drawback is, unsurprisingly, that they contain
mercury. This can present problems in disposing of them and can create
environmental nightmares in the very rare case where one
catastrophically self-destructs and spills the mercury inside. They are
otherwise functionally equivalent to an electro-mechanical relay and are
wired the same way.
The diagram above illustrates wiring where the power wires connected to the heating elements are separate from the wiring for the temperature controller. However, if necessary, one can easily tap into the line and neutral wires to run the controller, as is done in the first wiring diagram on this page. Although not shown in any of the diagrams, it would probably be a good idea to wire an MOV across the incoming power terminals of the temperature controller to eliminate any incoming voltage spikes, especially when it is powered off the same circuit as the heating elements, as in the first two diagrams.
A 240Vac system would simply use a two pole merc for power control, which is essentially two single pole mercury relays with their coils wired in series. Technically speaking, one could also use a single pole relay to run a 240Vac system, but it would leave one of the legs electrically live all the time. Obviously, this is potentially dangerous.
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