GL's "Eco-System" . Ideas for electrical controls
Notes about the heater and pump control
I've put together a few examples for you, starting from simple, moving up to more elaborate systems, to show how you can control you heater and pump more safely than having separate on/off controls for each.
I've included various options which allow
you to ensure that the heater can only activate under certain conditions, for example when the pump is on, or when the liquid level is
above a preset minimum, or when flow is present.
This control system offers more features than you'd find on a typical home made processor,
to try to improve operator safety. Each scheme is fairly simple and not that expensive to
make, so why not take a little time looking over each example, to understand how
they can benefit you.
I haven't finished the write-up on this yet, so feel free to ask questions if something isn't clear.
Basic pump / heater controller
This system helps to prevent accidental switch on of the heater, and only lets the heater operate if the pump is running.
(This helps to ensure that the heater has a constant cooling flow of liquid over it, which extends heater life and improves heating efficiency.)
It uses what I call the 'Think Twice' system. You need to push 2 buttons simultaneously to start heating. If you mount the buttons
well apart from each other, there's much less chance you'll accidentally switch on the heater.
A single push-switch lets you quickly kill power to the pump and heater.
The heater's temperature is controlled by an adjustable thermostat, which you set to 50C for processing, or 90C for de-watering / distilling.
I've used 'Contactors' in these designs. Contactors are simply heavy duty relays. The coil is shown as a circle with the contactor id number C1, or C2 etc. The contacts are shown as 2 parallel plates, also marked C1 or C2 etc.
What happens if the pump windings overheat for some reason? Or if the adjustable thermostat fails or is incorrectly set? Lets add some over-temperature cutoffs to let things cool off if something goes wrong...
Let's add thermal cutouts...
We'll add a pair of thermal cutoff switches in the feed to contactor C1. If either the motor or the tank goes above their limit tempaerature, the system will shut down and can only be re-started manually.
Now, what would happen if our tank was not full enough to completely cover the heating element? If the mixture of air and flammable vapour were just right, you could have an explosion, so lets add a
float switch to ensure the heater can't come on if the liquid level is too low...
Let's add a high voltage/current handling float switch ...
With this addition, if the liquid level should drop below the level of the float switch, the heater will switch off automatically. And, if you try to switch on the heater in this condition, nothing will happen. Power will only be
applied to the heater if sufficient liquid is above the element.
Important! Your float switch MUST be rated for the supply voltage in use and MUST have enough current rating to switch the contactor C2 on and off. Many float switches use low voltage and current rating reed switches which are NOT suitable for the wiring scheme I've shown.
You should fit a Varistor across the Contactor coil to minimise 'Back EMF' which can cause the switch contacts to burn and may in the worst case cause them to weld shut. You really DON'T want that to happen.
Varistors should be fitted across any Contactor Coil which relies on low power or critical function series switches for safety.
Varistors should be fitted across EVERY contactor coil in a circuit which includes semiconductor devices such as transistors, integrated circuits, electronic modules etc. This is because the 'Back EMF' which may be generated by the coil when it de-energises, can interfere with the low level signals in such devices. It can also damage such devices. Contactor coils without varistors are BAD NEWS for any nearby electronic equipment, as the 'Back EMF' can amount to many thousands of volts.
Surprisingly few electrical engineers take this effect into account, and I have often solved reported 'mystery' problems in control systems with one opening question ... "You have fitted varistors across your contactor coils, haven't you?". Most often, they haven't, and this proves to be the cure.
If you will be using a low voltage, low current reed switch, the next circuit will be suitable.
Let's add a low voltage/current float switch...
An alternative circuit, suitable for use with low voltage, low current float switches. An advantage of this scheme is the lower switching power in the float switch, thus less available arcing energy.
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