I have a basic circuit, supposed to be driven by 230 V AC, consisting basically of two heating wires and a DC motor driving a fan. There's a switch to turn the heating part and the fan on or off at the same time.
(Click on the image for a larger version)
The parts are as follows:
S1: A thermal switch, turning the whole circuit off when a given temperature (180 °C) is reached.
S2: A simple switch to manually power everything on or off.
F1: A Thermal fuse (192 °C).
H1: Heating wire 1 aka "main heating wire".
H2: Heating wire 2 aka "additional heating wire".
C1: A capacitor.
Some diodes.
L1: Inductor 1.
L2: Inductor 2.
M1: The DC Motor (24 V).
C2: Another capacitor.
What I want to achieve is controlling the heating part and the fan separately via an Arduino. Here's my plan:
Split the circuit at the mark labeled "Cut?".
Control the heating part via a Solid state relais (Sharp S216S02?).
Power the fan using a separate power source containing another relais to control the fan.
With that being said, here are the things I'm unsure about:
Is it really that simple? Or am I missing something?
Is there a simple way to avoid driving the fan from another power source?
You don't say what you're trying to achieve overall, but one practical consideration you will need to watch while developing and debugging your circuit is that if the motor doesn't run when it is intended to, you will rapidly overheat the heater coils and might quickly burn them out!
In passing, you might be interested to know that many years ago I wanted to use a fan heater controlled by an external thermostat to take the chill off the room I was sleeping in on winter nights, and didn't want to be woken by the noise when the fan heater started. I interfaced the fan heater to a single-board computer and wrote some software to remove mains cycles to slow the fan, so that the motor received only 15/16 cycles, 14/16 cycles, 13/16 cycles etc. The motor ran more quietly, but as I slowed the fan more and more the heater coil ran noticeable hotter, and endfloat in the bearings introduced a tapping sound, so the project wasn't really a success.
No doubt someone can help you with your detailed questions, but I would advise you to proceed with some care and to be careful not to start a fire!
Edit: Stating the overall aim of a project often makes it easier for people replying to provide suitable advice.
That circuit uses the heating element as a resistive divider to give a lower AC voltage into
the rectifier section. (Incidentally this is the only time when you can get away with
using a resistive divider as a voltage regulator - you already want the oodles of heat
that's produced).
So the motor relies on the heating element for its power - you won't get it running without
the heater element being on too.
A SSR can be used to control the motor's supply at the cut, and for the whole thing.
If you want to be able to power the motor without the heat, you'll have to find out what
voltage the current circuit supplies to the motor half of the circuit (it may vary a bit
with motor load and heating coil temperature). Be safe doing measurements like this -
if the motor has a ratings plate, read it first, it may say everything.
I want to be able to define some basic roasting profiles when using this. A roast profile basically says what temperature the coffee beans will be exposed to for what time. I.e. "150 °C for 5 minutes, 200 °C for the next 2,5 minutes, ..., cool down to room temperature".
, but one practical consideration you will need to watch while developing and debugging your circuit is that if the motor doesn't run when it is intended to, you will rapidly overheat the heater coils and might quickly burn them out!
I'm very well aware of this. But thanks for the heads up. This piece of information may be especially important for others that read this thread!
I need to control the fan independently from the heating part because I want to be able to run the fan for cooling, without any heat applied. Not the other way around.
No doubt someone can help you with your detailed questions, but I would advise you to proceed with some care and to be careful not to start a fire!
I'm quoting this, because this is another very important piece of information for other readers.
Edit: Stating the overall aim of a project often makes it easier for people replying to provide suitable advice.
MarkT:
That circuit uses the heating element as a resistive divider to give a lower AC voltage into
the rectifier section. (Incidentally this is the only time when you can get away with
using a resistive divider as a voltage regulator - you already want the oodles of heat
that's produced).
So the motor relies on the heating element for its power - you won't get it running without
the heater element being on too.
That is what I figured. But I wanted to make sure there is really no way to do this.
A SSR can be used to control the motor's supply at the cut, and for the whole thing.
Unfortunately I need it the other way round. I need to run the motor without the heat.
If you want to be able to power the motor without the heat, you'll have to find out what
voltage the current circuit supplies to the motor half of the circuit (it may vary a bit
with motor load and heating coil temperature). Be safe doing measurements like this -
if the motor has a ratings plate, read it first, it may say everything.
There's no way I'm going to tamper with the circuit while it is powered on. Fortunately I know it is this motor.
My plan is to run the motor at constant speed and control the temperature by switching the heat on and off. Basically a PID control using a relay as shown at Arduino Playground - HomePage.
I don't know whether switiching the heater element on and off repeatedly might have any adverse effect such as reducing its life?
Or whether your project is a one-off for your own use or maybe for production, in which case minimising the parts count would be important.
In the probably unlikely event that heater life would be significantly compromised, or if finer control of its temperature would be useful, the method I outlined in my first post of removing whole cycles from the mains periodically to control the temperature could probably be implemented easily. It would require a zero-crrossing detector but that shouldn't require many components.
Heater control via a solid-state relay is extremely common. In fact, it's easier on all the components than a simple switch (including thermal switches) since the SSR will likely use zero-crossing to connect and disconnect the AC source only when it crosses 0v.
PD: Do you have room for a 24v transformer? (Well, make that 16-18v, because of the recitification.)
SirNickity:
Heater control via a solid-state relay is extremely common. In fact, it's easier on all the components than a simple switch (including thermal switches) since the SSR will likely use zero-crossing to connect and disconnect the AC source only when it crosses 0v.
I don't know what the switching frequency of the proposed PID control would be: if it's high enough, that would probably be as effective in controlling the heater element temperature without any risk of element fatigue as my suggestion, and easier to implement .
Edit: The reference to a zero-crossing detector in my earlier post was to a need for a zero-crossing detector to generate an input to the Arduino for use in an algorithm to periodically withhold some cycles of the mains input supply to the heater element. Thinking about it now, I don't think that that input would be required if the output from the Arduino was to a solid-state relay using zero-crossing switching.
@ProfessionalDreamer:
SirNickity asked: "Do you have room for a 24v transformer? (Well, make that 16-18v, because of the recitification.)"
That would make it very easy to power the motor separately by simply connecting the transformer output at the cut you originally suggested.
