Dr.Al
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The last couple of posts have discussed this rather complicated control cabinet circuit:
This time, I'm going to talk about a much simpler (and much cheaper to build) version of the circuit:
It's worth noting that there are definitely cheaper and simpler ways of making it than this. I went with this layout for the purposes of the explanation as I think it covers a lot of the bases while also being affordable. Anyone implementing this themselves can make their own decisions on whether they want to include everything.
If you compare the two circuits, you'll see there are a lot of similarities. The biggest differences are that the controller is much simpler, there's no over temperature monitor (more on this later) and there's a solid state relay (SSR) instead of the thyristor pack. You'll also notice a different sort of diode near the contactor; again I'll discuss this below.
To give an idea of the cost of this control system, there are four major components involved:
A very basic PID controller and SSR can be bought as a package on amazon for £16. A DIN-rail mounted 24 V power supply with sufficient power output to drive a contactor coil can be found for a bit under £20. A three-switch contactor with a 24 V coil can be acquired for about £11 (contactors with mains voltage coils are also available, which would save the cost of the power supply, but I'd rather have the safety circuit as a low- voltage one).
Thus, all the major components can be bought for about £50.
Let's look at that schematic again:
If you compare this with the more complicated version at the top of this post, you'll hopefully see that the power circuit is essentially the same: circuit breaker, fuse, switch, contactor, heater, control device (this time an SSR rather than a current-controlling thyristor pack). I've moved the fuse to the other side of the switch, but the fuse location isn't that critical and (in both diagrams) I picked the location mainly based on what looked neat on the diagram!
The big difference comes with the controller. I'll talk more about the control strategy in the more complex circuit later, but let's look at the simple one first. In this system, the controller monitors the temperature from a single thermocouple and uses that output to decide whether to turn the SSR on or off. It may do more fancy things like turn it on and off quickly in order to get an **average** current that's a bit less than the maximum, but let's keep it simple and imagine it just turns it on and off according to temperature.
That's really all there is to this system. The controller has a fault output, so I've wired that up to the contactor as before, along with the emergency stop button and the door interlock (which ensures that the heater isn't live when the door is open). I'm not sure I'd entirely trust the controller's fault output and I'd prefer to have an independent temperature monitor, but I was going for a fairly cheap system with at least a bit of protection built in.
The last thing that's worth noting in this simple schematic is the diode I've drawn across the contactor/relay coil. It would work without that (and some relays even include the diode inside the package, although I think that's quite rare). However, an inductor (which is what the coil of a relay is, although it's a little more complicated than that) resists the **change** of current flowing through it. For the mathematically minded, the voltage-current equation of an inductor is:
In that equation L is the inductance and is a constant. What that means is that the voltage (V) is proportional to the **rate of change** of current. If the current changes from 1 A to 0 A in 1 second and the inductance is (implausibly) 1 Henry, the voltage across the inductor will be 1 V. If the current changes from 1 A to 0 A in half a second, the voltage will be 2 V. If you open one of those switches, the current effectively changes from (say) 1 A to 0 A in 0 seconds, resulting in a theoretically infinite voltage. That high voltage causes an arc across the switch contacts. Do that lots of times over and the switch contacts will become blackened and corroded and won't work any more.
Where the coil is driven with a DC supply (as it is here), putting a diode (known variously as a "flyback", "freewheeling" or "catch" diode) across the relay coil gives the current in the inductor an alternative path (rather than trying to arc across the switch contacts) and the current decays gracefully without damaging anything.
I'm planning to put a freewheeling diode in the more control cabinet, but I omitted it from the schematic diagram as it felt like it was getting cluttered enough without the diode!
That's it for this post. In the next post I'll probably talk about the fancy new controller and the (much simpler) over-temperature device. I'll also talk about why I've included so many thermocouples and what my ideas are for the block marked "Switches / Pots / LEDs".
This time, I'm going to talk about a much simpler (and much cheaper to build) version of the circuit:
It's worth noting that there are definitely cheaper and simpler ways of making it than this. I went with this layout for the purposes of the explanation as I think it covers a lot of the bases while also being affordable. Anyone implementing this themselves can make their own decisions on whether they want to include everything.
If you compare the two circuits, you'll see there are a lot of similarities. The biggest differences are that the controller is much simpler, there's no over temperature monitor (more on this later) and there's a solid state relay (SSR) instead of the thyristor pack. You'll also notice a different sort of diode near the contactor; again I'll discuss this below.
To give an idea of the cost of this control system, there are four major components involved:
- Power supply
- Controller
- SSR
- Contactor
A very basic PID controller and SSR can be bought as a package on amazon for £16. A DIN-rail mounted 24 V power supply with sufficient power output to drive a contactor coil can be found for a bit under £20. A three-switch contactor with a 24 V coil can be acquired for about £11 (contactors with mains voltage coils are also available, which would save the cost of the power supply, but I'd rather have the safety circuit as a low- voltage one).
Thus, all the major components can be bought for about £50.
Let's look at that schematic again:
If you compare this with the more complicated version at the top of this post, you'll hopefully see that the power circuit is essentially the same: circuit breaker, fuse, switch, contactor, heater, control device (this time an SSR rather than a current-controlling thyristor pack). I've moved the fuse to the other side of the switch, but the fuse location isn't that critical and (in both diagrams) I picked the location mainly based on what looked neat on the diagram!
The big difference comes with the controller. I'll talk more about the control strategy in the more complex circuit later, but let's look at the simple one first. In this system, the controller monitors the temperature from a single thermocouple and uses that output to decide whether to turn the SSR on or off. It may do more fancy things like turn it on and off quickly in order to get an **average** current that's a bit less than the maximum, but let's keep it simple and imagine it just turns it on and off according to temperature.
That's really all there is to this system. The controller has a fault output, so I've wired that up to the contactor as before, along with the emergency stop button and the door interlock (which ensures that the heater isn't live when the door is open). I'm not sure I'd entirely trust the controller's fault output and I'd prefer to have an independent temperature monitor, but I was going for a fairly cheap system with at least a bit of protection built in.
The last thing that's worth noting in this simple schematic is the diode I've drawn across the contactor/relay coil. It would work without that (and some relays even include the diode inside the package, although I think that's quite rare). However, an inductor (which is what the coil of a relay is, although it's a little more complicated than that) resists the **change** of current flowing through it. For the mathematically minded, the voltage-current equation of an inductor is:
In that equation L is the inductance and is a constant. What that means is that the voltage (V) is proportional to the **rate of change** of current. If the current changes from 1 A to 0 A in 1 second and the inductance is (implausibly) 1 Henry, the voltage across the inductor will be 1 V. If the current changes from 1 A to 0 A in half a second, the voltage will be 2 V. If you open one of those switches, the current effectively changes from (say) 1 A to 0 A in 0 seconds, resulting in a theoretically infinite voltage. That high voltage causes an arc across the switch contacts. Do that lots of times over and the switch contacts will become blackened and corroded and won't work any more.
Where the coil is driven with a DC supply (as it is here), putting a diode (known variously as a "flyback", "freewheeling" or "catch" diode) across the relay coil gives the current in the inductor an alternative path (rather than trying to arc across the switch contacts) and the current decays gracefully without damaging anything.
I'm planning to put a freewheeling diode in the more control cabinet, but I omitted it from the schematic diagram as it felt like it was getting cluttered enough without the diode!
That's it for this post. In the next post I'll probably talk about the fancy new controller and the (much simpler) over-temperature device. I'll also talk about why I've included so many thermocouples and what my ideas are for the block marked "Switches / Pots / LEDs".
