Although it may not seem like it, so far my posts on this project have been quite simple. However, from this update onwards, things are going to get a lot more complex, so grab a cup of tea and a slice of cake...oh and put your thinking cap on too
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Today I will be talking about the only thing on this project that MUST be 100% correct. The power supply unit which powers the entire electrical system for the cage. This update shows the first of four custom made circuit boards used on this project.
DISCLAIMER: Details included in this post are for
educational proposes only.
DO NOT attempt to recreate any part of this power supply. Parts of this power supply are connected directly to the building’s mains supply which runs at 230VAC, this level of voltage along with the current available can and will kill you or your pet instantly. I have around 20 years experience dealing with electrical systems as well as electrical qualifications. I possess the required practical and theoretical knowledge to make this power supply in a safe way.
Again,
DO NOT ATTEMPT TO RECREATE ANY PART OF THIS POWER SUPPLY.
This power supply is designed to step-down the voltage from 230V, convert the lower voltage from alternating current (AC) to direct current (DC) and then output the different voltages I need for the project, 1.3V, 3V, 5.06V, 10V and 15V.
For various reasons, I will use proper terminology when referring to parts of the electrical system. I will try to add the common name in brackets for the first instance. For example: a lamp (light bulb).
If I miss one or you still don’t know what something is, just ask me.
Low Voltage Side (mains):
The system controller has a flying cable (non-removable) with a standard 13amp plug which is fitted with a 3amp fuse. The cable then goes into the controller’s case through a cable gland which grips the cable and prevents it from being pulled out accidentally possibly causing a short circuit. The outer gray sheathing is then cut back to allow the wires inside to be terminated (attached). The cable is a three core cable, Phase (live (brown wire)), Neutral (blue wire) and CPC (circuit protective conductor (earth) ( yellow/green wire)). The phase wire first goes through a 250volt 0.25amp protection fuse, this fuse is a fast blow fuse meaning that if the current is more than 250milliamp, the fuse will blow and disconnect the power within 50milliseconds (1/20th of a second). The phase wire then goes through an on/off switch (rated at 250VAC at 5amp) and then finally into the transformer. The Neutral wire goes from the input cable directly to the transformer. The CPC is terminated into a round crimp connector. The transformer has a metal bracket around it and has mounting holes at the base. The CPC crimp connector is attached to the transformer with nuts/bolts that attaches the transformer to the controller’s case. This earths the transformer so in theory, if a fault occurs that energises the transformer’s inner iron core and the metal bracket. That power will go through the CPC, through my flats earthing system and into the ground. This will then trip the RCD (residual current device) in my flat’s distribution panel (fusebox) killing the power to the whole flat.
The transformer has a single primary winding (coil) and dual secondary windings. The primary side is rated at 250VAC. Both secondary windings are rated at 15VAC at 1.33amp.
It should be noted here for those who don’t know how a transformer works. There is no electrical connection between the primary and secondary windings, they are indeed completely separate from each other. A transformer uses the continuously varying magnetic field cause by the varying voltage of an AC source, to induce an electro-motive force (voltage although that’s technically the wrong word) into the secondary winding. The ratio between the number of turns of copper wire around the core of the primary winding and number of turns on the secondary windings is what determines the input and output voltages.
For example if the supply voltage is 100 volt, the transformer has 100 turns on primary with 2 turns on secondary, the output will be lets say 2 volt. If there's 100 turns on primary, 50 turns the on secondary, the output voltage will be 50 volt. This is just an example, in reality, transformers will have thousands of turns on each winding. There are also various other factors that determines primary voltage, secondary voltage/current such as transformer type, conductor (wire) thinkness ect.
Extra Low Voltage Side (electronics):
The output of the transformer has four terminals, two for each of the two secondary windings which we’ll call Output A and Output B. I will start with the simplest:
Output A; The phase wire from the transformer goes to a 250V, 1.0amp fast blow fuse before going to the power supply circuit board and into the first "~" pin on the bridge rectifier diode which converts the AC into DC. The rectified supply then goes to a PCB terminal block which the system controller is then connected to, marked as Output 0 on the labelled photo. Further details on where this goes will be in a later post. The neutral wire from the transformer is connected directly to the second "~" pin on the rectifier diode to complete the circuit.
Output B; The first part of this circuit is the same as Output A up to and including the bridge rectifier. It too is protected by a 250V, 1.0amp fast blow fuse. Both of these rectifier diode are rated at 80V 2amp.
However that’s where the similarity between the two ouputs ends. The output or “+” pin on the bridge rectifier diode splits into three traces (electrical pathways or wires on a circuit board). There is a small capacitor here as well which helps to ensure there is a smooth voltage. If the voltage drops for a split second, the capacitor will briefly take over until the voltage returns to its normal level. The capacitor will then recharge ready for the next time. The capacitors have a low amount of capacitance (the amount of energy stored) and will not run any part of the system on their own. Capacitors are not batteries and cannot be used as such.
Each of the three traces then goes to a LM317 adjustable voltage regulator. Along with is supporting circuitry the LM317 steps the voltage down from 15V to the required level, it also tries to maintain that output voltage even if the input voltage drops for a short time. The small blue square things are potentiometers (variable resistors) which are used to set the exact voltage I need. Each output (except output 4) has another capacitor on the output to help filter out any ripples from the voltage regulators. Output 3 has a larger capacitor as the main cage lighting are connected to output 3. Although the LM317’s are rated at 1.5amp each, they still need a heatsink to keep them cool. The heatsinks used are (at least for Output 1) slightly underrated for the current I’m drawing from the voltage regulator. That said, the voltage regulator remains well within its operating range (the heatsink peaks at somewhere around 40-45°C under full load and without the cooling fan, the LM317's have a thermal cut out which is factory set at 125°C).
The output pin of the voltage regulators are connected to a PCB terminal block to the right via underside traces or topside wires. The output voltages are as follows:
Output 1; 3.0V
Output 2; 5.06V
Output 3; 10.0V
Output 4; 1.3V
Pins 5 and 6 of the right hand terminal block are both neutral returns and are connected directly to the "-" pin on the rectifier diode.
Output 4 doesn't have its own voltage regulator, it "piggy backs" off output 1. The lower voltage is down to the voltage drop across the potentiometer and resistor. This can be done due to the power being drawn is less than the resistors rated power (1/2 Watt). The main lighting circuits for example pull to much power to just rely on a resistors voltage drop, hence the need for the LM317's.
The neutral terminal on the transformer's output B is again directly connected to the circuit board and to the second "~" pin on the output B rectifier diode.
As mentioned, I have fitted a 12V 80mm computer cooling fan to ensure that the voltage regulators, the transformer and other parts of the system remains cool. Despite the photos, the fan isn’t directly connected to output 3, I just had it like this while building it and conducting energised tests. The fan is switched with the “other colour” in normal operation and can be switched manually if need. The fan one can see here was just one I had lying around, it has now been replaced with a ball bearing fan which is almost silent.
I have attached four pictures with this post, two of which I have not embedded into the post due to the image size. The schematic is the same as the image embedded, only full size. The other is a labelled version of the photo that's embedded just full size and with labels highlighting the different parts of the power supply.
Power Supply.JPG
Photo of the power supply after completion.
Power Supply Schematic Small.jpg
Schematic diagram detailing the circuitry.
(please forgive the crude drawing, I do not normally draw schematics)
I should add that I have also since added LED’s on output 0 and output 2. This indicates that the outputs are running and makes it easier to tell if the fuses have blow as well as for system diagnostics. There's a blue LED for output 0, a pink LED for output 2.
As I said in the disclaimer above,
DO NOT try to recreate anything in this post. I have posted this information so you can understand what I have created,
NOT for you to try to recreate it.
Sorry this is a big update but I feel it’s justified due to its complexity and importance to the project. Sorry too if your brain is now hurting :P.
Take Care
Morgan + Archimedes