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Starter project link: https://github.com/lhr-solar/StarterProjects
Background:
The Problem
When we draw current from a battery it also produces heat, which induces a high current draw. If a battery gets hot enough, it will enter a thermal runaway where it gets so hot that it draws much more current than normal, producing more heat and drawing more current until the battery explodes (not good). There are a lot of preventative measures you can take to avoid thermal runaway like passive cell balancing. The simplest option to prevent the battery from getting too hot is controlling fans to dissipate heat in the battery box.
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My recommendation for starting this is to lay out every component you know you need, we know we have 4 2-pin connectors, 1 6-pin connector, and some number of MOSFETs. Place those components first and the vision will be clearer
For the NMOS use the IRLM0030 IRLML0030 symbol
Symbol Libraries:
We have many shared components for our boards, so we decided to have a separate repository just for storing shared components.
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Steps to add the UTSVT-KiCAD Libraries repo to your repository.:
Travel to your PCB directory in Git Bash
Get the clone link of the UTSVT-KiCADLibraries repo
Type this into bash:
Code Block git submodule add [clone link of the UTSVT-KiCADLibraries repo]Go to the schematic editor of the starter project
Go to Preferences->Manage Symbol Libraries
Go to the “Project Specific Libraries“ page
Press the + button at the bottom left of the screen
In Library path, select the “utsvt-chips.kicad_sym” and change the nickname to usvt-chips
{KIPRJMOD} is a variable that changes depending on the user.
Make sure to use the {KIPRJMOD} variable instead of your /Users/[username] directory
Repeat step 8 for every .kicad_sym file
Press “Ok“
To test if it worked, in the menu to add symbols type the nickname for one of the libraries you added and see if there is a symbol in that section
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Why don’t I just download thee UTSVT-KiCAD Libraries as a zip and then copy and paste it into the repo? |
Footprint Libraries:
Go to the PCB editor
Preferences → Manage Footprint Libraries
Go to Project Specific Libraries
Add each .pretty folder
Make sure the nickname stays as the the name of the footprints foder (without the .pretty name)
Protections:
Bypass capacitors:
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During layout keep the capacitors close to the pins you’re trying to filter noise to.
Add 2 100 uF capacitors in parallel with the 12V and GNDPWR line
Current Limiting:
The gate to drain of a mosfet acts as a weak capacitor (cuz semiconductor physics don’t ask me), so for various reasons, we need to limit current on the input of the gate, so add a resistor in series with the gate ( a 100k ohm resistor works fine).
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Look up the equation for current through a capacitor, given that equation why does switching the mosfet on and off induce high instantaneous current (assume current capacitance is staying constant). |
Pullup Resistors:
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To mitigate this we set the gate voltage of the gate to another voltage when no signal is being applied . Through through a pull-up resistor or pull-down resistor. A pull-up resistor pulls the signal to some voltage (usually VCC →, in this case, it’s 12V). A pull-down resistor pulls it down to GND.
Pullup resistor | Pulldown resistor |
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Pulls a voltage to Vcc12V | Pull a voltage to GND |
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You only need a pullup or a pulldown, why did you choose one |
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Stuff like motors and fans acts as an inductor, meaning they store current and then dump it all out. When we suddenly switch off an inductive load, the inductor will try to dump out all this stored current (because by opening the switch we’ve removed the current source essentially). To prevent this dump from causing big current spikes through our system we add a flyback diode in parallel with the inductive load (also called snubber diode, freewheeling diode, etc).
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To hold the boards down mechanically in the enclosures, we use mounting holes to screw into them. We need 4 mounting holes. The symbol is called Mouting Hole (the unconnected one).
Setting footprints:
womp wompWe try to use SMD components when we can since they are smaller, easier to solder with a heat gun, and you usually end up getting a better electrical component.
We use “0805“ components, which means they are .08 in by .05 inches (very small). We use that for most passive components (resistors, capacitors, etc).
The footprint name is formatted like this [footprint library]:[footprint name]
View the footprint menu by going to Tools → Assign Footprints.
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Final Checks:
When debugging problems, it’s nice to have status LEDs so add one for 12V power (make sure to add a current limiting resistor)
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PCB:
Make things symmetric :0
Footprint Libraries:
Make sure you do the symbol libraries setup step before this step
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Go to the PCB editor
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Preferences → Manage Footprint Libraries
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Go to Project Specific Libraries
Add each .pretty folder
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Setting up constraints:
The PCB manufacturer we use (JLCPCB) has specific machining requirements that we need to follow to have them order the board.
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A few tips:
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Wrapping Up:
Make a
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branch on the StarterProjects repo, push to it frequently, and make a Pull Request when you’re done
Add your name to the board in the silkscreen
Add funny images to the silkscreen:
Answer every question listed as ❓
DM your system lead to get a review
Relish in how smart you are