EV Precharge Circuit
over 50 have been made & sold
Precharge Circuits
Precharge circuits are used in a variety of high power applications, such as electric vehicles (EVs). If you tried to turn on an EV without a precharge circuit, it would probably weld the contactor closed! This is because the initial surge of electricity has so much energy that it heats up the contactor beyond its' melting point. So the precharge circuit extends this time to about a second long, keeping the contactor cool.
Read About Precharge Circuits
Design in CAD
The circuit had to be extremely insulative—shorting across the circuit board would render it completely useless. I chose FR4, a fiberglass + epoxy laminate for it's excellent insulative properties. Additionally, it needed to fit in the controller box and mount to it. The hardest part was coupling the insulative and space constraints.
Steel Mounting Brackets
I learned how much harder steel is than aluminum
Rivnuts for Mounting
Rivnuts are basically rivets with threading inside, like a nut inside a rivet. This means that you don't have to hold onto the nut to tighten it, which means you can use bolts on things where you can't reach the nut (such as below a circuit board).
More About Rivnuts
Resistors
Before this project, whenever I thought of a resistor, I thought of those tiny plastic things you stick into an Arduino breadboard, not giant metal things with heat sink. But this giant resistor only resists 150Ω. So what gives, then? Why's it so big? Well, because of the massive amount of power it's resisting, it gets very hot, so it needs a lot of surface area to release that heat.
HV Contactors
When working at high voltages (above 250V), relays are no longer viable switches. Instead, a high voltage contactor must be used. Though they work differently, you can essentially use a contactor in the same way. The particular HV contactors that I used are rated for up to 1000VDC and operate on just 12VDC!
Relays v. Contactors
Testing for Safety
Whenever there's high voltage, you need to be extremely careful as arcing is a very real and dangerous possibility. I tested the circuit for continuity BEFORE turning it on. After following all proper safety procedures to reduce any chance of injury, we turned on our HV power supply and conducted our tests.
Finished Product in Controller Box
