▲ 3 r/AskElectronics
Hi everyone,
I'm the Electronics lead for our Formula SAE team, and I'm running into a critical hardware failure on our Accumulator Pre-Charge V1 prototype. My low-side NPN transistor keeps failing short (Collector-Emitter dead short), causing the pre-charge relay to latch permanently as soon as the 12V LV system is powered on, even without a control signal.
Before I spin the V2 board, I would love some extra eyes on my schematic and my proposed V2 layout to see what I might be missing.
The Setup:
- Control Logic: LM339 Quad Comparator (5V logic).
- Isolation: 4N35 Optocoupler.
- Low-Side Switch: BC337 NPN BJT (TO-92).
- Load: 12V Automotive Relay (coil resistance ~400Ω, draws ~30mA).
- Protection: 1N4148/1N4007 flyback diode across the relay coil.
The Symptoms on V1:
- As soon as 12V is applied to the power side, the relay clicks and stays latched.
- Upon desoldering and testing the BC337, it measures a dead short between Collector and Emitter.
- This "Fail-Deadly" state happens almost instantly.
What I've tested so far:
- Logic Side is OK: I measured the forward voltage on the 4N35 LED and it sits at a perfect 1.12V when triggered. The LM339 is doing its job.
- Opto Current: I initially had a 470Ω base resistor, but increased it to 1kΩ (and later tested 2.2kΩ) to limit the current coming from the 12V rail through the 4N35 into the BJT base. The opto seems to be surviving just fine now.
- Dummy Load Test: When I replaced the relay with a resistive dummy load, the circuit didn't blow up, which points heavily towards an inductive spike issue or a routing problem with the flyback diode.
- Footprint Mismatch / GND Suspicion: I highly suspect our V1 PCB layout might have mixed up the BC337 pinout (routing C-B-E instead of the component's physical E-B-C) or had a flawed Emitter GND path, causing immediate Zener avalanche and melting the silicon.
Attachments: (Note: I shelved the GND polygons on the layout images for better visibility of the traces)
- Image 1: Schematic.
- Image 2: V1 Layout (Original/Failed) - Notice the Emitter routing and base resistor placement.
- Image 3: V2 Layout (Proposed Redesign) - My attempt to fix the grounding, footprint, and base resistor path.
- Questions:
- Validating the Failure: Looking at the V1 layout, do you agree that the Emitter path (being tied to the 470Ω resistor instead of direct GND) or a footprint mismatch is what instantly killed the BJT?
- Reviewing the Fix: Looking at my proposed V2 layout, did I route the Base resistor and the Emitter GND path correctly this time?
- Inductive Spikes/EMI: Is there anything wrong with how the flyback diode is routed relative to the relay coil in V2? Could EMI from the nearby DC-DC converter be causing issues, or was the V1 failure strictly a physical routing/junction breakdown issue?
Any feedback, roasts, or tips are highly appreciated! Thanks!
u/Impressive_Bank_JT — 15 days ago