FAQ
TL;DR: Add a shunt‑sensed pass‑transistor current limiter to the EEWeb‑style 12 V charger; the 2N3055 is rated 15 A, and “P = I²R.” [Elektroda, Steve Lawson, post #21666375]
Why it matters: It prevents blown SCRs/transistors and overheating when you ask, “how do I add current limiting to a 12 V charger?”
- Transformer: Typical 17 VAC secondary for a 5 A charger stage; verify rating under load. [Elektroda, Steve Lawson, post #21666372]
- Simple limit: ~5 A using a low‑ohm shunt (~0.10–0.12 Ω) and a 2N3055 pass transistor; heatsink required. [Elektroda, Steve Lawson, post #21666373]
- High‑current caution: At 10–20 A, dissipation rises fast; active‑region 2N3055 may overheat without aggressive cooling. [Elektroda, Steve Lawson, post #21666375]
- File sharing tip: Convert BMP to PDF for uploads by printing from Word. [Elektroda, Earl Albin, post #21666371]
- Original circuit reference shared by users for context and author contact. [Elektroda, Floy Viola, post #21666368]
Quick Facts
- Transformer: Typical 17 VAC secondary for a 5 A charger stage; verify rating under load. [Elektroda, Steve Lawson, post #21666372]
- Simple limit: ~5 A using a low‑ohm shunt (~0.10–0.12 Ω) and a 2N3055 pass transistor; heatsink required. [Elektroda, Steve Lawson, post #21666373]
- High‑current caution: At 10–20 A, dissipation rises fast; active‑region 2N3055 may overheat without aggressive cooling. [Elektroda, Steve Lawson, post #21666375]
- File sharing tip: Convert BMP to PDF for uploads by printing from Word. [Elektroda, Earl Albin, post #21666371]
- Original circuit reference shared by users for context and author contact. [Elektroda, Floy Viola, post #21666368]
How do I add a simple current limit to a 12 V car charger like the EEWeb design?
Use a low‑ohm shunt in series with the output and a sensing transistor that drives a 2N3055 pass transistor. The sense voltage across the shunt reduces base drive, clamping current near your target. Heatsink the 2N3055 generously. [Elektroda, Steve Lawson, post #21666373]
What shunt resistor value sets a 5 A limit?
Target about 0.10–0.12 Ω. At 5 A, that develops ~0.5–0.6 V to trigger limiting in the driver stage. Adjust slightly to reach full 5 A without premature limiting. Use a power‑rated resistor or calibrated wire loop. [Elektroda, Steve Lawson, post #21666373]
What transformer should I use for a 5 A charger stage?
Use a 17 VAC transformer that truly holds 17 VAC at a 5 A load. This supports the series‑pass stage while leaving headroom for rectifier drops and regulation. Confirm the current rating and temperature rise under load. [Elektroda, Steve Lawson, post #21666372]
Will a 2N3055 survive 10–20 A charging?
Not as a linear pass device without major cooling. Power is current times voltage drop, and as current rises, heat skyrockets. The 2N3055’s 15 A rating doesn’t guarantee safe dissipation in the active region. Use heavy heatsinking and forced air if you push limits. [Elektroda, Steve Lawson, post #21666375]
How hot is too hot for the driver transistor during testing?
Monitor device temperature. If a small driver gets too hot to touch, add a heatsink. One expert suggests adding a heatsink when it exceeds about 120 °C. “Use a non‑contact temperature probe.” [Elektroda, Steve Lawson, post #21666373]
What happens if the charger output is shorted?
The limiter clamps current near the setpoint, but the pass 2N3055 still dissipates heavy power. Expect it to get very hot quickly. A robust heatsink is mandatory for short‑circuit survival testing. [Elektroda, Steve Lawson, post #21666373]
Can I build the shunt from heavy copper wire?
Yes. A short loop of heavy wire can serve as a low‑ohm shunt if you calibrate it. One user successfully used a wire loop as an ammeter shunt in this project context. [Elektroda, Jerry C, post #21666374]
Is a crowbar circuit a good add‑on for protection?
It’s an option. A crowbar trips on over‑voltage and, with a resettable breaker, can protect the load. However, the contributor didn’t present a full crowbar design here; consider it complementary to current limiting. [Elektroda, Steve Lawson, post #21666372]
What’s an SCR and why protect it in a charger?
An SCR is a silicon‑controlled rectifier used for switching or control. In chargers, high surge or fault current can stress it. Adding a current limiter reduces fault energy and protects semiconductors downstream. [Elektroda, Steve Lawson, post #21666372]
What is a pass transistor in this context?
A pass transistor sits in series with the output and drops voltage to regulate current. Here, a 2N3055 acts as the pass element, controlled by a sensing stage tied to the shunt. [Elektroda, Steve Lawson, post #21666373]
How do I adjust the limit from 5 A to 10 A safely?
Lower the shunt resistance to raise the threshold, then re‑check dissipation. Remember, heat scales with I×V and copper losses with I²R. Plan for much larger heatsinks and airflow at 10 A. [Elektroda, Steve Lawson, post #21666375]
How do I share my schematic if BMP won’t upload?
Copy the BMP into a Word document and print to PDF, then upload the PDF. This workflow preserves clarity and reduces file size for forum sharing. [Elektroda, Earl Albin, post #21666371]
Why won’t my BMP upload, and which formats are preferred?
The forum rejects BMPs. Use GIF or PNG for diagrams to reduce size and keep lines crisp. These formats are better for electronics schematics. [Elektroda, Frank Bushnell, post #21666369]
Where’s the original 12 V car charger reference mentioned by users?
A member shared an external reference link to a 12 V car battery charger page and suggested contacting the author for modifications. [Elektroda, Floy Viola, post #21666368]
Quick 3‑step: How do I build the 5 A limiter stage?
- Insert a ~0.11 Ω shunt in series with the charger output.
- Add a sensing transistor driving a 2N3055 pass transistor with proper base resistor.
- Heatsink, then test with a dummy load and adjust shunt for 5 A. [Elektroda, Steve Lawson, post #21666373]
