User question
How many watts is R17 0.68 ohm
Artificial Intelligence Response
Direct answer to the question
- Most likely rating: 1 W.
- In DeWALT DCB107 chargers, R17 = 0.68 Ω is typically the primary‑side current‑sense (shunt) resistor, commonly implemented as an SMD 2512 (6.3 × 3.2 mm) part rated around 1 W. If space allows, using 2 W for extra margin is a safe repair choice.
Detailed problem analysis
- Role in the circuit:
- R17 sits in series with the primary switch (MOSFET) source and the return, providing a voltage proportional to the primary current for the PWM controller’s cycle‑by‑cycle current limit.
- This resistor sees short, repetitive current ramps each switching cycle rather than continuous DC load.
- Power in a primary current‑sense resistor:
- Current during the MOSFET on‑time is approximately triangular, rising from 0 to Ipk each cycle. The instantaneous dissipation is i(t)^2·R; the on‑time average per cycle is (Ipk^2·R)/3.
- Including the duty cycle D (fraction of the period that current flows), the long‑term average is roughly:
- \( P{\text{avg}} \approx \dfrac{D \, I{\text{pk}}^{2} \, R}{3} \)
- With typical small flyback chargers, Ipk is on the order of 0.5–1.2 A and D ≈ 0.2–0.4. For R = 0.68 Ω this gives only a few tens to a few hundred milliwatts average—consistent with 1 W parts selected for surge/abuse margin and temperature rise limits.
- Why 1 W (and not 0.25–0.5 W or 3 W):
- 0.25–0.5 W: Undersized for fault surges, start‑up/inrush pulses, and long‑term thermal reliability on FR‑4.
- ≥3 W: Unnecessary in this topology; typical boards use 2512 shunts (≈1 W class). Oversizing is fine during repair if space allows, but not required for proper function.
Current information and trends
- Tool chargers of this power class commonly use low‑ohmic SMD shunts (0.22–0.82 Ω) in 2010s–2020s designs, most often 1206–2512 footprints; 2512 is typical when R ≥ 0.5 Ω or when added pulse/thermal margin is desired.
- Controllers in this segment use cycle‑by‑cycle current limiting with sense thresholds in the few hundred millivolts to ~1 V range, making 0.68 Ω a reasonable choice for Ipk around an ampere.
Supporting explanations and details
- Physical ID cues:
- SMD: If the package measures ~6.3 × 3.2 mm and is marked “R68”/“0R68”, it is almost certainly a 2512 current‑sense resistor ≈1 W.
- Through‑hole (less common in this model): A ~8–10 mm long metal‑oxide/metal‑film body typically corresponds to ~1 W; ~12–15 mm suggests ~2 W.
- Tolerance/type:
- Use 1% if available (helps maintain current limit accuracy).
- Prefer low‑inductance current‑sense types (thick‑film or metal‑element) for switch‑mode use.
Ethical and legal aspects
- Safety first: This is a mains‑connected SMPS. Unplug before service, discharge HV capacitors, and use an isolation transformer and series bulb/variac for first power‑up after repair. Li‑ion charger repairs should only be performed by qualified personnel to avoid fire/shock hazards.
Practical guidelines
- If R17 failed, check likely root causes before replacing:
- Main MOSFET (Q1) for D‑S shorts.
- Gate drive path and PWM controller IC for damage.
- Input bridge/fuse and primary snubber network.
- Replacement specs (recommended):
- 0.68 Ω, 1% tolerance, SMD 2512, 1 W minimum; 2 W acceptable if it fits and the footprint allows heat spreading.
- Post‑repair check:
- Power via current‑limited source/series bulb.
- Monitor R17 temperature; sustained >100–120°C indicates undersizing or unresolved faults.
Possible disclaimers or additional notes
- Board revisions exist. Without your exact PCB photo/dimensions, 1 W is the best evidence‑based default; confirm by measuring the footprint or the original part size.
Suggestions for further research
- Identify the PWM controller marking on your PCB and consult its datasheet for current‑sense threshold; cross‑check that with \( I{\text{pk}} = V{\text{CS(th)}}/0.68 \) and use the power estimate formula to verify margin.
- If available, consult teardown photos of your exact DCB107 revision to visually confirm the package size.
Brief summary
- R17 (0.68 Ω) in the DCB107 is almost certainly a 1 W SMD 2512 current‑sense resistor. For repair, a 1 W part is appropriate; choosing 2 W adds thermal and surge margin if it fits. Before replacement, locate and correct the primary‑side fault that likely caused it to fail.
Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.
