FAQ
TL;DR: For HP 19V×4.74A (≈90W) chargers showing 0V, first check for GND–Vout shorts and connector faults; “A large proportion of faults are due to a simple bad connection.” [Elektroda, Frank Bushnell, post #21670808]
Why it matters: This FAQ helps DIYers quickly diagnose no‑output and diode faults in 90W HP-style laptop PSUs using only a meter.
Quick Facts
- Nominal spec: 19 V output, up to 4.74 A (≈90 W). Shorts at the output can clamp it to 0 V. [Elektroda, Luis Cruz, post #21670807]
- Functional silicon diode shows about 0.6–0.7 V forward drop on the meter’s diode mode. [Elektroda, Steve Lawson, post #21670810]
- Infinite resistance both directions indicates an open diode; 0 Ω both ways indicates a shorted diode. [Elektroda, Steve Lawson, post #21670810]
- Underpowered or sagging universal adapters may show “connected, not charging” and trigger shutdowns. [Elektroda, Frank Bushnell, post #21670814]
- Cable and plug fatigue can cause intermittent power; wiggle‑test the DC plug while monitoring output. [Elektroda, Frank Bushnell, post #21670816]
How do I test a charger diode with only a multimeter?
Power off and discharge capacitors. Use the meter’s diode mode. Forward bias should read ~0.6–0.7 V. Reverse bias should read OL. If your meter lacks diode mode, use resistance: one direction low, the other very high. “Infinite both ways means open.” [Elektroda, Steve Lawson, post #21670810]
Do I need to desolder diodes to test them accurately?
Often yes, because parallel paths can skew readings in‑circuit. Lift one lead to isolate the junction if results look ambiguous. Then repeat the forward/reverse test to confirm ~0.6–0.7 V forward drop and OL reverse. [Elektroda, Steve Lawson, post #21670810]
My HP 90W charger shows 0 V at the output—what’s the first check?
Check for a short between GND and Vout at the DC connector. A hard short will pull the output to 0 V and can trigger protection. Clearing the short restores normal voltage. [Elektroda, Luis Cruz, post #21670807]
What does 0 Ω both ways vs. infinite both ways mean on a diode?
0 Ω measured both directions indicates a shorted diode. Infinite resistance both directions means an open diode. Only one conducting direction is acceptable for a good junction. [Elektroda, Steve Lawson, post #21670810]
How can I trace the fault when the adapter reads 0 V?
Start at the output and move backward toward the transformer. Keep your black lead at ground and probe stages until voltage reappears. The failure lies near the transition point. [Elektroda, David Figueroa, post #21670812]
What does “rectifying AC” mean for the charger’s diodes?
Those diodes convert transformer AC into DC. In operation you can scope across them; powered off, use the meter’s diode mode. Expect about 0.6 V across a conducting silicon diode. [Elektroda, Frank Bushnell, post #21670808]
Universal 12–20 V adapter says connected but not charging—why?
It may be under‑rated or sags under load. Without the battery buffering, voltage droop can halt charging and cause shutdowns. Set the correct voltage and ensure current capacity matches 90 W. [Elektroda, Frank Bushnell, post #21670814]
Laptop powers off ~2 seconds after removing the battery on adapter power—root cause?
Your adapter cannot sustain load transients. The voltage dips and the laptop’s protection shuts it down about 2 seconds later. Use a 19 V, 4.74 A capable supply in good condition. [Elektroda, Luis Cruz, post #21670813]
How do I check for intermittent plug or cable faults?
Perform a wiggle test. Power the supply, connect a load or LED+resistor, and gently flex the DC plug and strain relief. Flicker or drops confirm a cable or connector failure. [Elektroda, Frank Bushnell, post #21670816]
Should I check the input fuse, and where?
Yes. The fuse sits on the AC input side of the PCB. Verify continuity. Replace if open, as supply fluctuations often blow it. If it’s good, continue downstream checks. [Elektroda, Waseem Bari, post #21670817]
What forward voltage should I see across a good diode in-circuit?
Expect around 0.6–0.7 V across a forward‑biased silicon diode. Read it with diode mode. Reverse‑biased should read OL or very high impedance. [Elektroda, Steve Lawson, post #21670810]
3‑step how‑to: Test a suspect diode quickly
- Power off and discharge. Lift one diode lead if possible.
- Meter in diode mode: forward ≈0.6–0.7 V; reverse OL.
- If 0 V both ways, it’s shorted; OL both ways, it’s open. [Elektroda, Steve Lawson, post #21670810]
Can output shorts alone explain a dead 19 V rail?
Yes. A direct GND–Vout short forces 0 V and can keep the PSU latched off. Clear the short and retest under light load. [Elektroda, Luis Cruz, post #21670807]
Is a fluctuating no‑load voltage normal on a universal adapter?
Small oscillations around 19 V (for example, 18.5–19.5 V) can appear, but large sags under load indicate regulation trouble or insufficient rating. Replace poor‑quality units. [Elektroda, Luis Cruz, post #21670815]
