How to Test and Replace Diodes in HP 19V 4.74A 90W Laptop Charger with 0V Output

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#1 21670807 09 Jan 2014 18:12

Luis Cruz Luis Cruz

Anonymous

Post #1
21670807 09 Jan 2014 18:12

Hi guys !

I'm an electronics engineer masters student, and i'm looking to fix an issue with my hp laptop charger (19V * 4.74A = ~90W).

It started to beep (error signal) yesterday and i started searching web to a possible reason. All connectors are good (both in & out), 0v at output don't know why! I saw someone saying that there is a diode that need to be switched.. I opened it and i looked to pcb and there is 3 big diodes next to transformer circuit and a small one after those 3.. Other side of the pcb there are a few but i don't think those are the problem.. I have a multimeter but i don't know how to test the diodes. Desoldering all diodes and test them may be the solution ?

Thanks in advance :)

EDIT: The diodes are on the center of the photo

EDIT2: SOLVED! Gnd and Vout were on short circuit !
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#2 21670808 09 Jan 2014 19:35

Frank Bushnell Frank Bushnell

Anonymous

Post #2
21670808 09 Jan 2014 19:35

Glad you got it fixed !
But as for testing diodes in circuit, if they are rectifying an AC, put an oscilloscope across them.
If they are just conducting, see if they have about 0.6 V across them.
If the equipment is off, just use your multimeter's diode test range.
(Possibly there may not be other components significantly affecting the reading.)

A large proportion of electrical and electronic faults are due to a simple bad connection - e.g. dry joint, dirty or corroded contact, loose connector.
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#3 21670809 09 Jan 2014 20:57

Luis Cruz Luis Cruz

Anonymous

Post #3
21670809 09 Jan 2014 20:57

Thanks for the info, but i found an easier way to test diodes:
Check the resistance on both ways of the diode, if you get 0 ohm on both ways its short circuit.
If you get infinite on both ways its short circuit too.
If you get infinite on one way and in another you get 0 ohm its good!

Sorry my bad english
#4 21670810 10 Jan 2014 04:20

Steve Lawson Steve Lawson

Anonymous

Post #4
21670810 10 Jan 2014 04:20

Infinite resistance both ways would mean the diode is open, not shorted. Also, some multimeters have a diode checking setting that, typically, gives a voltage reading. If it reads around .6 to .7 volts, that is a functional forward biased diode.
#5 21670811 10 Jan 2014 05:55

Luis Cruz Luis Cruz

Anonymous

Post #5
21670811 10 Jan 2014 05:55

Didn't know that ! Mine don't have diode checking...
Diode should only allow current in one way
But thanks for the info !
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#6 21670812 10 Jan 2014 09:23

David Figueroa David Figueroa

Anonymous

Post #6
21670812 10 Jan 2014 09:23

Luis
One way to test the circuit is to go form the output backwards, take your meter back to ground and red as your probe finder.. check each point going backwards until you get a voltage reading, form that point the problem is around that area....

Two check and see if you are getting a reading form your transformer output "if not", it could also be your transformer.
Now checking the diodes out of circuit is a good idea, your reading should be 0.7 volts one way and when you switch the leads you shouldn't get any reading... should read OL open circuit..

Try this and if you have any problems or want more information reach me here..

David
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#7 21670813 10 Jan 2014 15:12

Luis Cruz Luis Cruz

Anonymous

Post #7
21670813 10 Jan 2014 15:12

Hey David,

I've fixed it already ! But i think you can help me with another issue:

With another charger, an universal, 12-20V 3-4.74A, it seems to be functional but when i plugged into the laptop, it says connected but not charging, and if i have no battery attached, i turn off pc and past 2 seconds it shut down. Seems like he can't hold the 90W :s It's supposed to, i've opened it and the pcb seems to be fine, no short circuit...

Cumps!
#8 21670814 10 Jan 2014 16:09

Frank Bushnell Frank Bushnell

Anonymous

Post #8
21670814 10 Jan 2014 16:09

Could you confirm:
The laptop is shutting down when you remove the battery with the charger attached ?
The power pack is set to the correct voltage;

Here's one possibility that could explain your symptoms:
The charger is slightly under-powered, but in normal operation, the battery maintains the voltage level when the laptop is drawing more power than the charger supplies. (When running processor/memory/disk intensive programs.)
But without the battery in, the voltage drop under high power conditions results in automatic shutdown.
#9 21670815 10 Jan 2014 17:56

Luis Cruz Luis Cruz

Anonymous

Post #9
21670815 10 Jan 2014 17:56

Hey Frank,

Check and check, on charger output i'm getting 19V (oscillates between 18,5 and 19,5).

I agree with you! But i've charged my laptop with this transformer before, and i used it a few days... it seems to have poor quality (Trust brand)

Don't know what might be happening.. I think it should be components.
#10 21670816 10 Jan 2014 23:00

Frank Bushnell Frank Bushnell

Anonymous

Post #10
21670816 10 Jan 2014 23:00

Another common fault: wires can become loose inside the charger power plug going into the laptop. This is due to the cable being bent many times whilst in use.
It can appear as an intermittent fault, so test the output while wiggling the plug.

Testing method for this fault: wire an LED and series resistor to a spare input socket, plug the charger into it and wiggle the plug, observe if the LED remains constant or flickers.
If this is the fault, re-wire or replace the plug.

I have also encountered a problem with wires coming loose from the input socket inside the laptop, or the input socket becoming loose from the computer's circuit board.
#11 21670817 11 Jan 2014 13:05

Waseem Bari Waseem Bari

Anonymous

Post #11
21670817 11 Jan 2014 13:05

you should check the Fuse on the PCB. which will be the side from where AC supply enter to the charger . . . . . . . mostly fuse burned due to fluctuation in Supply . . . check it . . . and replace it if burned. . . . I hope your problem will solve by following this steps . .
#12 21670818 12 Jan 2014 20:13

Luis Cruz Luis Cruz

Anonymous

Post #12
21670818 12 Jan 2014 20:13

The fuse is good, at least making contact with both sides.

Still don't know what might be :s
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Topic summary

✨ An HP laptop charger rated at 19V, 4.74A (~90W) exhibited a 0V output and error beeping. The issue was initially suspected to be faulty diodes on the PCB near the transformer. Testing methods for diodes were discussed, including using a multimeter in diode test mode to check forward voltage (~0.6-0.7V), measuring resistance in both directions to identify shorts or opens, and using an oscilloscope for AC rectifying diodes. It was advised to test diodes out of circuit for accurate readings. Common faults such as dry joints, corroded contacts, blown fuses, and loose wiring inside the charger or laptop input socket were also highlighted. The original problem was resolved by identifying a short circuit between ground and output. A secondary issue with a universal 12-20V 3-4.74A charger (Trust brand) not charging the laptop properly was discussed, likely due to underpowered supply or poor quality components. Testing for voltage stability and cable integrity was recommended.

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]