Dead SMPS Repaired: Low ESR Capacitor Was the Culprit

 

🛠️ Dead SMPS Repaired: Low ESR Capacitor Was the Culprit

Switched-Mode Power Supplies (SMPS) are commonly found in a wide range of electronic equipment due to their efficiency and compact design. Recently, I repaired a dead SMPS unit that showed no output and zero power-on indication. This article explains the fault diagnosis process, how I identified the root cause — a capacitor with low ESR (Equivalent Series Resistance) — and how replacing it brought the unit back to life.

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🔍 Symptom: Dead SMPS

The power supply came from a set-top box, rated for 12V, 2A output. When connected to AC mains, there was:

• No DC output

• No voltage at the output capacitors

• No switching noise or pulse at the MOSFET

• Primary side appeared dead

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🧰 Tools Used

• Digital Multimeter (DMM)

• ESR Meter

• Capacitance Meter

• Oscilloscope (for final verification)

• Soldering Iron and hot air rework station

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🧪 Step-by-Step Diagnosis

1. Visual Inspection

The board looked clean — no burnt components, no bulged capacitors. Fuse was intact, and NTC thermistor was in good condition.

2. Primary Side Testing

I checked:

• Bridge rectifier — OK

• Bulk capacitor (400V) — Charged properly to 310V DC

• Startup resistor — Within range

• Power IC (PWM controller) — Had supply voltage, but no output

This pointed to either:

• A faulty PWM controller (unlikely, as voltage was stable)

• Or a feedback loop issue

3. Checking the Output Side

I measured ESR of the secondary side capacitors using an ESR meter. One 1000µF / 16V capacitor near the output diode showed very low ESR (<0.01 ohms), and almost no capacitance.

⚠️ Note: This is unusual — low ESR is normally good, but in this case, it was abnormally low due to an internal short inside the capacitor, causing it to behave like a low-value resistor.

4. Replacing the Faulty Capacitor

I replaced the suspect capacitor with a new 1000µF/25V low-ESR type, rated at 105°C. The old capacitor, when tested out of circuit, showed only 12µF instead of 1000µF.

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✅ Power-On Test

After replacing the capacitor:

• The SMPS started switching

• Output voltage stabilized at 12.1V

• Ripple was within safe limits

• Load test passed with a 1A and 2A dummy load

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📌 Conclusion

In SMPS repair, electrolytic capacitors with abnormal ESR values can silently kill the circuit. Most people associate high ESR with faulty capacitors, but this case proves that very low ESR — especially due to internal shorting — is equally dangerous.

🔧 Takeaways:

• Always check ESR and capacitance

• Don't rely only on visual condition

• Use an ESR meter — it's a vital tool in modern power electronics repair

• Replace secondary caps with good-quality, low-ESR, high-temperature rated capacitors

After a long time i am Presenting one of my interesting story How i repaired an expensive Cable Receiver which is  being supporting in my local cable network for TV Channels.

Since i am an electronic professional i will be having  Regular touch with local electricians, Cable operators, Videographers  ets. Because they are all my regular Costumers.

These kind of receivers are high  critical and cable operator cannot keep extra standby because of its cost  high expensive .Every local cable network area depends only on this equipment which receives low db signals and then amplified, distributed to Users. If such kind of critical equipment's breakdown then the cable operator is in big trouble. His mobile phone will engage in messed of incoming calls and he is not able to serve the costumer. The only solution to him is to get repaired within low time.

One of  my cable costumer faced above situation and he came to me with tension and requested me to get repaired within low time. Every one who do the repair work knows they cannot repair all the different equipment's, Only specialist can do that within the time because they are well experienced, trained and equipped with proper instruments.

For me its really new thing and for the first time i am opening this expensive RF equipment and very querisity to see the inside design. I am very lucky in this  because i have seen many new things, New design ,new technology in my work shop which i have not seen so for at that time .Really this activity is nothing but technology booster to me.

Come let us start the story how i operated and solved this expensive equipment. When i received this equipment as a symptom its temporarily ON for few second and goes to dead. Removed the mains chord and re-Inserted resulted the same symptoms and the cycle repeated. Suspected on SMPS working Issue.

Generally maximum SMPS  are designed multiple DC OUT depending  on the requirement. In this device i noticed +24V,+5V and -5v DC With common ground.

As a thumb rule if  SMPS stops working First we need to check cold test with the help of Multimeter to the Components handle high stress area like Power transistor/mosfet/IGBT/Top switch etc. Hear in this Receiver its purely STR Power supply design. I check the STR For shorts thank god its OK, Checked the PWM IC For shorts ok, Checked the DC Run cat no shorts ok, start up circuit resistors found ok.Now my next step is to check secondery side for shorts ,Step by step checked the secondery diode for the shorts found ok,Feed back voltage divider ckt ok ,Replaced the opto Isolator  IC817

no result .When every thing ok then why this cycling turn ON/OFF ???.

Really its very headach to me.I did not understood the fault route cause.Ultimatly i replaced all the electrolitic secondary capacitor and the equipment regained the life.