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Smoke Alarm Repair

By on August 21, 2021
smoke alarm repair








A 40-year-old mains powered smoke alarm started occasionally triggering without smoke and the prime suspect (other than dust) was the main 4.7 μF 50V electrolytic capacitor.

4.7uf 50 volt capacitor

The capacitor was checked with a Fluke 110 multi-meter

fluke 110 meter

which reported a healthy 5.18 μF and a cleanup stopped the false alarms (dust is detected as smoke); but it would still beep occasionally on windy days when the mains voltage was perhaps more variable than usual.

Now came the interesting aspect of this repair, namely to check the main capacitor again but with a superior meter, the Peak ESR70

peak esr70 meter

which now reported just 1.58 μF and an ESR (effective serial resistance) of 12.6 Ω which located the problem with the aged capacitor (a new capacitor had an ESR of just over 1 Ω).

This illustrates the problem of DC capacitor testing (the Fluke) over AC testing (the Peak), although it might be argued that one should simply replace 40-year-old electrolytic capacitors; getting hold of axial lead capacitors is still possible, so the repair looks very clean.

andy sheikh anwar

This article was written by Anwar (Andy) Shiekh originally from London, England; he repairs things to help make an income go further and presently teaches Physics in Colorado, U.S.A.

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Note: You can check his previous repair article on Nook Repair And Battery Replacement




  1. Albert van Bemmelen

    August 21, 2021 at 8:33 pm

    Fluke meters are overpriced and in my opinion are surpassed by cheaper and better instruments that also do not carry so much weight with them either. And I'm sure it is not even a good ESR meter at all!

    • Tim Waldner

      August 21, 2021 at 9:11 pm

      Fluke does not market a handheld esr meter. I have a fluke 289, which is a excellent meter, but for caps I use a DER-EE5000. Low cost, does the job.

      • Imoudu Onwumah

        August 22, 2021 at 5:03 am

        I don't understand your reading.You say ur peak reads 1.58uf while what is showing on the metre is 22.20uf.I think there is misplaced of figures.

        • Anwar Y Shiekh

          August 22, 2021 at 8:46 pm

          I just used generic pictures from the Web of the devices used and reported the value I measured.

  2. Anwar Y Shiekh

    August 21, 2021 at 9:18 pm

    The Fluke only measured the capacitance and not the ESR

  3. Parasuraman S

    August 22, 2021 at 12:37 am

    Excellent trouble shooting. Learnt about a new Peak Atlas tool! Many thanks!

    • Anwar Y Shiekh

      August 23, 2021 at 2:59 am

      My ESR70 unit uses a 12V battery, but the new version uses a AAA which is much nicer.

  4. Waleed Rishmawi

    August 23, 2021 at 7:53 pm

    thanks for sharing your repair with us. I have the same fluke meter but mine can measure amps as well. I use the ESR meter for testing capacitors not the Fluke since it is not very accurate. have a very blessed day

    • Anwar Y Shiekh

      August 24, 2021 at 10:22 am

      I got the volts only version as this way, even when fatigued, I am less likely to make an error and measure current when I should be measuring voltage.

      I also purchase variacs from and get a free digital multi-meter with each $50 spent, so I now have way too many multi-meters.

      I like variacs as they allow me to run the inductive motors in room fans slow enough to be silent so I can run them in the bedroom.

      Have also moved to rechargeable 9V lithium batteries for the meters to save on battery cost.

  5. Yogesh Panchal

    August 27, 2021 at 2:58 pm

    I have seen many technicians who repair Laptops and other electronics equipment are using cheap multi meters and it works. it is not always higher price or Brands means good quality their equipment/material also Fails.


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