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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 21st Feb 2019, 1:06 pm   #1
60 oldjohn
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Default Dusk- dawn sensor Capacitor

Bought 3 light units from China, they all failed after 3 months and I thought it was the relays. Forum members suggested could be the 330nf 400v capacitor used to drop the voltage, they were right, it measure a lot less than 330nf. Replaced with a Vishay, the capacitors last around 9 months. Not much space when the unit is squeezed into the waterproof housing.


I am hoping to find a more reliable Capacitor? Or a more reliable circuit?


John.
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Old 21st Feb 2019, 1:25 pm   #2
Argus25
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Default Re: Dusk- dawn sensor Capacitor

I have been through this problem many times myself, if it is the same problem.

When X2 style capacitors are used in voltage dropping circuits, it is a problem waiting to happen. Yet, many commercial designs use them. The X2 capacitor slowly drops its uF value with time, as its conductive layers are eaten away.

As I mentioned on another thread, I first noticed this when I used some to drop the voltage for some synchronous clock motors, then the more I looked for the problem, the more I saw it in other appliances. It is like noticing that a 6V electrical system VW Beetle has one headlight dimmer than the other, once you see it they are everywhere (well back in the 70's at least, but of course if you can remember the 1970's you were not actually there).

The only way I have found around this is to trash the X2 capacitors (easy call as they are not trustworthy) and go for 1200 to 1500V rated poly caps, which are physically at least twice the size, then you will not have any issues with them in the application.
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Old 21st Feb 2019, 3:33 pm   #3
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Default Re: Dusk- dawn sensor Capacitor

Yes use a "normal" capacitor with a VDR or some other spike limiter across it and add a few 10's of ohms in series.
 
Old 21st Feb 2019, 6:33 pm   #4
G8HQP Dave
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Default Re: Dusk- dawn sensor Capacitor

My experience is that X caps used as droppers roughly halve their value in about 5-6 years. I guess it depends on how spiky your mains supply is. Lots of items will become landfill when all they need is a new capacitor.
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Old 22nd Feb 2019, 12:42 am   #5
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Default Re: Dusk- dawn sensor Capacitor

The Vishay F1772 series is designed for X2 or dropper applications.
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Old 22nd Feb 2019, 8:40 pm   #6
60 oldjohn
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Default Re: Dusk- dawn sensor Capacitor

Thanks Guys, I think I will give the F1772 series a try, depending on size. The cap in photo is 25% bigger than the original, but only just fits in the case.

John.
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Old 23rd Feb 2019, 3:10 pm   #7
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Default Re: Dusk- dawn sensor Capacitor

We used to use Mullard (later BC Components, probably now Vishay?) MKT368 series capacitors as droppers, with fusible resistors in series for surge limiting and protection against short-circuit failure (not that I ever observed a case of that; but you need belt, braces and sticky knickers when working with gas).
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Old 24th Feb 2019, 12:27 pm   #8
60 oldjohn
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Default Re: Dusk- dawn sensor Capacitor

I think that is the range of capacitor that I am using, What value of Fusible resistor would you recommend ?




John.
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Old 24th Feb 2019, 3:46 pm   #9
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Default Re: Dusk- dawn sensor Capacitor

We used 2 * 470Ω NFR25H in series with a 330nF dropper, but you will really have to select a value according to your application. You don't want too great a switch-on current (a discharged capacitor is a dead short) but you want the fusible resistor to be running as close as possible to its rated power under the normal running current. Remember the current through a capacitive dropper is constant; whatever the load is not drawing gets swallowed up by the Zener diode.
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