On Fri, 30 Oct 2015 18:48:26 -0000, Ian Field wrote:



"Tough Guy no. 1265" wrote in message
news
On Fri, 30 Oct 2015 17:10:19 -0000, Tough Guy no. 1265
wrote:

https://www.dropbox.com/s/fy5xm8lmd3....jpg?dl=0&s=sl

This supply came with a 2 foot 9W strip light of LEDs. After 1 week it
began flickering badly. 240V AC in on the right. The circuit outputs
75V DC on the left, with a 3.5V AC ripple on it (with the 100uF capacitor
included, which I verified is ok). With a 680uF capacitor in its place,
there is no visible flicker.

What's likely to have broken in this circuit?

What's the chances of it lasting if I run it with the larger capacitor?

Forgot to add, I checked the voltage and current output with the larger
capacitor, and it was virtually identical.

When you first switch on, the fully discharged capacitor looks like a dead
short for the split second it takes to charge up. The bigger capacitor will
cause the turn on surge to last nearly 7x longer.

The LEDs light up very slowly with the bigger capacitor (for half to one second perhaps), so I think it's being limited. Also, when I turn it off, won't the capacitor stay charged to the voltage just below what's required to make the LEDs conduct, so it's not going to be empty at the next startup?

If the circuit has a NTC thermistor inrush limiter, you're probably OK.
I usually look for them in any equipment I scrap, when I still used filament
bulbs - one of those added behind the switch plate makes the bulb last
years.

They look similar to a disc ceramic capacitor about 10 - 15mm diameter, they
tend to be dull green or black and have a matt finish as compared to
capacitors that are usually shiny finish.

I can't see one. There's a resistor connected directly to neutral on the bottom right of the photo. In series with that resistor are the two capacitors (blue and yellow), going across to live. Connected across those two capacitors is the input to the bridge rectifier made up of four diodes (under the blue capacitor). The red capacitor is across the output of the bridge. I can't easily make out what's connected to what for the rest of it, but there's a microchip, and the current going to the LEDs is very constant no matter what capacitor I put on it, so I'm guessing good regulation. These are also the only LEDs I've ever bought which stay the same brightness when I change the input mains voltage by 20 volts (which is what my UPS does when the voltage reaches 250 - I've got a very crap mains supply here since they changed the substation and they won't do anything about it as it's "within legal parameters", so my lighting circuit goes though the UPS).

Cut a track in the AC live side and bridge it with the thermistor.

If the donor equipment also includes an MOV surge arrestor, that's also
worth adding to your kit.

Similar physical description, but they're usually beige colour or black and
most often shiny.

Its easy to tell the difference - the NTC will show continuity at low
voltage, the MOV should stand normal mains without conducting.

--
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Therefore, they must "bitch" or they will blow up.