"Tough Guy no. 1265" wrote in message
news
On Fri, 30 Oct 2015 20:30:40 -0000, Ian Field
wrote:



"Tough Guy no. 1265" wrote in message
news
On Fri, 30 Oct 2015 20:06:02 -0000, Ian Field
wrote:



"Tough Guy no. 1265" wrote in message
news On Fri, 30 Oct 2015 19:04:07 -0000, Ian Field
wrote:



"Tough Guy no. 1265" wrote in message
news 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.

Post a photo.

The link is at the top of this post, you must have missed it.

Can't see anything that looks like an NTC, or a MOV, adding them
wouldn't
hurt.

The NTC can be spliced into the mains live feed without doing anything
to
the board, the MOV should be added across the AC input to the bridge
rectifier.

If the electrolytic capacitor on the left was too small, the SMPSU
circuit
would try to even out the voltage regardless - if it has overcurrent
trip;
too big a capacitor would keep tripping it every time it started up
again.
But it might eventually get going.

It appears to be current limited. It takes some time to charge the
larger
capacitor. The LEDs gradually brighten.

A constant voltage supply driving LEDs would let the magic smoke out.

Well most of the LED lamps I've got change brightness depending on the
mains voltage. So they can't be that constant current. Maybe they just
have a voltage as a fraction of the mains, then rely on a resistor to
limit the current?

I bought various LED lamps from a discount store just to crack open and see
what's in them.

One had a 4 terminal component close to the pads for the LED wires, I
couldn't identify the part but it probably has to do with current
regulation.

Among hobbyists, there is a circuit for powering regular LEDs from a single
1.5V cell called a "Joule thief" its basically a flyback converter with no
regulation, but the LED only conducts on the flyback peaks so there is a
sort of limiting.

Another trick is the so called "wattless dropper" - a capacitor in series
with the mains so capacitive reactance is large compared to the dynamic
resistance of the LED load, with large Xc, current doesn't change much with
varying mains - some commercial LED bulbs use it.