RE: Re: PCB Design

["Keith Doxey" <ukha@xxxxxxx>]

Hi David,

You've just reminded me of what happened to one of my Disco lighting
controllers.

It was rated for 1000W per channel (actually it could handle more than that
but not if all 3 channels were on at the same time due to 3kW max from 13A
plug), and I decided to use it to control some festoon lighing outside an
amusement arcade.

I knew the lights were working OK as they had been lit constantly before I
fitted the controller. A quick count of the number of bulbs 50 x 15W gave
me
the total wattage of 750W which was well below the maximum and I had
regularly driven 1kW per channel no problem.

I was somewhat disturbed when I turned it on..... there was an extremely
loud bang from inside the case and no lights came on :-(

Removing the case revealed the cause of no lights.... 3 legs where the
triac
should have been and a big scorch mark inside the case. When I went to
replace the triac I also found that a PCB track had been vaporised as well.

Whilst the total load was only 75% of the maximum, I had forgotten that the
cold resistance of a bulb is much lower leading to a higher than expected
inrush current. Because of the 50 bulbs connected this effectively became a
dead short killing my controller :-(

Didnt make that mistake again :-)

Keith

www.diyha.co.uk
www.kat5.tv


> -----Original Message-----
> From: David Buckley [mailto:db@xxxxxxx]
>
>
> augment the trace with single core wire soldered onto the trace
> all the way along to make it a "thick" trace.  This is often
done on
> mains light dimmers; 4 x 10A needs a lot of current carrying
> capability.
>

> Oh yes, there is one other thing.  Get a short across the load, and
> your track width or fusing wrong, and the PCB trace vaporises.
> Whilst not as loud as an electrolytic cap letting go, it is
> adequately loud and scary, and you _will_ jump some distance :-))
>
> David.
>