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'[EE]:Smoking fresh crowbar experience'
2002\11\04@075245 by Morgan Olsson

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Hi,

I have a demo setup i am... or WAS... gonna demonstrate wednesday.
Now, the minute after i came back from lunch i heard a crack and a lod pop, and when i got to my desk there was a small cloud rising from the demo setup i had left running...

The cloud came from the power supply, a DIN rail Comatec 24V 800mA out.  Opening it revealed it had popped i´ts 35V output electrolytic.  So it had probably been very high output voltage.  I found it had no crowbar thingy at all.  Why do I buy crap??.  And of course i don´t want a new such electrinics killer on the guarantee!

Anyway, it managed to kill almost all electronics, a PLC and a cirquit i have made.

Now i have repaired my cricuit:

I had a 1N4002 series diode on input.  It hade *exploded* open.
After it there was a noise suppression choke, then 220uF 50V cap and 33V 1,4W (?) zener to GND.  The zener had short cirquited firmly, protecting my design :)  What impresses me is that the zener was stronger than the 1N4002. (what also impresses me is that the (madly running) 800mA supply (with only 470uF output cap) cracked the 1N4002 open...)   The Zener is a axial glass Motorola 1N4752A.

To be wise i should also also have a fuse instead of relying on a "short cirquit proof" tagged supply!

Now, i´ll dig into the PLC...

/Morgan

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2002\11\04@084019 by Roman Black

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Morgan Olsson wrote:

> The cloud came from the power supply, a DIN rail Comatec 24V 800mA out.
> Opening it revealed it had popped i4ts 35V output electrolytic.
> So it had probably been very high output voltage.
> I found it had no crowbar thingy at all.  Why do I buy crap??.


The 3 rules of electronics real estate;
1. run test
2. run test
3. run test
;o)

I've seen 1w resistors that when run at 1W got so
hot the solder melted and they fell out of circuit.
"3 amp" power supplies that caught fire running at
2.5 amps for an hour. And my pet hate are small
power chips rated for 1A or 1.5A that self destruct
when run at their so called rated current. Some
manufacturers are no better than crooks, the worst
being some of the lesser-known Asian brands using
other manufacturers datasheet specs but making
parts nowhere near that good.

I'm still struggling to find good schottky 1N5819
diodes. So far i've tested 3 brands marked as
1N5819, with huge differences between the saturation
voltage Vf and even differences in the shape of the
curve.

It's getting nasty, the days of buying a part by
number and expecting it to perform are long gone...
:o(
-Roman

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2002\11\04@090105 by Reinaldo Alvares

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Parts made in Asia don't have branded logos and usually cost a fraction of
the price for the real ones. So spend some $, and check for quality.
{Original Message removed}

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