Can the PIV of a diode be "safely" exceeded?

[KILOWATT]

Hi everyones thanks to read. Since a few weeks, i'm using the LightKeeper
Pro from Ulta-Lit Tree Company ( http://www.lightkeeper.biz/default.asp ) to
test and repair miniature lights sets (series-wired). I've found the Quick
Fix Trigger utility very useful. as you can see on their website, it's based
on a piezo igniter that sends a high voltage pulse through the lights set to
activate (short) the shunt inside the bulb, that didn't do so when the
bulb's filament burned out. I wanted to see how this system is built so i
opened the unit. Here's two photos plus a schematic i've drawed for the
igniter section. (Sorry, the symbol for the piezo igniter is probably
wrong...i don't know the correct one).
http://www3.sympatico.ca/kilo.watt/images/lightkeeper1.JPG
http://www3.sympatico.ca/kilo.watt/images/lightkeeper2.JPG
http://www3.sympatico.ca/kilo.watt/images/hv_pulse_gen.bmp
As i can see, the four diodes allow a peak reverse voltage of approximately
4Kv, wich is sufficient to break down the shunt's insulation inside the
burned bulb(s) and complete the series circuit. Once completed, half of an
AC cycle can flow through the four diodes, allowing the set to glow (dimly)
and show which bulbs are burned and needs replacement.
What leads me to my question (please look at the schematic) is: why the
reverse voltage doesn't seem to damages those diodes? According to the piezo
igniter's website ( http://www.yiqiang-piezo.com/english/production2.htm ),
the output voltage is at least 15Kv. With the four 1n4007 wired in series, i
should get a peak reverse voltage of about 4Kv isn't? For what i know, a
diode is destroyed once it's PIV is exceeded. I think i missed something
when studying the basic operation theory of a diode. ;-) TIA for any
useful reply.
--
Alain(alias:Kilowatt)
Montréal Québec
PS: 1000 excuses for errors or omissions,
i'm a "pure" french canadian!
Come to visit me at: http://kilowatt.camarades.com
(If replying also by e-mail, remove
"no spam" from the adress.)

 

[Bob Myers]

"KILOWATT" <kilowatt"nospam"@softhome.net> wrote in message
news:Sfemd.7126$rc.616816@news20.bellglobal.com...

For what i know, a
diode is destroyed once it's PIV is exceeded. I think i missed something
when studying the basic operation theory of a diode. ;-) TIA for any
useful reply.

No, the PIV is just the point (well, minus some margin)
where the diode should be expected to "break down" and
conduct in the "wrong" direction. But "break down" may be
giving you the impression that the diode is damaged in the
process, when that is not at all the case. Diodes may
safely be operated past the reverse breakdown point
IF the current through the diode is limited (to whatever
value is safe depending on the specifications of the diode
in question). This is, in fact, the normal operating mode
for a "Zener" diode, which is simply a diode which has a
very abrupt reverse-breakdown point (and usually one
which has been intentionally designed to be a relatively
low (and therefore useful, for such things as regulators)
voltage.

The way a diode is generally used as a rectifier, though,
there would not be any current limiting provided, and
exceeding the PIV would in general be a one-time
event...

Bob M.

 

[KILOWATT]

Yess... i normally avoid crossposting because some people see this action as
"bad" even if i perfectly agree with you, Rich.

KILOWATT, please cross-post instead of multi-posting this sort of
question, so that the answers can propagate to both groups for
the benefit of more people.

You'll note that I've crossposted this one.

If a thread becomes more group-specific, there are more than enough
people around USENET who will be more than happy to kindly remind you
to set your followups appropriately. ;-)

Thanks,
Rich

 

[rayjking]

Alain,

You are observing the action of avalanche. Most modern high voltage diodes
are designed to withstand various amounts of reverse avalanche energy
absorption. Diodes that are applied in series should be tested for this
avalanche characteristic.
The information given for the piezo out put voltage of 15kv does not happen.
The diodes clamp the voltage at about 4500 volts.

Ray







"KILOWATT" <kilowatt"nospam"@softhome.net> wrote in message
news:Sfemd.7126$rc.616816@news20.bellglobal.com...

Hi everyones thanks to read. Since a few weeks, i'm using the LightKeeper
Pro from Ulta-Lit Tree Company ( http://www.lightkeeper.biz/default.asp )
to
test and repair miniature lights sets (series-wired). I've found the Quick
Fix Trigger utility very useful. as you can see on their website, it's
based
on a piezo igniter that sends a high voltage pulse through the lights set
to
activate (short) the shunt inside the bulb, that didn't do so when the
bulb's filament burned out. I wanted to see how this system is built so i
opened the unit. Here's two photos plus a schematic i've drawed for the
igniter section. (Sorry, the symbol for the piezo igniter is probably
wrong...i don't know the correct one).
http://www3.sympatico.ca/kilo.watt/images/lightkeeper1.JPG
http://www3.sympatico.ca/kilo.watt/images/lightkeeper2.JPG
http://www3.sympatico.ca/kilo.watt/images/hv_pulse_gen.bmp
As i can see, the four diodes allow a peak reverse voltage of
approximately
4Kv, wich is sufficient to break down the shunt's insulation inside the
burned bulb(s) and complete the series circuit. Once completed, half of an
AC cycle can flow through the four diodes, allowing the set to glow
(dimly)
and show which bulbs are burned and needs replacement.
What leads me to my question (please look at the schematic) is: why the
reverse voltage doesn't seem to damages those diodes? According to the
piezo
igniter's website (
http://www.yiqiang-piezo.com/english/production2.htm ),
the output voltage is at least 15Kv. With the four 1n4007 wired in series,
i
should get a peak reverse voltage of about 4Kv isn't? For what i know, a
diode is destroyed once it's PIV is exceeded. I think i missed something
when studying the basic operation theory of a diode. ;-) TIA for any
useful reply.
--
Alain(alias:Kilowatt)
Montréal Québec
PS: 1000 excuses for errors or omissions,
i'm a "pure" french canadian!
Come to visit me at: http://kilowatt.camarades.com
(If replying also by e-mail, remove
"no spam" from the adress.)