P
Peter
Guest
What does the Ifsm rating of an scr mean?
P
Phil Allison
Guest
"Peter"
** I = current, f = forward, s = single, m = maximum.
Generally it refers to the largest, once in a while, *half cycle 50/60 Hz *
current pulse allowable.
......... Phil
** I = current, f = forward, s = single, m = maximum.
Generally it refers to the largest, once in a while, *half cycle 50/60 Hz *
current pulse allowable.
......... Phil
P
Peter
Guest
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:4dtg9nF1bta8hU1@individual.net...
So I have a power supply that has a zener diode across the output to
trigger an SCR in the event of over voltage.......
The power supply's fuse is , say 10 amp. Does the SCR's rating have to be
at least 10 amp or will a lower rating one do given that the fuse in the
event of a short to ground through the SCR will fail pretty quickly
Peter
news:4dtg9nF1bta8hU1@individual.net...
Thanks
So I have a power supply that has a zener diode across the output to
trigger an SCR in the event of over voltage.......
The power supply's fuse is , say 10 amp. Does the SCR's rating have to be
at least 10 amp or will a lower rating one do given that the fuse in the
event of a short to ground through the SCR will fail pretty quickly
Peter
P
Phil Allison
Guest
"Peter"
** Knew there was a catch to this.
Post the full schematic and all the details an ABSE.
Or else piss off .
........ Phil
** Knew there was a catch to this.
Post the full schematic and all the details an ABSE.
Or else piss off .
........ Phil
T
Terry Given
Guest
Peter wrote:
half-sine-wave"
If you know something about your fuse, you *can* determine whether or
not the SCR will clear the fuse without failing, clear the fuse but
short-circuit in the process, or just plain old explode.
A fuse is characterised by the amount of energy it takes to melt it; the
term used is "I-squared-t" or I^2t. If you have an I^2t figure for your
fuse, you can compare it with the SCR's I^2t rating to see which one wins.
If SCR I^2t >> fuse I^2t, the SCR will happily blow the fuse, and keep
on ticking. This is what you really want to have happen.
If SCR I^2t << fuse I^2t, the SCR will blow itself to bits (or something
else will fail, eg a pcb track fry), without blowing the fuse. This is a
bad thing.
If SCR I^2t = fuse I^2t (or close to it) then the SCR will clear the
fuse, but will probably fail short-circuit in the process.
OK, so how do you calculate SCR I^2t? well, there are 2 ways:
1) read it off the datasheet. For really big diodes, this is an
incredibly important number - you really dont want to fuck up fuse
selection when directly connected to a 20MVA distribution transformer.
Look at the datasheet for a semikron SKKD15, I^2t is listed underneath Ifsm.
2) calculate it from Ifsm. You know its a sine wave, of say 30 Amps peak
(I'm looking at a UF400x which specs Ifsm for 8.3ms so f = 60Hz).
Square it, giving [30*sin(2*pi*f*t)]^2 = 900*sin^2[2*pi*f*t]
integrate it over one half period, giving:
I^2t = 900A^2*1/(4*60Hz) = 3.75A^2s
the general formula we have just worked out is:
I^2t = 0.5*Tfsm*Ifsm^2
because Tfsm = 0.5*1/f = 1/2f
lets check with the semikron SKKD15 diode:
http://www.semikron.com/internet/ds.jsp?file=795.html
Ifsm = 320A, 10ms (f = 50Hz, they are Germans) at 25C, so we calculate
I^2t_calc = 0.5*10ms*320^2 = 512A^2s.
the datasheet specs I^2t = 510A^2s at Tj = 25C
OK, lets get all brave and try it at Tj = 125C:
Ifsm = 280A
I^2t_calc = 392A^2s, the datasheet says 390 A^2s.
Voila.
Just a word of caution though:
fuse I^2t is NOT even vaguely linear. take a look at this:
http://www.cooperet.com/library/products/ABC_Specs.pdf
on page 2. You need to calculate I^2t rating from this graph, under the
same conditions as the SCR figure. 10ms = bottom of graph, so for a 30A
ABC fuse, 10ms I^2t = 450A^2*0.01s = 2000A^2s (this graph is RMS amps
not peak amps, and 0.5*peak^2 = rms^2).
the spec'd I^2t is 1429 A^2s, but thats specd at 1000A (ac interrupt
rating), which is off the sheet. if we run the calculation backwards,
1429 = (1000^2)*t so t = 1.4ms, which looks like a pretty good curve fit.
HTH
Cheers
Terry
Ifsm is defined as "non-repetitive peak forward surge current 8.3ms
half-sine-wave"
If you know something about your fuse, you *can* determine whether or
not the SCR will clear the fuse without failing, clear the fuse but
short-circuit in the process, or just plain old explode.
A fuse is characterised by the amount of energy it takes to melt it; the
term used is "I-squared-t" or I^2t. If you have an I^2t figure for your
fuse, you can compare it with the SCR's I^2t rating to see which one wins.
If SCR I^2t >> fuse I^2t, the SCR will happily blow the fuse, and keep
on ticking. This is what you really want to have happen.
If SCR I^2t << fuse I^2t, the SCR will blow itself to bits (or something
else will fail, eg a pcb track fry), without blowing the fuse. This is a
bad thing.
If SCR I^2t = fuse I^2t (or close to it) then the SCR will clear the
fuse, but will probably fail short-circuit in the process.
OK, so how do you calculate SCR I^2t? well, there are 2 ways:
1) read it off the datasheet. For really big diodes, this is an
incredibly important number - you really dont want to fuck up fuse
selection when directly connected to a 20MVA distribution transformer.
Look at the datasheet for a semikron SKKD15, I^2t is listed underneath Ifsm.
2) calculate it from Ifsm. You know its a sine wave, of say 30 Amps peak
(I'm looking at a UF400x which specs Ifsm for 8.3ms so f = 60Hz).
Square it, giving [30*sin(2*pi*f*t)]^2 = 900*sin^2[2*pi*f*t]
integrate it over one half period, giving:
I^2t = 900A^2*1/(4*60Hz) = 3.75A^2s
the general formula we have just worked out is:
I^2t = 0.5*Tfsm*Ifsm^2
because Tfsm = 0.5*1/f = 1/2f
lets check with the semikron SKKD15 diode:
http://www.semikron.com/internet/ds.jsp?file=795.html
Ifsm = 320A, 10ms (f = 50Hz, they are Germans) at 25C, so we calculate
I^2t_calc = 0.5*10ms*320^2 = 512A^2s.
the datasheet specs I^2t = 510A^2s at Tj = 25C
OK, lets get all brave and try it at Tj = 125C:
Ifsm = 280A
I^2t_calc = 392A^2s, the datasheet says 390 A^2s.
Voila.
Just a word of caution though:
fuse I^2t is NOT even vaguely linear. take a look at this:
http://www.cooperet.com/library/products/ABC_Specs.pdf
on page 2. You need to calculate I^2t rating from this graph, under the
same conditions as the SCR figure. 10ms = bottom of graph, so for a 30A
ABC fuse, 10ms I^2t = 450A^2*0.01s = 2000A^2s (this graph is RMS amps
not peak amps, and 0.5*peak^2 = rms^2).
the spec'd I^2t is 1429 A^2s, but thats specd at 1000A (ac interrupt
rating), which is off the sheet. if we run the calculation backwards,
1429 = (1000^2)*t so t = 1.4ms, which looks like a pretty good curve fit.
HTH
Cheers
Terry
T
Terry Given
Guest
Terry Given wrote:
but they will tell you in the datasheet.
or 10ms,
but they will tell you in the datasheet.
M
Mike
Guest
In article <44798e88$0$7875$5a62ac22@per-qv1-newsreader-01.iinet.net.au>, pmhughes@nospamozemail.com.au says...
So in your sentence above you are speculating a lower rated SCR will
not fail (or presumably encounter any harm) whilst the fuse will, by
maybe restating the question and critiquing your own logic you may
well have answered it.
--
Regards
Mike
* GMC/VL Commodore, Calais VL Turbo FuseRail that wont warp or melt !
* High grade milspec ignition driver electronics now in development
* Twin Tyres to suit most sedans, trikes and motorcycle sidecars
Web site under construction http://niche.iinet.net.au
Semiconductors are well known for being great fuse protectors !
So in your sentence above you are speculating a lower rated SCR will
not fail (or presumably encounter any harm) whilst the fuse will, by
maybe restating the question and critiquing your own logic you may
well have answered it.
--
Regards
Mike
* GMC/VL Commodore, Calais VL Turbo FuseRail that wont warp or melt !
* High grade milspec ignition driver electronics now in development
* Twin Tyres to suit most sedans, trikes and motorcycle sidecars
Web site under construction http://niche.iinet.net.au
P
Peter
Guest
Is there some unwritten rule about posting to this NG that I've missed
somewhere?
P
Phil Allison
Guest
"Peter"
** Cos YOU are bloody TROLLING !!!!!
....... Phil
** Cos YOU are bloody TROLLING !!!!!
....... Phil
P
Peter
Guest
How is a request for information trolling?
The power supply in question is a Johnson Unit that I've repaired for a
friend.
http://members.ozemail.com.au/~pebarhug/radios/Johnson_1.jpg
The semiconductors only had in house type numbers on them.
My p was a genuine question. What seems elementary to you is not to me.
P
Phil Allison
Guest
"Peter"
What does the Ifsm rating of an scr mean?
** I = current, f = forward, s = single, m = maximum.
Generally it refers to the largest, once in a while, *half cycle 50/60 Hz *
current pulse allowable.
** Knew there was a catch to this.
Post the full schematic and all the details an ABSE.
Or else piss off .
** Cos YOU are bloody TROLLING !!!!!
** DO NOT snip out the context - arsehole !
YOU are bloody TROLLING !!
YOU are wasting people's valuable time and abusing their good will.
If YOU have no FUCKING idea how to design things or do repairs - that is
100% YOUR problem !!!
Go annoy the wankers at "sci.electronics.design or "
sci.electronics.repair."
....... Phil
What does the Ifsm rating of an scr mean?
** I = current, f = forward, s = single, m = maximum.
Generally it refers to the largest, once in a while, *half cycle 50/60 Hz *
current pulse allowable.
** Knew there was a catch to this.
Post the full schematic and all the details an ABSE.
Or else piss off .
** Cos YOU are bloody TROLLING !!!!!
** DO NOT snip out the context - arsehole !
YOU are bloody TROLLING !!
YOU are wasting people's valuable time and abusing their good will.
If YOU have no FUCKING idea how to design things or do repairs - that is
100% YOUR problem !!!
Go annoy the wankers at "sci.electronics.design or "
sci.electronics.repair."
....... Phil
P
Peter Howard
Guest
"Peter" <pmhughes@nospamozemail.com.au> wrote in message
news:447aa47e$0$7899$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
Is that Phil Allison you are having a discussion with?
If so, you are wasting your time trying to reason with him. Just killfile
the graceless prick. I did, six months ago and have not missed him.
His technical knowledge is quite good but he turns to psychotic abuse for no
reason. Or maybe his reason is the gratification of getting a shocked
reaction. Whatever. He is a troll and is not worth your trouble.
PH
news:447aa47e$0$7899$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
Peter,
Is that Phil Allison you are having a discussion with?
If so, you are wasting your time trying to reason with him. Just killfile
the graceless prick. I did, six months ago and have not missed him.
His technical knowledge is quite good but he turns to psychotic abuse for no
reason. Or maybe his reason is the gratification of getting a shocked
reaction. Whatever. He is a troll and is not worth your trouble.
PH
P
Phil Allison
Guest
"Peter Howard"
** Piss off - you know nothing IDIOT.
....... Phil
** Piss off - you know nothing IDIOT.
....... Phil
P
Peter
Guest
"Peter Howard" <bbrover109@bbbigpond.net.au> wrote in message
news
Rzeg.13102$S7.8223@news-server.bigpond.net.au...
news
Rzeg.13102$S7.8223@news-server.bigpond.net.au...
A
atec77
Guest
Peter wrote:
file to be ignored.
(wink wink) so best if you're thin skinned to push it over in the wanker
file to be ignored.
A
atec77
Guest
Phil Allison wrote:
black
Queer
Kettle
black
Queer
P
Peter
Guest
"atec77" <atec77##@hotmail.com> wrote in message
news:447af7dd$0$17550$61c65585@un-2park-reader-01.sydney.pipenetworks.com.au...
individual.
news:447af7dd$0$17550$61c65585@un-2park-reader-01.sydney.pipenetworks.com.au...
Yeah - done that...no more nonsense from that quarter. What a sad,pathetic
individual.
M
Mark Harriss
Guest
Peter wrote:
went on to design SCR's for GEC or someone like that in the States.
This is a sore point for "I could have been a genius" Phil.
You weren't to know that Phil's good buddy who DID complete Uni
went on to design SCR's for GEC or someone like that in the States.
This is a sore point for "I could have been a genius" Phil.
P
Phil Allison
Guest
"Peter"
** Bloody TROLLs like Peter always squeal like stuck pigs when they are
OUTED !
........ Phil
** Bloody TROLLs like Peter always squeal like stuck pigs when they are
OUTED !
........ Phil
M
Michael A. Terrell
Guest
Phil Allison wrote:
Very true, and you have never stopped squealing, have you, troll?
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
Very true, and you have never stopped squealing, have you, troll?
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida