trigger capacitor
Jussi
I'm using a 28 nF capacitor to trigger a 60 Ws flashtube, driven by a
680 uF, 300 V main capacitor. The triggering seems to work fine.
However, I've noticed that somewhat larger trigger capacitors are often
used. Is there any use to change it to larger?
Another thing, would it be possible to use an SCR in series with the
flashtube to control the duration of the pulse with for example NE555
chip?
Mike Poulton
> I'm using a 28 nF capacitor to trigger a 60 Ws flashtube, driven by a
> 680 uF, 300 V main capacitor. The triggering seems to work fine.
> However, I've noticed that somewhat larger trigger capacitors are
> often used. Is there any use to change it to larger?
The trigger capacitor, or the energy storage capacitor? If the trigger
cap works, then there is no point in using a bigger one. However, you
are only giving your 60J flash tube a bit over 30J. It could handle
almost twice that much flash energy.
> Another thing, would it be possible to use an SCR in series with the
> flashtube to control the duration of the pulse with for example NE555
> chip?
Not easily. There are ways of shortening the flash pulse, but they do
not involve using just a series SCR and timer. When an SCR turns on, it
latches -- you can't turn it off again until current flow through it
stops. You could forcibly turn it off using a series pulse transformer
to produce a negative spike that reverse-biases the SCR. That would
certainly be possible, but is a bit complex. Varying the storage
capacitor voltage is easier, if you just want less energy per flash. If
you need to be able to control pulse width while keeping peak power
high, then you would need to use some kind of semiconductor switch in
series with the flashlamp. For 60J at 300V, mosfets may be more
practical than forced commutation of an SCR.
--
Mike Poulton
MTP Technologies
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Live free or die!
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Sam Goldwasser
. On 25 Nov 2002, Jussi <jika...@cc.hut.fi. said:
.
. . I'm using a 28 nF capacitor to trigger a 60 Ws flashtube, driven by a
. . 680 uF, 300 V main capacitor. The triggering seems to work fine.
. . However, I've noticed that somewhat larger trigger capacitors are
. . often used. Is there any use to change it to larger?
.
. The trigger capacitor, or the energy storage capacitor? If the trigger
. cap works, then there is no point in using a bigger one. However, you
. are only giving your 60J flash tube a bit over 30J. It could handle
. almost twice that much flash energy.
Yes, but at what life? 60J may be the explosion energy.
. . Another thing, would it be possible to use an SCR in series with the
. . flashtube to control the duration of the pulse with for example NE555
. . chip?
.
. Not easily. There are ways of shortening the flash pulse, but they do
. not involve using just a series SCR and timer. When an SCR turns on, it
. latches -- you can't turn it off again until current flow through it
. stops. You could forcibly turn it off using a series pulse transformer
. to produce a negative spike that reverse-biases the SCR. That would
. certainly be possible, but is a bit complex. Varying the storage
. capacitor voltage is easier, if you just want less energy per flash. If
. you need to be able to control pulse width while keeping peak power
. high, then you would need to use some kind of semiconductor switch in
. series with the flashlamp. For 60J at 300V, mosfets may be more
. practical than forced commutation of an SCR.
Another approach is to use a parallel SCR or quenchtube to divert the
current into a capacitor for long enough to allow a series SCR to turn
off. Some energy is wasted but the net effect is to save most of it.
A series MOSFET would need to handle hundreds of AMPS! SCRs can do it
on a single cycle basis.
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Bill Jeffrey
> > Another thing, would it be possible to use an SCR in series with the
> > flashtube to control the duration of the pulse with for example NE555
> > chip?
>
> Not easily. There are ways of shortening the flash pulse, but they do
> not involve using just a series SCR and timer. When an SCR turns on, it
> latches -- you can't turn it off again until current flow through it
> stops. You could forcibly turn it off using a series pulse transformer
> to produce a negative spike that reverse-biases the SCR. That would
> certainly be possible, but is a bit complex. Varying the storage
> capacitor voltage is easier, if you just want less energy per flash. If
> you need to be able to control pulse width while keeping peak power
> high, then you would need to use some kind of semiconductor switch in
> series with the flashlamp.
A lot of cameras are able to truncate the flash when the exposure sensor
determines that enough light has been received. I am under the
impression that they do this by connecting an SCR in parallel with the
main flash capacitor (or maybe in parallel with the flash tube). When
this SCR is triggered, it dumps all the remaining stored energy to
ground. Not too efficient if your flash is battery-powered and you want
multiple flashes - but certainly simple.
One can also buy GTO (gate turn-off) thyristors, as I recall. Similar
to an SCR or triac, but with another layer of semiconductor in the
structure somehow. A Google search on the phrase "gate turn off
thyristor" brings up several hundred hits.
Bill
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