W
whit3rd
[About using a flyback coil to store energy for an LED
pulse generator for stroboscope use... }
Yep, that'd be awkward, allright. With higher frequency input, you
still
want full charging of the inductor, so lets only allow 10ms (this
would
let the constructor reproduce 60 Hz and below, which is a useful
range).
To get a 1.5 mH inductor up to 0.3A with 5V (less a bit for a switch,
and for sense resistor and coil resistance), takes
t = L (0.3A)/(4.5V) = 0.1 ms
meaning that 1.5 mH is about one hundred times too small. It also
means that the turnoff transient at the regulated +5V supply is 300
mA.
Neither is good, so instead, use ten times as many turns on the
primary winding (the 5V one) as on the secondary winding (the 1.5 mH
one that drives the LED). Just as a car spark coil goes from 12V
on the primary (with -300V spike at points opening), while the HV
winding
generates 30 kV, so the doubly-wound core will have the
low-slew 5V side that ramps up, and the high-slew LED side that
quickly dumps the energy into that LED. The 'blocking diode' in
series
with the LED will have to hold off about 50V during charging.
Ten times the turns means instead of 1.5 mH, the primary winding has
N**2 * 1.5 mH,
or 150 mH inductance, and takes 0.030A instead of 0.300A at full
charge,
so the transient when it turns off is only 30 mA. I'd still prefer to
run this
off a filtered unregulated supply if that's available.
pulse generator for stroboscope use... }
Could you expand a little on this? I have some very vague things in
mind, but I'm falling short and could use a few pointers to consider.
I'm assuming for a moment that the Ron is about 3
ohms or so with a Vfwd of 3.3V. That suggests a dI=(3V-2V)/3 ohms or
(1/3)A. The dt is 200us. So the dI/dt, or V/L, is a little more than
1500. With a mean V of about 2.5V, this is on the order of 1.5mH.
Once the 200us has expired, it would be desirable to ramp up the lost
(1/3)A of dI. But over the much longer period of something on the
order of as long as 50ms (the OP mentioned 20Hz.) This suggests a V
across the L of about 10mV
Yep, that'd be awkward, allright. With higher frequency input, you
still
want full charging of the inductor, so lets only allow 10ms (this
would
let the constructor reproduce 60 Hz and below, which is a useful
range).
To get a 1.5 mH inductor up to 0.3A with 5V (less a bit for a switch,
and for sense resistor and coil resistance), takes
t = L (0.3A)/(4.5V) = 0.1 ms
meaning that 1.5 mH is about one hundred times too small. It also
means that the turnoff transient at the regulated +5V supply is 300
mA.
Neither is good, so instead, use ten times as many turns on the
primary winding (the 5V one) as on the secondary winding (the 1.5 mH
one that drives the LED). Just as a car spark coil goes from 12V
on the primary (with -300V spike at points opening), while the HV
winding
generates 30 kV, so the doubly-wound core will have the
low-slew 5V side that ramps up, and the high-slew LED side that
quickly dumps the energy into that LED. The 'blocking diode' in
series
with the LED will have to hold off about 50V during charging.
Ten times the turns means instead of 1.5 mH, the primary winding has
N**2 * 1.5 mH,
or 150 mH inductance, and takes 0.030A instead of 0.300A at full
charge,
so the transient when it turns off is only 30 mA. I'd still prefer to
run this
off a filtered unregulated supply if that's available.