J
Jon Kirwan
Guest
On Sun, 12 Jul 2009 05:11:36 -0700 (PDT), fungus
<openglMYSOCKS@artlum.com> wrote:
make anything out of that except that it looks about like what I'd
expect as the voltage at the top of the LED stack with the capacitor
in place (__if__ the amplitude were small... on the order of tens of
millivolts.)
This is instead the voltage across a 4 ohm resistor (which should also
look like the voltage at the top of the LED stack), but the ripple
height should be low -- on the order of millivolts, too.
sounds very bad to me.
okay. (1/4 second or more.) I hope it has a high enough voltage
spec, too! If not, bad news there.
Perhaps the better thing to do right now is to replace your LEDs with
a resistor! And play it safer until the darned thing is working
right.
With 6 LEDs running on say 30mA and call it 21V, you need 21V/30mA or
700 ohms. That's going to burn off over 1/2 watt, too! So make it a
2 watt resistor. (Don't use an 1/8th watt, unless you use a LOT of
them paralleled up.) 700 ohms is hard to find, so use something in
that area or parallel a few to get close to it. Just keep in mind
that power figure! It's a lot.
.....
Maybe someone can do somewhat better than me, looking at your
pictures. There is a sharp bottom in both pictures. But I don't know
if that is located at zero volts, or not. However, I would guess it
isn't at zero volts because of the sharp bottom. But there is 250mA
variation here (1V peak to peak) you say. So whatever the bottom
voltage is, that will tell us the minimum current reached. But we do
know there is a ripple in the current of 250mA, which is a lot. It is
possible you are hammering your LEDs with a lot more than that, given
that the low point doesn't flatten out.
Can you read off the voltage at the bottom, there? That will give a
little more information. (Assuming things don't change again!)
.....
In the meantime, see about getting some toroid cores. Now here I'm
out of my water depth. I don't know a good supplier for these and I'm
not well versed on materials, generally. Here are a couple of sites
that describes the various ferrite materials. (They are numbered.)
http://www.bytemark.com/products/ferrmat.htm
http://www.bytemark.com/products/fermgprp.htm
Personally, I'd prefer something called "high volume resistivity" and
the highest mu you can get is is probably material #43. The high
volume resistivity helps ensure you won't short things out with the
core, itself. However, if you are using insulated wire and winding
smoothly, I suppose any of the core materials are fine so long as they
don't have high losses at the frequencies of interest. (On that
score, if they rate losses at frequencies below 1MHz, you probably
want to stay away from it.) This limits you to #43, #61, #64, and
#67. The lowish permeability of #67 might be a bit restrictive,
though. Here's another page that talks a little about the materials
and something called AL.
http://www.bytemark.com/products/ferral.htm
The AL value is used for figuring out roughly what kind of inductance
you are likely to get with some number of windings. The fuller
formula for that is here:
http://www.bytemark.com/products/sectwo.htm
A comment about the cores. Here's a page on their sizes:
http://www.bytemark.com/products/ferrphys.html
An F-23 is just big enough to wind one layer of perhaps 30 windings
using #32 wire and maybe 14 windings of #26 wire. Unless you get
something with a large AL figure, you won't get too close to 200-400
uH with a single layer. You can always wind more, of course. Larger
cores let you wind more windings on the first layer and the inner hole
size is what will limit you on total windings, even if stacked.
Let's say you want to limit yourself to winding no more than 50 turns
for primary and secondary, each, and that you want 400uH for each.
Reading this again:
http://www.bytemark.com/products/sectwo.htm
You can see that you want AL=1E9*L/N^2, if L is in henries. With N=50
and L=400e-6, you get AL=160 (or more.) So you need to find something
around that value. Looking back at:
http://www.bytemark.com/products/ferral.htm
You can see that F-23 with #43 material fits the bill. The problem
will be winding that many turns on something that small. That means
fine wire and stacked windings, probably. But it would be tiny. An
F-240 with #61 looks about right. But I bet they will be expensive.
Another possibility is to look at increasing the frequency a bit
(reduces the inductance.) With L=200e-6, you only need AL=80 with 50
turns per winding. And so on.
Take a look at: (no recommendation here, I've never used them)
http://www.cwsbytemark.com/index.php?main_page=index&cPath=206_221
Anyway, finding the right core and the right supplier is a process.
Jon
<openglMYSOCKS@artlum.com> wrote:
Without a scale, both of these look about the same to me. I couldn't
make anything out of that except that it looks about like what I'd
expect as the voltage at the top of the LED stack with the capacitor
in place (__if__ the amplitude were small... on the order of tens of
millivolts.)
This is instead the voltage across a 4 ohm resistor (which should also
look like the voltage at the top of the LED stack), but the ripple
height should be low -- on the order of millivolts, too.
Okay... that's good.
Okay... that's bad. This suggests 1V/4ohm = 250mA variation. Which
sounds very bad to me.
Okay. That will take some serious time to charge up, if it is working
okay. (1/4 second or more.) I hope it has a high enough voltage
spec, too! If not, bad news there.
Sounds like a changing situation. Parts are getting hammered.
Perhaps the better thing to do right now is to replace your LEDs with
a resistor! And play it safer until the darned thing is working
right.
With 6 LEDs running on say 30mA and call it 21V, you need 21V/30mA or
700 ohms. That's going to burn off over 1/2 watt, too! So make it a
2 watt resistor. (Don't use an 1/8th watt, unless you use a LOT of
them paralleled up.) 700 ohms is hard to find, so use something in
that area or parallel a few to get close to it. Just keep in mind
that power figure! It's a lot.
.....
Maybe someone can do somewhat better than me, looking at your
pictures. There is a sharp bottom in both pictures. But I don't know
if that is located at zero volts, or not. However, I would guess it
isn't at zero volts because of the sharp bottom. But there is 250mA
variation here (1V peak to peak) you say. So whatever the bottom
voltage is, that will tell us the minimum current reached. But we do
know there is a ripple in the current of 250mA, which is a lot. It is
possible you are hammering your LEDs with a lot more than that, given
that the low point doesn't flatten out.
Can you read off the voltage at the bottom, there? That will give a
little more information. (Assuming things don't change again!)
.....
In the meantime, see about getting some toroid cores. Now here I'm
out of my water depth. I don't know a good supplier for these and I'm
not well versed on materials, generally. Here are a couple of sites
that describes the various ferrite materials. (They are numbered.)
http://www.bytemark.com/products/ferrmat.htm
http://www.bytemark.com/products/fermgprp.htm
Personally, I'd prefer something called "high volume resistivity" and
the highest mu you can get is is probably material #43. The high
volume resistivity helps ensure you won't short things out with the
core, itself. However, if you are using insulated wire and winding
smoothly, I suppose any of the core materials are fine so long as they
don't have high losses at the frequencies of interest. (On that
score, if they rate losses at frequencies below 1MHz, you probably
want to stay away from it.) This limits you to #43, #61, #64, and
#67. The lowish permeability of #67 might be a bit restrictive,
though. Here's another page that talks a little about the materials
and something called AL.
http://www.bytemark.com/products/ferral.htm
The AL value is used for figuring out roughly what kind of inductance
you are likely to get with some number of windings. The fuller
formula for that is here:
http://www.bytemark.com/products/sectwo.htm
A comment about the cores. Here's a page on their sizes:
http://www.bytemark.com/products/ferrphys.html
An F-23 is just big enough to wind one layer of perhaps 30 windings
using #32 wire and maybe 14 windings of #26 wire. Unless you get
something with a large AL figure, you won't get too close to 200-400
uH with a single layer. You can always wind more, of course. Larger
cores let you wind more windings on the first layer and the inner hole
size is what will limit you on total windings, even if stacked.
Let's say you want to limit yourself to winding no more than 50 turns
for primary and secondary, each, and that you want 400uH for each.
Reading this again:
http://www.bytemark.com/products/sectwo.htm
You can see that you want AL=1E9*L/N^2, if L is in henries. With N=50
and L=400e-6, you get AL=160 (or more.) So you need to find something
around that value. Looking back at:
http://www.bytemark.com/products/ferral.htm
You can see that F-23 with #43 material fits the bill. The problem
will be winding that many turns on something that small. That means
fine wire and stacked windings, probably. But it would be tiny. An
F-240 with #61 looks about right. But I bet they will be expensive.
Another possibility is to look at increasing the frequency a bit
(reduces the inductance.) With L=200e-6, you only need AL=80 with 50
turns per winding. And so on.
Take a look at: (no recommendation here, I've never used them)
http://www.cwsbytemark.com/index.php?main_page=index&cPath=206_221
Anyway, finding the right core and the right supplier is a process.
Jon