Flash Light Project (Feb 08 N&V)

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
klamp
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Post by klamp » Fri Mar 07, 2008 10:15 pm

Bigglez wrote:Greetings Mrai,
MrAl wrote:If you could post the circuit that would help others that dont have access to it understand what you are asking.
Here you go!

Image
Image

Comments Welcome!
Not trying to hi-jack this post but anyone know if C2, above, the 1uf 35v cap, is a disc or tantalum? I just wondered if it was a mis-print in the "Projects", as it is listed as a ceramic and I'm never heard of a 1uf ceramic !!..............

Bigglez
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Post by Bigglez » Sat Mar 08, 2008 8:17 am

Greetings (No First name Supplied),'
klamp wrote:Not trying to hi-jack this post but anyone know if C2, above, the 1uf 35v cap, is a disc or tantalum? I just wondered if it was a mis-print in the "Projects", as it is listed as a ceramic and I'm never heard of a 1uf ceramic !!
Good question! The symbol is non-polarized, but one
microfarad is high (as you pointed out). Perhaps it's
a 100nF (o.1uFD) used for spike supression.

Although not discussed earlier, I thought at the time
the capacitor is not needed because the feedback
loop would operate with either DC or a step voltage
across the sense resistor (R2).

Comments Welcome!

klamp
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Post by klamp » Sat Mar 08, 2008 9:52 am

C2 is polarized, shows ground in pix and I know C1 is for sure a tantalum.

Bigglez
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Post by Bigglez » Sat Mar 08, 2008 10:42 am

Greetings (Still No First Name Supplied),
klamp wrote:C2 is polarized, shows ground in pix and I know C1 is for sure a tantalum.
Then use a tantalum.
(BTW, Neither capacitor symbol in the article is polarized).

Comments Welcome!

klamp
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Post by klamp » Wed Mar 12, 2008 8:23 pm

your right symbol is not, circuit shows a ground. I just wondered if anyone has built this and knows what C2 is..:smile:

Keith

Robert Reed
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Post by Robert Reed » Wed Mar 12, 2008 9:26 pm

Klamp
1 uF MLC ceramics are as common as dirt. I stock over 20 of them at any given time. Every major and minor supplier stocks these componenents. Quite cheap in the "y" series. expect to pay more in the "XR7" series. They are a minimum of 50 volts, then step up to 63 and 100 volts.

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MrAl
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Post by MrAl » Thu Mar 13, 2008 4:03 am

Hi there,

Many circuits like this are built with a variable duty cycle so that the
current output stays fairly constant when current is sensed. It does
this by decreasing the duty cycle so that less energy is delivered
to the output. Even with a circuit like that however there is a point
where the circuit gets to its max duty cycle and can put out no more
additional energy. In a case like that you dont need feedback because
the duty cycle can stay at its max without a problem.
That's about the same thing that is happening with the circuit shown
here. The circuit runs at max duty cycle and when the current (sensed
by R2) gets too high it eliminates some cycles so that the average
current stays low enough to drive the LED(s) safely. At some point
however, the same thing happens: the circuit even at max duty
cycle can only put out so much energy and once there are enough
LEDs to handle that energy no feedback is needed. At this point
adding additional LEDs might not do anything as the energy will
still be the same. This means there will be some ideal number of
LEDs for a given input voltage.
Be aware however that without feedback there is no control when
the input power goes up and down with battery usage and replacement.
A new battery will cause a higher output current than a low used battery,
so the circuit would have to be tested with a new battery to make sure
the LEDs dont get overpowered and tested with a used battery to make
sure enough light is still produced to be useful.
Also be aware that when using a diode as a voltage reference instead
of a 'voltage reference diode' made for that, that the output will vary
somewhat with temperature... about 3 or 4 percent for every 10 degrees
C change in ambient temperature.
LEDs vs Bulbs, LEDs are winning.

klamp
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Post by klamp » Fri Mar 14, 2008 6:43 pm

Robert Reed wrote:Klamp
1 uF MLC ceramics are as common as dirt. I stock over 20 of them at any given time. Every major and minor supplier stocks these componenents. Quite cheap in the "y" series. expect to pay more in the "XR7" series. They are a minimum of 50 volts, then step up to 63 and 100 volts.
Ok, I'v never used them, tons of poly-film but never a ceramic 1uf. Even in the ole-days I remember large round paper caps non electrolitic of course that were 1uf, thanks for the post......... :smile:

klamp
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Post by klamp » Fri Mar 14, 2008 6:53 pm

Robert Reed wrote:
Klamp
1 uF MLC ceramics are as common as dirt. I stock over 20 of them at any given time. Every major and minor supplier stocks these componenents. Quite cheap in the "y" series. expect to pay more in the "XR7" series. They are a minimum of 50 volts, then step up to 63 and 100 volts.

Ok, I'v never used them, tons of poly-film but never a ceramic 1uf. Even in the ole-days I remember large round paper caps non electrolitic of course that were 1uf, thanks for the post......... , by the way the parts list call for a "disc-ceramic" not a MLC or surface mount I do know what they are.....

Robert Reed
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Post by Robert Reed » Fri Mar 14, 2008 9:02 pm

Hi Klamp
1 MF discs would be quite large and rare. Today the discs have two advantages - they generally come in higher voltage ratings - up to 5 KV and the lower valued ones ( up to 200 pf or so ) are available in an NPO style - very stable. I do a lot of RF design and have found that ceramics provide the best coupling and bypass capacitors. Head and shoulders above polyester. For purposes I described above, either disc or MLC work equaly well. The MLC come in a much smaller package though which is why I generally pick them. Since your parts list calls for a 35 volt rating, I see no reason not to use the MLC style.

gatoruss
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Revisiting the LED Flash Light Circuit

Post by gatoruss » Wed Dec 17, 2008 6:36 pm

I know that this thread is dated, but I have been revisiting this article. Can someone point me toward - or provide me with :) - an explanation of D1's purpose in this circuit and how it works. I gather that it sets Vr to 0.6V, but I am having trouble understanding how that is accomplished.

Thanks.

Robert Reed
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Post by Robert Reed » Wed Dec 17, 2008 7:20 pm

Once the diode is into full conduction, it will have a forward drop of 0.6 volts across its junction regardless of increased current - sort of a mini zener. At least that would be the design plan. In reality the junction drop is usually 0.65 - 0.7 volt and changes in current will alter that voltage. However modest changes in diode current (20%) will have little effect on the voltage so that node will remain at a reasonably constant level in spite of changes in supply voltage. The junction voltage will also vary somewhat with temperature but again this is insignificent for most applications.

Bigglez
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Re: Revisiting the LED Flash Light Circuit

Post by Bigglez » Thu Dec 18, 2008 2:32 am

gatoruss wrote:I know that this thread is dated, but I have been revisiting this article. Can someone point me toward - or provide me with - an explanation of D1's purpose in this circuit and how it works. I gather that it sets Vr to 0.6V, but I am having trouble understanding how that is accomplished.
Refering to figure 5 on page 34 (NandV, Feb 2008),
the silicon diode, D1, is forward biased by resistor R1,
from the bulk DC voltage provided by the battery.

The forward voltage drop of the diode, approx, 680mV
at 25degC is used by the AVR uC as a voltage ref
for the internal comparator. The other input of the
comparator is fed from the voltage developed across
R2, which in turn monitors the current in the LED.

Further, the uC pumps the voltage up to drive the
LED, and establish a current in the sense resistor R2.

LED current = V(d1)/R2 = 0.68/27 = 25mA

gatoruss
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Post by gatoruss » Thu Dec 18, 2008 3:54 am

Thanks. This is very helpful.

BTW, the BOM on page 36 indicates that R1 is 100K ohms. This gives a current of 0.0382 mA. This seems like too much resistor? In the 1N914 data sheet (if I am reading it correctly), at 0.0383 mA the Vf is around 450mV? Wouldn't a 1000 ohm resistor be more appropriate?

Thanks again.

Robert Reed
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Post by Robert Reed » Thu Dec 18, 2008 7:07 am

I have to agree that is a stingy current. When I use this application, I like to see at least 1 or 2 ma flowing thru the diode. Unless battery load is a major consideration, I would go with somewhat higher current than the 100K resistor could provide.

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