Re: How does this circuit keep a constant 20ma going to the LED
- From: "petrus bitbyter" <pieterkraltlaatditweg@xxxxxxxxxxxxxxxxx>
- Date: Wed, 28 Nov 2007 12:07:42 +0100
"Chris W" <1qazse4@xxxxxxx> schreef in bericht
news:1p83j.3662$KK1.2400@xxxxxxxxxxxxxxx
I purchased a Ramsey Electronics BL1 kit. It is a very simple LED blinker
kit. All it does is alternately blink one led then the other. It seems
pretty obvious how it does the blinking. But the kit says it will work on
any where from 3V to 15V. What I can't understand, is how it can do that
and still provide the same 20ma to the LEDs. Or maybe it doesn't do that
but the book sure makes it sound that way.
Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?
You can see the schematic here...
http://hp15c.org/BL1.gif
Q1 and Q2 are 2N3904
R1 and R2 are 100 ohms
R3 and R4 are 47k ohms
C1 and C2 are 4.7uf
--
Chris W
KE5GIX
"Protect your digital freedom and privacy, eliminate DRM,
learn more at http://www.defectivebydesign.org/what_is_drm";
Ham Radio Repeater Database.
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You left out the most important component value: The LEDs. Unless they're
special types, the current through a LED will rise to about 130mA at 15V.
That's high, way too high for an ordinary 20mA LED. Even if you look only to
the average, so 65mA, it's too high. If you don't blow them up at once, they
will live short at that high a current. Now there are numerous
extra-ordinary LEDs these days. Types that can handle over 100mA. But I
don't expect that types to appear in a simple, lowcost kit like the one at
hand seems to be.
petrus bitbyter
.
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