Re: A reminder

Bruce In Bangkok <decypher.address@xxxxxxxx> wrote in
news:4nr2t45h3rm1q7uq31pgus8shbdsoadunc@xxxxxxx:

Any thoughts on using a 555 as a pulse width modulator to drive a LED
from a 12 volt source? I am presently using a 7806 and a resister but
that ends up drawing about as many amps as an incandescent light.



That's exactly how an LED taillight on a vehicle or trailer works. When
you turn on the taillight circuit wire, the lights are pulsed with a
square wave, giving you the illusion of "dim" from your eye's optical
persistence, how you see this message on your screen. When the brake
wire is fed, the LED comes on full DC brightness. There's only one set
of LEDs in them.

The 555 should drive a powertab transistor so you can run as many LEDs
off it as you wish, limited only by the powertab's peak current rating.
You won't need much of a heat sink on this transistor because you'll
make SURE that when it is on it is fully saturated by selecting its
series base resistor. A normally-off power MOSFET used in small
inverters eliminates any load at all on the 555 and is more efficient as
it saturates in a snap. There are lots of models. I'm currently
working on a switching audio power amp with such MOSFET output
transistors. The resting feed with no audio input is a square wave fed
to each gate one N-channel one P-channel. The average DC output fed to
a series choke with capacitance on its output towards the speaker load
is zero...50% on, 50% off. The frequency is around 100 Khz and the L-
filter blocks this frequency because its rolloff is around 25 Khz above
the audio we want out. The speakers feel nothing.

When audio is applied to the pulse width modulator, it varies the width
up and down around 50% duty cycle and the output fed through the
averaging circuit L-filter on the output is a VERY low impedance, up to
90V peak-to-peak audio. The transistors don't even get warm when the
audio is painfully loud! These MOSFET switchers are rated for
continuous saturated current of THIRTY AMPS! Their switched off Source
to Drain voltage is rated at 60 or 80 VDC, as I remember.

Make sure you switch the 555 at a very high rate, ABOVE the audio range
so if anything is near the wires that's magnetic, it won't make it
"sing" when dimmed. LEDs can switch on and off at fantastic rates and
STROBE LIKE HELL so you want to make SURE you can't detect their
strobing or it will drive you crazy to sit in their light....like being
in a store or gym lit by stupid Mercury vapor or Metal Halide gas lamps
on 60 Hz. The frequency isn't critical as it's no where near how fast
the 555 can switch it for you. Don't go crazy, however, as you'll end
up with inductive spikes from the wiring.

Next time you're on a street at night and a car with LED taillights is
going away from you so you can see it without its brake lights on, move
your eyes rapidly back and forth and you can see their flashing on and
off at a rapid rate in taillight mode....

Series analog regulator, thankfully for the house batteries, are
HISTORY!

I bet somebody already makes an LED dimmer that works this way....

Here's a custom IC that's cheap and the application note:
http://www.scribd.com/doc/11968289/Multichannel-LED-Dimmer-with-
CapSense-Control

Another source Google found:
http://www.docstoc.com/docs/4107556/Multichannel-LED-Dimmer-with-
CapSense-Control

Of course, Someone in Vienna is leddimmer.com:
http://leddimmer.com/dateien/Deutsch/struktur/home.html

Ah, here's a nice looking one for $15 from ebay in Singapore:
http://cgi.ebay.com.sg/DC-12V-8A-LED-Dimmer-adjustable-brightness-
controller_W0QQcmdZViewItemQQitemZ360136683352

Easy to mount, will look nice if the dimmer is behind a panel with only
its knob protruding through. Price is right.....(c;]

I'm sure Asia has millions of other examples and hits closer to home.

.

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