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QUESTIONS & ANSWERS
I want to replace an
unit with this circuit
on my outdoor pole
sign. The sign has 76
LED bulbs (120V <
3W each), so max 26
bulbs on at a time.
Do I need to add a
resistor between the
triac gate and MT2
terminal? Do I need a
snubber network of
39 ohm res and 0.01
µF across MT1 and
MT2? Can you
improvement in the
circuit? Do you think
there will be a
problem with EMR
— Ankur Bhakta
AThere is already 180 ohms in series; I
assume that is adequate. At any rate,
you don’t need a second resistor. The
load is non-inductive, so a snubber is
not needed but it might reduce EMR.
The current is low and the photo-triac
module switches at zero crossings, so
if the wires to the bulbs are twisted,
EMR should not be a problem.
Adding a power line filter at the input
will reduce conducted EMI on the
■ FIGURE 2.
AModel trains use universal motors and will run on AC or DC. To stay with AC, you could use a variac variable
transformer but a five amp unit is
expensive. A light dimmer could work,
but they are not usually rated for so
much current. I favor DC power
because it is quieter (less EMI) and
cheaper. A simple circuit is shown in
The speed and current draw is
determined by the back EMF of the
motor; if the motor stalls, a fuse could
be blown. However, I believe that
model train transformers are
impedance protected; that is, if you
short the output, it will not produce
enough current to burn up — at least
not right away. Even so, you should
have a fuse.
QI have significant hearing loss. To aid in listening to the TV, I use a SurfLink Media device that plugs
into the audio output of the TV and
transmits a signal throughout the
room. For power, it uses a wall wart
that outputs 5V DC at 1.0 amps.
I would like to use this device in
my work shop to listen to the radio.
QCould you publish a circuit for an electric train speed controller? The input from a transformer is 20 volts AC.
The controlled motors run on AC at
zero to 20 volts at up to five amps.
— Al Rothman
■ FIGURE 3.
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