■ FIGURE 4
■ FIGURE 5
the second is just for
safety. The resistor, R1, is
just to provide a path for
the leakage current of the
two LEDs.
You can build it on
perf board, soldering the
leads together.
ATo use a resistor, a known
constant load is required. In
your case, the load varies so
the resistor will not work. The
ECG184 is okay, but it does not have a
hole to fasten it to a heatsink and it is
an NPN. I prefer to use a PNP in this
application in order to have negative
common. Another solution would be
to use a switching regulator, but it would
be complicated and no advantage un-
■ FIGURE 6
26 January 2008
less the efficiency was better than 75%.
In Figure 4, the PNP transistor
dissipates 4. 5 watts (3V X 1.5A), so a
heatsink is needed. The heatsink in the
parts list is rated 32 degrees C rise at
six watts, so it will be adequate. The
LEDs — D1 and D2 — are green to give
the proper voltage drop. Connecting
them in parallel is not recommended
but in this case, the normal current
will not exceed the rating of one LED;
DIMMING
THERMOSTAT
QI want to build
a dimming thermostat type circuit to control a
50 to 100 watt bulb at 240
VAC. It needs to have a settable range
between 70 to 100 degrees F. When
the set temperature is reached, the light
should dim down to hold the desired
temperature. I’ve looked at some
simple diac/triac dimmer circuits. Is it
possible just to add a thermistor in line
with the potentiometer?
This is a very useful type of circuit
for many animal/reptile keepers.
Many thanks for any advice you
may have.
— Tony Flynn, Ireland
AIn a triac dimmer,
the temperature
increases as the resistance is decreased;
the thermistor resistance
decreases as the temperature
increases so it is not suitable to
put in series with the potentiometer. I suggest using an
LM34 temperature sensor
which outputs 10 millivolts per
degree F. An LED and cadmium
sulfide cell could be used to
control the triac, but I think
rectifying to DC and using a
