ELM440
Example Application
The ELM440 can be easily configured to
Using a rectifier circuit for the reference signal
can pose some problems if one is not careful. Full-
wave circuits can be particularly troublesome as
all of the diodes may cease to conduct after the
peak of the waveform, and the input voltage level
is then essentially floating. In this case, at
minimum, a large value (470KW) resistor should
also be connected between the sync input and
VSS, to provide a ground reference during these
times.
provide a 60Hz output as shown in Figure 1 below.
A 3.579545MHz crystal is connected to the
integrated circuit, along with suitable loading
capacitors, and the 60Hz output is then taken from
pin 7. Pin 5 is grounded, as no reference signal is
supplied. As usual, it is also good practise to place
a bypass capacitor across the power supply as
well.
Figure 2 shows how easily this circuit can be
modified to operate in synchronism with the AC
power line. A reference signal is taken from one
half of a 16Vrms CT transformer winding through a
47KW resistor. Both circuits share a common VSS.
The resistor was chosen to limit peak currents in
this circuit to about 0.24 mA, which is well within
the IC’s capabilities. The ELM440 will continue to
use this signal as it’s reference, as long as it is
available. Upon loss of the AC, the circuit will
automatically switch over to using the crystal as a
reference.
Both circuits shown are simple, leaving little
room for change. One modification that might be
considered for either is to replace one of the fixed
value capacitors with a variable one, in order to be
able to trim the oscillator frequency for greater
accuracy.
+5V
0.1µF
1
2
3
4
8
7
6
5
60Hz Out
3.58MHz
27pF
27pF
Figure 1. Typical Connections
+5V
0.1µF
1
2
3
4
8
7
6
5
60Hz Out
3.58MHz
27pF
27pF
47KW
8V
8V
+
Figure 2. Using the Sync Input
ELM440DSB
Elm Electronics – Circuits for the Hobbyist
4 of 4
< http://www.elmelectronics.com/ >
ELM [ ELM ELECTRONICS ]
