Brochure

ESG
8.508.50
8.508.50
8.50
IC and other drive sources
Most CMOS and NMOS logic families will not directly
interface with SSRs, except for a few specially designed
types. However, a CMOS buffered gate can reliably drive
an SSR that has low input power requirements (i.e.
>1500 ohms at 5 volts) and is also driven in the sink
mode the same as TTL.
Fig. 7 shows 1/6 of 4049 (inverting) or a 4050 (non-
inverting) CMOS hex buffer driving such an SSR with a
common 5 volt supply. CMOS can, of course, operate at
higher voltages, but care must be taken not to overstress
the gate with excessive dissipation.
Integrated circuits with open collector outputs are also
commonly used to drive SSRs, as in Fig. 8. The open
collector IC has an output transistor without an active
(transistor) or passive (resistor) pull-up and generally
has enough power to drive an SSR directly. Open collec-
tor outputs can also be logically ORed like discretes, so
that the SSR may be controlled by any one of the many
outputs. Furthermore, the SSR supply voltage does not
have to be the same as the IC Vcc, provided that one side
is common and the transistor and SSR maximum
voltages and currents are not exceeded.
SSRs do not generally require pull-up or shunt resistors
for noise reduction or any other functional reason. An
open input, if not assigned to a particular logic level,
produces an open or off state in the output (unless
otherwise designated). Input lines would have to be
extremely long and through noisy environments before
noise of any significance would appear at the input
terminals to cause the SSR to change state.
Sink
(0)
Source
(1)
V
DD
SSR
(1) (2,5 V Source)
(0) 0,4 V (Sink)
3,2 mA max.1,2 mA max.
CMOS buffered gate
1500 ohm Minimum
impedanz at 5 V
Fig. 7: Buffered CMOS gate driving a high input impedance SSR
V
CC
+
Input
Output
SSR
+
–
O/C
Out
(a)
O/C
Out
(b)
O/C
Out
(c)
+
–
I
C
(a) or
(b) or (c)
–
+
–
Fig. 8: Open collector IC driving SSR in logically ORed configurati-
on