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ESG
8.488.48
8.488.48
8.48
Driving the SSR
To activate an SSR output, a voltage greater than that
specified for maximum turn-on is applied to the input (3
volt dc typical). The off state occurs when zero or less
than the minimum turn-off voltage is applied (1 volt dc
typical). For an AC input type, the typical values would be
90 volts RMS for on, and 10 volts RMS for off. For an
SSR designated as normally closed or form B, the
previous on-off conditions would be reversed. Generally,
normally open is the accepted, but undesignated,
standard for the SSR.
dc is considered as being a steady-state dc voltage of
one polarity, and ac is a reasonably well shaped sinusoi-
dal waveform.
Due to consideration of input to output isolation, the
switch controlling the input to an SSR can be placed in
series with either of the two input terminals, assuming
polarity is observed (dc). The same flexibility applies to
the output side, where the load may also be placed in
series with either output terminal. There are a few
specialised types, usually with more than two input or
ouptut terminals, that have dedicated functions (i.e. Vcc
logic input and common).
The activating signal may be derived from mechanical
contacts or solid state devices such as those shown in
figure 6 The minimum supply voltage through these
contacts may be equal to the SSR turn-on voltage (3
volts dc typical), whereas the positively or negatively
referenced transistors require a minimum supply
voltage a few tenths of a volt above the specified turn-on
threshold, say 3.5 volts
Solid State Relay characteristics
Systems offers an extensive range of Solid State Relays
in various package styles, mounting options, terminal
types and switching capability.
Selecting the ideal SSR
In a bid to specify the exact SSR for an application, it is
important to consider the:
• input drive requirement
• output current
• load or output current
• the isolation and installation requirements
In many instances the load power will dictate whether
the SSR is PCB, panel, or DIN rail mounted. In loads
higher than 5 to 7 amps, a heat sink becomes necessary
to remove heat from the SSR body. Certain SIG Positec
Systems designs include integral heath sinks, while
others have dissipation characteristics that are inherently
within the products.
dc. This is because of their approximate 0.2-0.4 volt on
state voltage drop when driven in the grounded emitter
(saturating) mode.
+3,5 V min.
PNP
Input
Output
SSR
+
–
+3,5 V min.
Input
Output
SSR
+
–
NPN
+4,5 V min.
Input
Output
SSR
+
–
Sink mode
Source mode
TTL
Right
Wrong
Fig. 5C: TTL gate
Fig. 5B: NPN transistor
Fig. 5A: PNP transistor