Brochure

ESG
8.468.46
8.468.46
8.46
SSR Operation
For a better understanding of SSRs, an SSR Operational
description is included. It has to be said that an in-depth
understanding of the internal circuitry of an SSR and
how it functions are not in themselves a prerequisite to
the use of SSR in many applications.
Most SSRs in the higher current ranges are offered with
either ac or dc control options. Indeed many have some
form of current limiting at their input in order to provide
a practical operating voltage range.
dc inputs
Figs. 2A und 2B illustrate two typical dc input circuits for
controlling current through the photocoupler LED. The
low end of the input range is tailored to provide the
minimum input current required to operate the SSR, at
the specified turn-on (must on) voltage (typically 3 volts
dc). The high end of the range by dissipation in the
current limiting component (typically 32 Vdc).
ac inputs
ac inputs models are usually suitable for both 120 und
240 Vac line voltages, with a typical operating range of
90 to 280 Vac and 60 kΩ input impedance. Full wave
rectification is used, followed by capacitive filtering and
dropping resistors, as shown in Figs. 3A und 3B. While
both circuits work equally well, the circuit in Fig. 3B is
favoured as being more reliable and fail safe, since two
or more components would have to fail to create an
unsafe situation.
In the circuit of Fig. 3A, a single diode breakdown would
place a dead short across the incoming line, thus crea-
ting a possible heat hazard.
Either of the ac input circuits in Fig. 3 is also capable of
operating from a dc source and, therefore, might be
considered as ac-dc; however, SSR inputs are rarely
characterised in that way. The circuit of Fig. 3B should
operate with a dc control range similar to that of the ac
(RMS) source. On the other hand, the circuit of Fig. 10A
might have dissipation problems with the input resistors,
since they would no longer operate at a 50% duty cycle.
In both cases, the SSR would have the uniqueness of
operating from a dc signal of either polarity.
Well designed ac input-output SSRs can operate from
separate power sources operating at different frequen-
cies, as long as they are both within the specified limits
of voltage, frequency and isolation. Line frequency for
both input and outputs is typically specified as 47 to 63
hertz, the upper limit of which is not critical for the input
control power since the input is rectified and filtered.
However, the upper frquency limit for an output is less
flexible, especially for a triac, which has definite limitati-
ons, related to its ability to commutate off. An SCR
+
–
Protective diode
(parallel)
DC control
+
–
Protective diode
(series)
DC control
Fig. 2A Dropping resistor
Fig. 2B Constant-current resistor
AC control
AC control
Fig. 3B: Bridge input
Fig. 3A: Two-diode input
As a precaution against inadvertent voltage reversal, a
series or inverse parallel diode is usually included in the
input circuit. With an inverse parallel diode resistor, so
brief voltage transients of a higher magnitude will not
damage the diode or LED. However, the series diode is
favoured because it also raises the level of voltage noise
immunity by a value equal to its forward voltage drop.
phase angle and frequency of the line, and in the case of
the zero voltage/current types, may be deliberately
prolonged. In the case of ac input control, the operating
speeds of both the EMR and SSR are similarly extended
due to phase angle and filtering considerations.