Application Note 045 Current Limiting vs. Current Interrupting

Application Note 045
Current Limiting vs. Current Interrupting
Solid State Optronics, Inc. has developed two types
of technology which help solid state relays (SSRs)
limit the flow of current through their output structures.
This application note will attempt to clarify the
differences between the two and describe how each
one performs.
(For the AD6C111, 120mA is the maximum
continuous load current), and causes the current
limiting and interrupting functions to be activated.
The AD6C111-L has the current limiting function
designed into it. The AD6C111-N is designed with a
current interrupting function. Behavior of both
functions is depicted below in Figure 2:
Current Limiting is a mechanism where the load
current is held to a certain value even if it tries to
exceed that value. For instance, if the limiting value
is 150mA, then during a fault condition of 300mA, the
relay will attempt to limit the current to 150mA. The
idea behind Current Limiting is to maintain a steady
load current to downstream circuitry in situations
where line impedance might be lowered momentarily
creating a slight increase in current.
Current (mA)
Time (S)
Figure 2: Device Behavior
From the graph in Figure 2, the difference in behavior
can clearly be seen. Current Limiting, present in
AD6C111-L units, limits the current to a preset value.
This value will gradually decrease over time as
package temperature begins to rise. Current
Interrupting, present in AD6C111-N units, actually
pulls the load towards zero and partially shuts the
device down.
In both types of relays, power and heat dissipation
becomes an important issue. In the case of the
AD6C111-L devices, much more heat will be
generated because the device is fighting to maintain
high levels of current even while the voltage across it
continues to rise. In the case of the AD6C111-N, less
heat is generated because the current value allowed
through the device drops as voltage rises. If no
voltage suppressor devices (MOV, TVS) are placed
across the devices, heat generated by the
components will eventually destroy the devices.
100 Ω
Amp Meter
Volt Meter
Figure 1 depicts a test circuit used to evaluate the
performance of both current limting and current
interrupting in a fault condition. The relays used were
AD6C111 devices, some with current limiting, some
with current interrupting.
Current Interrupting is a mechanism where the load
current through the SSR is rapidly pulled towards
zero during a fault condition. The Interrupting feature
is intended to protect downstream circuitry during
rapid, large transient spikes such as a lightning strike
along telephone lines.
48 VDC
Figure 1: Test Circuit
As shown in the diagram, a load of 480mA was
applied to the relay. This current exceeds the
maximum allowable continuous current for the device
AppNote 045