Delta RMC151 - Manual (Page 1182)

RMC70/150 and RMCTools User Manual
Outputs can be wired in either a high-side or low-side configuration. Because all the
outputs share the Output Common, all outputs on the same module must be wired the
same.
Using Outputs with Resistive Loads
Figure 1: SSR switching resistive load: low-
side configuration.
Figure 2: SSR switching resistive load:
high-side configuration.
The load resistance must be sized such that the maximum current through the SSR does
not exceed 50mA. The maximum current is calculated with the following equation:
Current = V
cc
/ R
Load
For example, if the supply voltage V
cc
is 24V, and the load resistance is 480Ω, the current
will be:
Current = 24Ω / 480Ω = 50mA
which is right at the 50mA limit. The load resistance should not be decreased any further
with a 24V supply voltage.
Using Outputs with Inductive Loads
External fuses should be used to protect the SSRs if there is a possibility of over-
current. When switching inductive loads, it is important to place a diode or tranzorb
across the load to protect the switch when transitioning from an ”ON” to an ”OFF” state.
Otherwise, the collapsing magnetic field can cause a reverse voltage spike in excess of
the 30 V rating of the SSR. See figures below for details.
Figure 3: SSR switching inductive inductive load: high-side configuration.
Example: Calculating maximum current for inductive load.
To calculate the maximum current through the SSR in the above diagram, we assume
zero SSR resistance:
Max. current = 24V / 480Ω = 50mA
Max. current = 12V / 480Ω = 25mA
In the 24V case, the maximum current is right at the maximum allowed by the SSRs. The
outputs may be overpowered if the coil resistance is reduced further. To calculate the
typical current through the SSR, we use the typical SSR resistance of 25Ω:
1162 Delta Computer Systems, Inc.