G&L MMC-A2, Hardware Manual

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Appendix A
-
Operation Details
G & L Motion Control Inc. MMC Hardware Manual 173
A.3 DC Outputs
A DC Output consists of a solid state switch. It turns on or off according to the
logic state sent to it by the application program. If the application program sends it
a logic 1, the switch closes and the device is powered. If the application program
sends a logic 0, the switch opens and power to the device is cut off. The
IMPORTANT
Switching devices can sometimes have a leakage current that ex-
ceeds the IT
min
(current allowed when off) of an input module. In or-
der to use such a device, an impedance (typically, a resistor) needs to
be used in parallel with the input.
For example, some of the newer proximity switches use two wires in-
stead of three. The third wire was used for a power or ground line.
Without the third wire, the switch is easier to install. However, it re-
quires more leakage current in the off state to power its internal cir-
cuitry.
As a conservative estimate, use the following formula to calculate an
external resistance value. It keeps the input voltage at or below 2.4V
when the switching device is in the "off" state.
If the switch leakage specification is
≤
1.7 mA, then:
Use a resistor less than or equal to 2.5 K
Ω
. Be sure that the wattage
is adequate for the resistor when the switching device is in the “on”
state remembering that:
2.4 V
Switch Leakage 0.75 mA –
--------------------------------------------------------------- R
≥
2.4 V
1.7 0.75 mA –
-------------------------------- 2.5 K
Ω ≥
P
V
ON
2
R
---------- =