Recognition Systems HANDKEY II Manual page 18

Planning an Installation
HandReader outputs, when measured to ground, generally show around 4.5
volts when they are inactive and no load is attached. This voltage is developed by
a combination diode and series resistor pull up to the internal +5 volt supply of
the HandReader. These outputs are pulled up internally to insure that they
remain in a known condition if used to output Wiegand or magnetic stripe data
to some external device.
If one of the outputs is shorted to ground, there will be approximately 5 mA of
current flowing through the short, but no damage will occur. Because of the
open collector structure of the outputs, each output is free to float to whatever
external voltage is applied (when inactive). For example, if one side of a relay coil
is connected to an external +12 volt power source and the other side of the relay
coil is measured with respect to the ground of the external power source, the
measurement will be +12 volts.
If the ground of the external power source (+12 volt return) is tied to the ground
of the HandReader, and the free relay coil wire connected to the LOCK output,
the LOCK output pin will read +12 volts also (when inactive). This is because the
LOCK output is not active and free to "float" to whatever external voltage is
applied. When a hand is verified, the LOCK output becomes active and
essentially looks like a short to the HandReader ground. This "short" causes the
full +12 volts of the external power source to be placed across the relay coil,
energizing the relay. The ground of the external +12 volt source must be tied to
the HandReader ground to make a complete circuit path.
All HandReader outputs are rated at +24 volts DC maximum with a maximum
current draw of 100 mA. This means that it is acceptable to use up to a +24 volt
DC external power supply to energize external devices. Whatever external relay
is used should be chosen to match the external power supply voltage. For
example, if the external relay coil is rated at 15 volts, a 15 volt external power
supply should be used. In no case should the external voltage be higher than
+24 VDC.
Each HandReader has a protection mechanism built in to protect against voltage
transients (spikes) coming back into the HandReader from an external relay coil.
Transients from an "opening" or de-energizing relay coil can reach several
hundred volts. This protection is on all HandReader outputs and will limit
reverse spikes to approximately 28 volts to protect the open collector transistor
driver. HandReader outputs are NOT designed to switch AC voltages. DC
voltages MUST be used and the correct polarities MUST be maintained.
Relays or devices connected to the lock and auxiliary outputs must not exceed
NOTE
0.1 A current draw.
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