Keithley KPCI-PIO24, User Manual, Page: 28 / 66

KPCI-PIO24 User’s Manual Installation 3-11
Boosting relay drive current
Some relays require higher drive currents than standard solid-state relays. The maximum output
low sink current for each I/O line of a KPCI-PIO24 board is 64mA at 5V. If your relay requires
more than 64mA or more than 5V, you can boost the drive current and/or voltage for relay control
using the circuit shown in Figure 3-6.
Figure 3-6
NPN transistor relay control for an output of a KPCI-PIO24 board
For drive-current requirements between 15mA and 100mA, select an NPN transistor
with appropriate current capacity. The power transistor must be rated for the required
supply voltage and must have a collector current rating no higher than 0.5A. If higher current
is needed, substitute a Darlington NPN transistor.
Using +5 VDC from the computer power supply
CAUTION Do not connect the +5 VDC outputs to an ex5VDC supply.
This may damage the external supply, the board, and
the computer.
Do not draw more than 1.0 A, total, from the board to power
external circuits. Drawing more than 1.0 A may damage
the board.
The board extends power from the +5 VDC computer supply to the I/O connector (see
Figure 3-1). This power is convenient for use in light external circuits, such as pull-up resistors.
If you ensure that the following conditions are maintained, this power may also be used to
energize external accessories:
• The total current drawn to power the board and all external circuits must not overload the
computer power bus.
• The total current drawn to power all external circuits must be less than 1A.
KPCI-PIO24
Board
470
Ω
Min.
Digital
Output
# n
NPN
Transistor
Surge
Protection
Diode
Digital
Common
GND
To Relay
DC Power
Supply
+
−
Relay