Choosing an input resistor for a microelectronic relay Oct 06, 2004 | PDF | 44 kb

Application Note AN-101
Choosing an Input Resistor
for a Microelectronic Relay
by Bill Collins, International Rectifier
Table of Contents
Page
Introduction ..........................................................................................1
Procedure.............................................................................................1
The International Rectifier Photovoltaic Relay (PVR) devices are current-controlled
microelectronic relays with a specified current which must be supplied for turn-on. Therefore, a
current limit resistor is necessary when operating from a voltage source. This application note
gives the procedure for determining the proper resistor to program the microelectronic relays to
operate from any control voltage.
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AN-101
cover
APPLICATION NOTE
AN-101a
International Rectifier • 233 Kansas Street, El Segundo, CA 90245 USA
Choosing an input resistor for a Microelectronic Relay
by Bill Collins
Introduction
The International Rectifier Photovoltaic Relay (PVR) devices are current-controlled microelectronic relays with a specified current which must be supplied for turn-on. Therefore, a current limit
resistor is necessary when operating from a voltage source. This application note gives the procedure for determining the proper resistor to program the microelectronic relays to operate from any
control voltage.
Procedure
The selected resistor must be of sufficiently low value that the specified turnon current flows at the minimum signal voltage and lowest operating temperature.
Note that the input circuit shown in Figure
Figure 1. Input
1 consists of the internal Light Emitting
Diode (LED) plus the external resistor which is being selected.
Circuit
To determine the maximum allowable value of RC, the maximum LED forward voltage drop at the
coldest operating temperature should be determined from the input characteristics curve found in
each respective technical data sheet. An example is shown here as Figure 2.
RC <
=
Ein - E LED
Ic (turn-on current)
Input Current (mA)
The value normally used for -40oC operation is
1.6VDC. The following equation expresses the
maximum allowable value for RC .
Example: Ein (Min.) = 4.5 VDC;
o
Ic = 5 mA; TA >
= -40 C
RC <
=
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4.5V -1.6V
.005A
<
= 580 Ohms
LED Forward Voltage Drop (Volts DC)
Figure 2. Input Characteristics (Current Controlled)
1
AN-101a
A minimum allowable value of RC is set by the
necessity of not allowing the input current to
exceed 25 milliamperes at the highest signal
voltage and maximum operating temperature. A
high temperature LED drop of 0.9 volts is most
commonly used.
In the above examples a resistor in the calculated range and near the maximum allowable
value would be selected, for example 500 Ohms.
Ein - E LED
RC >
= I (Max. allowable current)
c
Example: Ein = 6.0V Max; Ic
o
Max. = 25 mA; TA <
= 85 C
6.0V - 0.9V > 204 Ohms
RC >
=
=
.025A
of RC corresponding too 25 mA maximum input
current also is plotted. These steps should be
followed to determine an appropriate input resistor.
1.
Determine the minimum available input volt
age and read the maximum allowable RC
from the plot corresponding to the selected
signal current (in this case, 6 mA or 12 mA).
2. Read the maximum allowable input voltage
for the selected resistor value by checking
the bottom “minimum allowable RC” plot. The
allowable input signal voltage range has now
been determined. Note that by reading horizontally across a given input resistor value
from the “Signal Plot” to the “Minimum
Allowable RC” plot the allowable input voltage
range can be directly observed.
Figure 3 is a plot of the above equations for two
commonly used input currents: specifically,
6 mA and 12 mA. The minimum allowable value
Figure 3. Input Resistor For Two Typical Input Currents
2
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