Cautions for Use of Solid State Relays
ASCTB400E 201806-T
2. Over-Current Protection
Load Type Description
An SSR circuit operated without overcurrent protection may
result in damage to the device. Design the circuit so the device’s
rated junction temperature is not exceeded for a continuous
overload current.
(e.g. Surge current into a motor or light bulb)
The surge-on current rating applies to over-current errors which
occur less than several tens of times during the service life of a
semiconductor device. A protection coordination device is
required for this rating.
Methods of over-current protection include the following:
1) Suppressing over-currents
Use a current limiting reactor in series with the load power
supply.
2) Use a current shut-off device
Use a current limiting fuse or circuit breaker in series with the
load power supply.
Example of executing fuse selection of over-current protection
cooperation
10
No. of cycles at 60Hz
Fuse cut-off current
Surge ON current
10
100
1,000
1
100
1,000
(A peak)
NHR15 (fuse 15A)
AQ-A (15A type)
NHR10 (fuse 10A)
1. Heaters (Resistive load)
The SSR is best suited to resistive loads. Noise levels can be
drastically lowered with zero-crossing switching.
2. Lamps
Tungsten or halogen lamps draw a high inrush current when
turned on (approximately 7 to 8 times the steady state current for
zero-crossing SSRs; approximately 9 to 12 times, in the worst
case, for random type SSRs). Choose an SSR so the peak of
the inrush current does not exceed 50% of the SSR surgeon
current.
3. Solenoids
AC-driven solenoid contactors or solenoid valves also draw
inrush current when they are activated. Choose an SSR such
that the peak of the inrush current does not exceed 50% of the
SSR surgeon current. For small solenoid valves and AC relays in
particular, a leakage current may cause the load to malfunction
after the SSR turns off. In such an event, use a dummy resistor
in parallel with the load.
• Using an SSR below the specified load
4. Motors load
When starting, an electric motor draws a symmetrical AC
starting current some 5 to 8 times the steady-state load current,
superimposed on a DC current. The starting time during which
this high starting current is sustained depends on the capacities
of the load and load power supply. Measure the starting current
and time under the motor’s actual operating conditions and
choose an SSR so the peak of the starting current does not
exceed 50% of the SSR surge-on current.
When the motor load is deactivated, a voltage exceeding the
load supply voltage is applied to the SSR due to counter-EMF.
This voltage is approximately 1.3 times the load supply voltage
for induction motors, and approximately 2 times that for
synchronous motors.
• Reversible motor control
When the direction of motor rotation is reversed, the transient
current and time required for the reversal far exceed those
required for simple starting. The reversing current and time
should also be measured under actual operating conditions.
For a capacitor-starting, single-phase induction motor, a
capacitive discharge current appears during the reversal
process. Be sure to use a current limiting resistor or reactor in
series with the SSR.
Also, the SSR should have a high marginal voltage rating, since
a voltage twice as high as the load supply voltage develops
across the SSR in the reversal process.
For reversible motor control, carefully design the driver circuit so
the forward and reverse SSRs do not turn on at the same time.
5. Capacitive load
A capacitive load (switching regulator, etc.) draws an inrush
current to charge the load capacitor when the SSR turns on.
Choose an SSR so the peak of the inrush current does not
exceed 50% of the SSR surge-on current. A timing error of up to
one cycle can occur when a switch used in series with the SSR
is opened or closed. If this is a problem, use an inductor (200 to
500
μ
H) in series to the SSR to suppress dv/dt error.
6. Other electronic equipment
In general, electronic equipment uses line filters in the primary
supply circuit.
The capacitors used in the line filters may cause the SSR to
malfunction due to dv/dt turn on when the equipment is turned
on or off. In such an event, use an inductor (200 to 500
μ
H) in
series with the SSR to suppress dv/dt turn on.
Load power supply
Load
SSR
Output
Dummy resistor
ー 11 ー
