Download Q - Crouzet

CROUZET SOLID STATE RELAYS – FAQ’s
Click here to return to the FAQ home page
Product Questions (Click on question to view the answer):
Q.
Q.
Q.
Q.
Q
Q.
Q.
Q.
Q.
Q.
Q.
Q.
Q.
Q.
Q.
Q.
Q.
Q.
Q.
Q.
What does the “Z” and the “R” mean in the GA3 series solid state relays?
What is meant by zero-voltage turn-on?
What is meant by instantaneous or random turn-on?
In what application would I use a zero voltage turn-on v. a random turn-on solid state
relay?
Why does the output of the relay not measure zero Ohms on my Ohmmeter when I apply
control voltage to the solid state relay?
I am using an AC output solid state relay for use with a DC load, but the relay will not
shut off. Why?
I have a solid state relay that turns on but will not turn off. Why?
How do I select an adequate heat sink for my application?
What is the minimum input current necessary to turn on a solid state relay?
What is the torque specification for Crouzet’s solid-state relays?
What is a snubber network and when would I use one?
What is minimum Off-State DV/DT?
What is I²T?
What is thermal resistance/impedance?
Why does a solid state relay have off-state leakage current?
What is a transzorb?
What is a MOV and how does it compare to a transzorb?
Why would I need EMC compliance?
Why are SCR output solid state relays better than Triac output SSR’s?
When would you use a SCR solid state relay? A Triac solid state relay?
Can’t find the answer you are looking for? Simply click here “Crouzet TechTalk” and visit Crouzet’s community bulletin board. Here you can post
questions or comments regarding our products, personnel, company…you
name it. You can also view posts from our other members and read the
responses they have received from within the community. The bulletin
board is monitored continuously by our staff, so any question you post will
be quickly answered.
Don’t want to share your question or comment with the community? No
problem! Simply private-message one of our staff members (P.M. button
within the member’s profile). Your message will be sent directly to their
inbox.
CROUZET SOLID STATE RELAYS – FAQ’s
Return to Top
Q.
A.
What does the “Z” and the “R” mean in the GA3 series relays?
“Z” stands for zero voltage turn-on (zero-cross or ZC) while “R” stands for random turnon.
Q.
A.
What is meant by zero-voltage turn-on?
The relay will only turn on when the line (output) voltage is near zero.
Q.
A.
What is meant by instantaneous or random turn-on?
When the control voltage is given the relay output will turn-on regardless of what the line
voltage is.
Q.
A.
In what application would I use a zero voltage turn-on v. a random turn-on relay?
Zero-crossing relays are used with resistive loads while random turn-on relays are used
with inductive loads (motors, transformers, coils etc.).
Zero crossing relays may also be used with inductive loads, but consideration must be
given to the power factor of the load. If the load is too inductive, then the output of a ZC
relay may half-wave. If this occurs, than a random-fire SSR should be used in the
application.
A random-fire SSR may also be used in resistive applications. Some applications require
that the load only be energized for a portion of the AC cycle. This can be accomplished
with the random SSR’s (due to it’s relatively fast turn-on time), given that an appropriate
controller is available. However, the initial surge current will be significantly higher due to
the SSR switching power when the line voltage is closer to peak.
Q.
A.
Q.
A.
Q.
A.
Why does the output of the relay not measure zero Ohms on my Ohmmeter when I
apply control voltage to the SSR?
All solid state relays require a minimum amount of voltage across the output before the
SCR’s will conduct. Most multi-meters are designed to simply provide a small amount of
voltage in order to measure the ensuing current draw. Therefore, when measuring a
solid-state device, the meter will typically report an open circuit (or very high impedance).
I am using an AC output SSR for use with a DC load, but the relay will not shut off.
Why?
AC output SSR’s are zero current turn-off relays and thus must cycle through the zero
current before it can turn off. You must use a DC output relay to control a DC load.
I have an SSR that turns on but will not turn off. Why?
First measure the control voltage. The input of a DC input SSR must go below one volt to
guarantee that the relay will shut off (10Vrms for AC input SSR). If there is less than one
volt across the input (10Vrms for an AC SSR) or if the input is completely disconnected,
then the problem most likely resides in the output of the relay.
The two most likely reasons for an SSR failing to turn off are: the SCR’s have been
damaged, or there is an over-temperature condition. When SCR damage occurs, the
relay will typically half-wave. This is due to the fact that usually only one of the SCR dies
will become damaged. This can result from excessive surge currents, transients on the
AC mains, or other electrical phenomena. To determine if this has occurred, allow the
relay to adequately cool down (approximately 15 minutes) and connect to a small load,
such as a 100W bulb. If the bulb is illuminated without the control signal, then the relay’s
output is shorted. Likewise, if the bulb is only partially illuminated when the control signal
is applied, then the relay is half-waving.
CROUZET SOLID STATE RELAYS – FAQ’s
Return to Top
I have an SSR that turns on but will not turn off, continued;
The relay will also fail to turn off when the output SCR’s exceed their maximum rated
temperature of 125ºC. To determine whether a relay is “running” hot, measure the
temperature of the base plate near the mounting screw. The temperature should not
exceed 80ºC to 90ºC. If the relay exceeds 80ºC, then the heat sink of the application
must be reevaluated. Also, ensure that there is an adequate amount of thermal
compound between the base plate and the heat sink. However, it is also important to
note that too much heat sink compound will also impede the relays ability to adequately
transfer heat to the heat sink.
Q.
A.
How do I select an adequate heat sink for my application?
You must first determine the amount of power being dissipated by the relay in the
application. You can calculate this multiplying the load current by the voltage drop (Vf)
across the output of the SSR. The Vf of a SSR can be found in its specification sheet or
can be measured individually. Once this is determined, then you must calculate the
difference between the ambient temperature within the panel during operation and 80ºC
(recommended max base plate temperature). Then, divide the temperature difference by
the power being dissipated to determine the maximum thermal impedance of a heat sink
for the application.
For example, a relay is carrying 50 amps in an application with a Vf of 1.1Vpk, resulting in
55 Watts of power being dissipated. The ambient temperature inside the panel is 35ºC,
leaving a 45ºC difference between ambient and the maximum recommended base plate
temperature of 80ºC. If you divide the temperature differential of 45ºC by the 55 Watts of
power being dissipated, the result will be 0.818, or a 0.82ºC/W heat sink for the
application. To be prudent, you should always deduct 0.1ºC/W from the result to account
for the thermal compound used in the assembly, and then round down to the nearest
tenth. So, a 0.82ºC/W heat sink less 0.1º is 0.72ºC/W. Rounding down, you now know
that you need at least a 0.7ºC/W heat sink for your application.
Keep in mind that this is the most conservative way to calculate your heat sink
requirements. If the duty cycle is not 100%, then you may need less of a heat sink. The
best way to determine the “head room” in your application is to measure the temperature
of the base plate of the relay within the actual application, keeping in mind the 80ºC
recommended maximum base plate temperature.
Q.
A.
What is the minimum input current necessary to turn on a solid state relay?
This varies from family to family. Most of our solid-state relays have a regulated input,
which means that the input will attempt to draw it’s “nominal” current rating across the
entire voltage range of the relay. However, some SSR’s still have a straightforward
resistive input, which means that the minimum input current for an “on” state is
dependent upon it’s input impedance.
The DC input GN series SSR’s will draw between 10mA and 12 mA (maximum of 14ma,
minimum of 7ma) across the full 3-32Vdc input range while the GNA5 will draw minimum
of 9.5ma to a max of 14ma. The AC input GN series SSR’s will draw minimum of 5mA to
max of 10mA across the entire 90-280Vac input range while the GNA5 will draw a
minimum of 5ma to a maximum of 8.5ma. The GA3 three-phase SSR’s also have
regulated inputs, but require up to 20mA for the DC input models, and 10mA for the AC
input models.
CROUZET SOLID STATE RELAYS – FAQ’s
Return to Top
Q.
A.
What is the torque specification for Crouzet’s solid-state relays?
Most of our relays use the same size screws.....# 6 for input and #8 for output and
mounting. The torque settings should be high enough to retain the wires on the
terminals, but not so high to distort the clamps. Likewise, on the mounting screws, they
need to be high enough to compress the thermal interface material, but not so high as to
distort the base plate. So long as the materials for the screws, terminals, and base plates
are similar, then similar torque settings can be specified. Here are our recommendations:
for # 6 input screws: 12 in-LB, +/- 2 in-LB
for # 8 output screws: 15 in-LB, +/- 2 in-LB
for # 8 mounting screw: 15 in-LB, +/- 1 in-LB
Note: there may be circumstances where these numbers may not be the best, but
overall, they are close and a good place to start.
Q.
A.
What is a snubber network and when would I use one?
A snubber network is a resistor and capacitor in series and is placed in parallel with the
output of the ssr. Typically snubbers are required when the relay is placed in an
environment that has electrical noise. Most manufactures incorporate a snubber network
into their ssr’s. Crouzet uses an inductor along with transorbs to fill this role instead of a
snubber network.
Q.
A.
What is minimum Off-State DV/DT?
The rate of rise of applied voltage across the output terminals that the SSR (AC) can
withstand without turning on in the absence of a turn-on control signal. Usually expressed
as a minimum value at maximum rated voltage in terms of “volts per microsecond” (V/us)
Q.
A.
What is I²T?
It is the nonrepetitive pulse-current capability of the SSR; used for fuse selection.
Expressed as “amperes squared seconds” (A²s) with a stated pulsewidth, typically
between 1 and 8.3 milliseconds.
Q.
A.
What is thermal resistance?
Expressed as “degrees Celsius per watt” (°C/W), this value defines the temperature
gradient between the output semiconductor junction (Tj) and the SSR case (Tc) for any
given power dissipation. Basically, it’s the difference in temperature between two points
on an SSR dissipating a specific amount of power. Thermal impedance is used to
calculate the temperature of the SCR die in an application with a known heat sink
impedance and Rjb (resistance; junction-to-base plate). The Rjb can be found in the
relays specification sheet.
For example, a 25A relay with a thermal impedance of 0.25ºC/W mounted to a 2.0ºC/W
heat sink is carrying 15 amps in a 30ºC ambient. With a Vf of 1.0Vpk, the total power
dissipation is 15 Watts (15 amps x 1.0 V). With a 2.0ºC/W heat sink, the total temperature
rise of the base plate above ambient will be 30ºC (15W x 2.0ºC/W), which puts the base
plate at 60ºC (30ºC rise + 30ºC ambient). The difference between the base plate and the
SCR die is 30 Watts x 0.25ºC/W (Rjb), or 7.5ºC. When this is added to the calculated
base plate temperature, we find that the SCR die is operating at 67.5º.
CROUZET SOLID STATE RELAYS – FAQ’s
Return to Top
Q.
A.
Why do some SSR’s have off-state leakage current?
Leakage current comes from the snubber network. The amount of leakage current
(typically 10-15 mA) is dependent on the size capacitor used in the snubber network.
Crouzet uses inductive snubber networks instead of resistor-capacitor networks in most
of their relays, which will have a typical leakage current is less than 1mA.
Q.
A.
What is a transorb?
It is a small device much like a diode and it is used to protect electronic devices from
transients. Transorbs work by clamping any excessive voltage to a certain limit. They do
this by conducting when excessive voltage is present between the two terminals. For a
short time, the transorb will absorb a high amount of power. The transorb also reacts
within a few billionths of a second making it superior to any traditional solution.
Q.
A.
What is a MOV and how does it compare to a transorb?
A MOV is a metal oxide varistor, which is used to protect a SSR from transients, which
can cause the SSR to turn-on without the control voltage applied. MOVs are typically
placed in parallel with the output of the SSR. The customer usually installs the MOV.
After a MOV reaches its life expectancy it typically fails shorted. By contrast, a transorb
typically does not fail shorted and is installed at the factory when the SSR is built.
Q.
A.
Why would I need EMC compliance?
More than likely, you probably don’t need EMC compliance. The European Unions EMC
Directive for electrical equipment being placed on the European market only requires this.
However, products that are EMC compliant are typically more reliable, as they are
designed to operate in harsh electrical environments.
Q.
A.
Why are SCR output SSR’s better than Triac output SSR’s?
SCR die are more capable of handling inrush currents and electrically-fast transients.
However, given the right application, triac output SSR’s are quite reliable,
Q.
A.
When would you use a SCR SSR, Triac SSR?
Triac output SSR’s are general-purpose relays typically used for resistive type loads only.
The SCR output solid state relays are used to switch resistive or inductive loads,
especially those with high inrush currents.