General Questions & Answers

Q Under what circumstances is an RC snubber network required?

A An RC snubber network is required to protect the SSR from the effects of dv/dt. By allowing sudden increases in voltage to travel through the RC network, the SSR will not see the full force of the transient and therefore does not misfire or activate at an appropriate time.

Q What does dv/dt mean?

A Dv/dt is the rate of rise of voltage that is applied across the output of a SSR. (Change in voltage over change in time.)

Q Is there any reason not to include snubber networks with all SSRs?

A Yes. Snubber Networks introduce large AC leakage currents relative to SSRs that are not equipped with them. For many applications a small leakage current is acceptable, however in a circuit that is sensitive to leakage current, the snubber network should not be included. In relays that are equipped with snubber networks, the snubber circuit itself accounts for the majority of the leaked current.

Q What is the difference between back to back SCRs and triacs?

A Typically SCRs are used to switch DC, however, when 2 are connected in a "back to back" configuration they can quite effectively switch AC. Triacs are designed to switch AC and are essentially the above mentioned combination of 2 SCRs wired together and housed in a single package. Back to back SCRs are often chosen for SSRs because they have lower voltage drops, higher breakdown voltages and better thermal properties,however they are more expensive than triacs.

Q What is the difference between a zero turn on (Also known as a zero cross.) SSR and a random turn on SSR?

A A zero turn on SSR turns its output on after receiving a signal on the input terminals and sensing that the AC voltage is at or very near to zero. (Within preset limits.) A random turn on SSR will turn its output on when it receives a signal on the input terminals regardless of what voltage the AC signal is at

Q What loads require a random turn-on vs. a zero-cross fired relay?

A Resistive loads tend to work well with a zero cross SSR, however loads that are highly inductive work best with the random turn on SSRs. There are many conditions that dictate which style of SSR is best suited for a particular application. These conditions include, but are not limited to inrush current, power factor, type of control functions in use on the input side and many more.

Q Can I measure a DC current load using an HBC-138 (Current transducer.)?

A No. There are many methods of measuring DC loads, some of the most common being a DC shunt, however the HBC-138 series of current transducers can only be used for AC. They work similarly to a transformer and require a rising and falling current that crosses zero in order to obtain a measurement.

Q Why do I need a heat sink with my relay?

A Solid state relays, while being called relays, are quite different in many ways. In many applications they are far more desirable than standard electromechanical relays due to features such as much faster response time and MTBF (Mean Time Between Failure). One of the characteristics of a SSR that is unlike a standard relay is the fact that it gives off heat proportional to the number of amps being switched. This heat is generally on the order of 1.2W/switched amp. This heat needs to be removed from the SSR or it will continue to increase the internal temperature of the die resulting in thermal runaway that will destroy the SSR. One of the best ways to remove this heat is with a heat sink. Heat sinks are available in many sizes and shapes to accommodate requirements such as power output of the SSR and mounting applications. NPH and it's SSR partner has years of experience in this area and will be happy to help you specify the correct size or even work on a custom design for your application.

Q What parameters do I need to know to determine the proper SSR to use in my application?

A The following is a list of common parameters that are helpful to know ahead of time for proper SSR selection:

Line voltage, Load type, Load rating, Switching mode, Control voltage, Control current, Mechanical packaging
Ambient temperature

Q What ambient temperature are your assemblies rated to operate at?

A Unless otherwise stated, NPH assemblies are rated to an ambient temperature of 40oC (104oF) with a continuous duty cycle.

Q What is an MOV, and why should I have one?

A MOV stands for Metal Oxide Varistor. A MOV is used with an SSR to reduce the effects of voltage transients. A varistor will divert a surge current and protect the SSR from damage under many conditions. Varistors do not degrade over time however they do degrade when subjected to repeated transients. A varistor absorbs and dissipates
the transient into its environment in the form of heat. If the transient is large enough that the varistor can not dissipate the heat, it will go into thermal runaway and damage to the MOV will occur. Over time if the MOV is subjected to frequent transients it will degrade. Proper line quality should be maintained and the MOV should be used as a backup for an unforeseen failure.

Q How close can your assemblies be mounted to each other on a DIN rail?

A Our assemblies are rated to be mounted directly next to each other assuming that the DIN rail is mounted horizontally. Our custom heat sinks utilize the chimney effect, allowing the heat to rise through the heat sink and out the top.

Q What is the duty cycle of NPH assemblies?

A We rate our assemblies to be utilized at our maximum current and voltage ratings with a 100% duty cycle.

Q What is the projected MTBF of your assemblies?

A The MTBF (Mean Time Between Failure) for our SSRs when properly installed and operated within the specifications is estimated to be between 20,000,000 and 400,000,000 hours. The main factor in SSR failure is heat. If the SSR is operated within its thermal ratings, one can expect an extremely long operational life. The best conditions for an SSR are being operated at a relatively constant current load. Frequent current jogs will lead to frequent changes in temperature of the internal parts of the SSR. These thermal changes can lead to internal mechanical stresses on solder joints and often appear where dissimilar materials are joined together.

Q For three phase applications, when should I use two pole or three pole control?

A Two pole control is used for delta loads. With two of the three poles opened, current can not flow through the remaining pole since each pole of a delta is connected to the other two and there is not a reference to ground common to any of the poles.

Three pole control is used for wye loads because all three poles have an individual reference to ground in the center of the wye. If only two poles were opened, the third leg of the wye would still be conducting.

Q Can I get a single packaged relay that will control 2 or 3 poles?

A Yes. NPH has many SSR assemblies that will accommodate a large range of three phase applications up to 100A per pole using our standard assemblies and many more applications from our custom, application specific design engineering department. NPH also offers many three phase control applications utilizing soft starts, logic control, PLC inputs, Phase control, time proportioning and much more. Call us for details or to discuss an application.

Q What is the range of inputs NPH assemblies can accept?

A Our standard assemblies can accept the following inputs:

0-5Vdc, 3-32Vdc, 3-15Vdc, 4-10Vdc, 15-32Vdc, 90-140Vac, 90-280Vac

Our Phase and Time proportioning power controllers respond linearly proportional to the following:

0-5Vdc, 0-10Vdc, 4-20mA, 0-135Ohm, Potentiometer
(And many more custom values available. Call us for details.)

Q When do I need to use soft start control?

A Soft start should be used in any application where there is a high inrush of current. Inrush is typical of loads such as motors and certain heaters that appear like a short when first turned on and current is only internally limited upon constant operation or the steady state. These applications need to be current limited for the initial start up period and NPH offers a wide variety of soft start application based equipment.

Q Are SSRs suitable for panel mounting?

A Yes, typically the industry standard hockey puck style SSR can be panel mounted if it does not require a heat sink for its particular application. If a heat sink is necessary, NPH offers a variety of mounting options including panel mount brackets and DIN clips.

Q What is the normal failure mode of SSRs and why?

A There are 2 failure modes; failed in the open state and failed in the closed state. Closed failures are the more common failure mode and this form of failure most often occurs due to overheating of the SSR which leads to an internal melting of components which end up as a closed circuit. The open failure is most often caused by a separation of solder joints due to extreme temperature changes over the life of the SSR.

Q How can I protect against an SSR failure?

A SSR failures are usually due to the relay being subjected to a condition outside of its specifications, typically over heating is the result. Great care should be taken to ensure that the SSR is mounted in a location with proper ventilation and airflow so that the assembly is not operated at a current higher than its rating for the ambient tem
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