Application Guide catalog - Watlow - #89

• No moving parts, no mechanical failure

• Resistant to shock and vibration

• Can withstand a direct short circuit with no damage

• Non-isolated alone in most cases; requires additional isolated contactor, opto-isolator or switch wired to it

• Must match the specifications for the internal and external switching devices

solid state relay. The input specifications of the power switching device must match those listed for the solid state switch output. The solid state switch is an open collector, switched dc signal that provides a minimum turn on voltage of 3V-(dc) into a minimum 500Q load; maximum on voltage not greater than 32V-(dc) into an infinite load. The switched dc output is in most cases a non­isolated output. The device this output drives must provide isolation to prevent interaction with other power, input or output circuits. Watlow's dc input solid state power controllers provide this isolation.

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The Switched DC Control Output Since solid state switches have no moving parts, they have no mechani­cal failures. These solid state switches are more resistant to shock and vibra­tion than mechanical relays. The absence of moving parts provides silent operation.

Watlow's switched dc output provides a small dc signal to trigger an external power switching device such as a

power device such as a silicon controlled rectifier (SCR) or equipment requiring a current or voltage input. Specific versions of the Watlow QPAC, VPAC, POWER SERIES and DIN-A-MITE® will interface directly with the process output from the Watlow temperature controller.

The control provides the electrical

signal (4-20mA or 0-5V-(dc)) to drive the final load device. No external low volt source is required. The process output in most cases is a non-isolated output. The device this output drives must provide isolation to prevent interaction with other power, input or output circuits.

The process output on Watlow con­trollers is wired through a device requiring a 4-20mA/0-20mA input with an input impedance of 600-800O maximum; or a 0-10V=(dc),1-5V=(dc) or 0-5V^(dc) input with an input impedance of 1-5KQ minimum. This may be a valve positioner, variable

The key to selecting the correct Wat-low controllers solid state relay output is knowing the application. What downstream device will you switch with the controller's solid state relay output?

If you are switching a solenoid, a mercury displacement relay (MDR), or a mechanical relay or contactor, you need Watlow's 0.5A solid state relay (SSR) with RC (Quencharc® brand) suppression for noise immunity. This integral RC network dampens any noise generated by the downstream output device.

On the other hand, if you are switch­ing an ac input solid state relay (SSR), an ac input SCR (Watlow DIN-A-MITE, QPAC), or other high impedance loads (typically 5KO, like a piezoelectric buzzer, or neon lamp), you need Watlow's 0.5A solid state relay (SSR) without contact suppression. The absence of the Quencharc® RC network eliminates output leakage across the contacts, and thus the possibility of false firing.

Another way to phrase the question is: Do you need RC suppression with your solid state relay control output, or will your output device tolerate output leakage? Solenoids, MDR, and mechanical relays or contactors tend

to generate noise spikes that require RC suppression. AC input solid state relays (SSRs), ac input SCRs, and other high impedance loads are subject to output leakage and false firing—choose solid state relay without RC suppression.

If you need more information, includ­ing how to remove or add noise sup­pression to or from a solid state relay control output, call you Watlow sales agent, authorized distributor, or a Watlow application engineer.

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