Open Collector Output 3h5q1i

Open Collector Output Many Signet instruments and sensors feature “Open Collector Outputs” for purposes of signal transmission, alarming, control signal output, etc. Although such outputs allow for a lot of wiring flexibility, care must be taken not to destroy the circuits via incorrect polarity, over-voltage, transients or current overload. 1. Function Open Collector (“OC”) outputs are low powered, solid state switches. Although the term “Open Collector” stipulates the use of bipolar transistors (NPN-type or PNP-type) as a switch, nowadays Field Effect Transistors (FET or MOSFET) are used. Unlike electromechanical switches (e.g. push buttons or dry relays) these OC switches are very fast, use little power, are inexpensive, do not bounce and do not wear.

2. Sensor Wiring A typical example of the need for high speed switching capability is the OC frequency output of Signet flow sensors like 3-2536 or 3-2540. Signal frequencies can reach several hundred pulses per second while voltage and current requirements are small enough, allowing the use of a transistor switch. For each output pulse this switch connects the signal output to the negative supply or ground terminal of the sensor and is therefore an “NPN” style output.

Below is an explanation of proper wiring and dimensioning of related circuit components. Please note that the following recommendations may or may not apply to other manufacturer’s equipment.

However, OC’s are also more limited in of voltage and current rating as well as being polarised (i.e. they have a “plus” and “minus” terminal and thus DC only switching capability). They are less tolerant to overload abuse than electromechanical devices. Usually these switches have higher resistance and voltage drop.

Signet does not produce sensors with PNP style outputs (which connect the signal output internally to the positive supply terminal). Most indicating instruments or control system inputs require a signal voltage of 0 to 5 V (TTL or CMOS logic levels) or 0 to 24 V. Therefore, Open Collector output circuits must be complemented with a “PullUp-Resistor” to function properly. Please see the following example diagram for wiring with a PLC input:

+5V to +24V

V+

Do not exceed the absolute maximum voltage rating of the OC output as listed in the sensor specifications, normally 27 or 30 Volt, DC only. This includes changes to power line fluctuations, transients or power supply instability, otherwise damage to the OC will occur.

Signet Sensor/Instrument OC+ Switch

Gnd

OC-

Pull-UpResistor

Gnd

Power Supply

Input

Gnd

PLC

Please note that the voltage connected to the positive sensor supply (V+) must correspond to the required high-level PLC input voltage (i.e. if the high-input voltage of the PLC is 24 V, then the pull-up must be supplied with 24 V). If the input is “TTL-Level” or “CMOS-Level”, that means 5 V for high level, then the pull-up should not be connected with a supply higher than 5 V.

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Open Collector Output (continued) 3. Instrument Output Wiring Open collector control and alarm outputs on Signet instruments (i.e. ProcessPro® or ProPoint® series) are electrically isolated from the instrument’s power

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supply. That means these can be used in the above mentioned NPN configuration as well as in PNP configuration, if required. Below are a few sample circuits:

• PLC Wiring “PNP” style Signet Instrument/Sensor OC+

Flow

+5V to +24V

Power Supply

Open Collector Switch

Gnd Input

OC-

PLC

Pull-Down Resistor

Gnd

• PLC Wiring “NPN” style +5V to +24V

Signet Instrument/Sensor OC+ Switch

OC-

Pull-UpResistor

Gnd

Power Supply

Input

PLC

Gnd

• Alarm circuit or alarm lamp wiring to a single Signet instrument Signet Instrument

Alarm Circuit

OC+

V+

Switch Gnd

OC-

Power Supply

• Alarm circuit or alarm lamp wiring to serve multiple Signet instruments - Triggers the alarm if any one of the instruments open collector outputs are on.

Signet Instrument

Signet Instrument

OC+

Switch

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OC+

OC+

OCSwitch OC-

Signet Instrument

OCSwitch

Alarm Circuit V+

Gnd

Power Supply

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Open Collector Output (continued) 4. Voltage and Current Limitation As mentioned before, the supply voltage in the OC output circuit MUST be limited to the specified maximum OC voltage (see operating manual for specific instrument). The use of a quality regulated 5 V, 12 V or 24 V (depending on the application) power supply is recommended.

The current through the Open Collector switch must be limited. Typical OC outputs allow only for 10 to 50 mA switch current (please consult manual). Exceeding this current limit can burn out the OC output components immediately. Please see the following section on how to dimension the loads.

5. Load and Pull-Up/Down Resistor Considerations By utilizing basic arithmetic and Ohm’s law, one can determine the safe limits of load resistance. When the OC switch is closed, almost the entire supply voltage is applied to the load, (i.e. the pull-up or pull-down

resistor, the alarm horn input, a potential power relay coil or annunciator lamp). The resulting current through the load and through the OC switch, as well, can be calculated as:

(Current) = (Supply Voltage)/(Load Resistance) • Example 1: The supply voltage is 24 V and a pull-up-resistor of 10 kΩ is used. Current is 24/10,000 = 2.4 mA

(If the OC current rating is 10 mA, then in this example, it would be considered safe.)

• Example 2: The supply voltage is 12 V and a horn with a resistance of 100 Ω is used Current is 12/100 = 120 mA (Even if the OC current rating is 50 mA, this load will damage the instrument)

6. Transient Protection There are several “difficult” load cases that must be considered: • Inductive loads: These can be power relay or other solenoids, motors, alarm horn coils, etc. Such loads generate very high voltage spikes every time the load switches. If such a load is unavoidable, the use of transient suppression components, or Signet RCfilters (3-8050.396), or snubbers, wired parallel to the load is required. This is critical, as a single transient pulse may destroy the output.

7. “Active High” and “Active Low” Setting Depending on the desired function of the circuit attached to the OC output, it may be necessary to have the OC output switch turned “on” or “off” when the criteria for the activation of this output are met. By default, Signet instruments are set to operate in “active low” mode. This means when the -defined condition for the activation is met (e.g. exceeding of an alarm limit) the OC switch is turned “on”.

• Capacitive loads: This type of load should be rare but can occur if the load contains an internal power supply/ regulator that is fed from the output circuit. In such a case, it must be assured that the in-rush current does not exceed the OC current rating. • Incandescent lamps: Such lamps have a very high start-up current until the filament glows and the current settles to the specified value. The use of incandescent lamps on an OC output is not recommended. An LED type annunciator should be used instead. If wired as standard “NPN-style” output (see previous page) the logic level of the attached control system or PLC input consequently becomes “low” logic level. If a high input logic level is required for activation, it can be accomplished by changing the OC output function to “active high” in the menu system of the instrument. Most Signet instruments allow for this option.

8. Fail-Safe Behaviour No matter what the setting, most OC outputs of Signet instruments turn off when the instrument loses power. This must be taken into when evaluating system failure consequences. If the system layout requires a “closed” or “on” condition for the output in case of power loss, a mechanical dry relay (NC s) must be used instead of the OC output. 362

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Open Collector Output (continued)

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Control Outputs Many Signet products offer control outputs that can be categorised into three categories: Mechanical Relay, Solid-State Relay and Open Collector. Each control output offers benefits and limitations based on the application requirements. See below for comparisons.

Flow

Open Collector Benefits: • Longer life than a Mechanical Relay • No moving parts • Can switch DC voltage only (typically < 30 VDC) • Faster ON/OFF switching capabilities than Mechanical Relays Considerations: • Can only be used with DC voltage • Polarity very important when wiring • Not recommended for use with inductive loads • Lower voltage and current ratings than Mechanical Relays • Typically should not apply current > 25 mA Solid-State Relays Benefits: • Has isolated outputs (optically) • Can switch DC voltage (typically > 30 VDC) • Can switch AC voltage (typically > 42 VAC) 50 mA DC / 50 mA AC • Longer life than a Mechanical Relay • No moving parts • Faster ON/OFF switching capabilities (Equal rise/fall times) Considerations: • Not recommended for use with: • Inductive loads (ex. Solenoid, Pumps) • If using inductive loads, snubbers (RC Filter) can prevent relay damage • Lower voltage and current ratings than Mechanical Relays Mechanical Relays Benefits: • Can switch line voltage (typically > 120 to 240 VAC) • Can switch DC voltage (typically < 30 VDC @ 5A) • Has a large current rating (typically 5 A) • Larger voltage and current ratings than Solid-State Relay and Open Collector Outputs

Technical Reference

Considerations: • Slower ON/OFF switching capabilities than Solid-State Relay and Open Collector Outputs • Mechanical s can burn/wear over time • Snubbers (RC Filter), Signet 3-8050.396, can prolong life

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