Pulse & Relay Outputs

Outputs may be transistor, voltage pulse or a relay contact. The period T of a continuously occurring pulse is the time to complete one cycle while the frequency is the inverse of the period can be measured as a frequency f.

Find a Product Output ⇒ Function ⇒ Input

Pulse/Frequency Output

Pulse outputs are produced using solid state means while relay outputs have a mechanical component. Pulse outputs may have a specified amplitude or may be require external components to complete the circuit before the output signal can be measured which is typical of an open collector output.

The diagram shows a block diagram of an APC153 connected to an output load between terminals 8 and 10.

The output load represents the input of the receiving equipment or a pull up resistor.
Current Ip flows when the output transistor is ON or is zero when OFF.
The APC153 conveniently provides an isolated power supply for use as the pull up voltage, this power supply could be part of the receiving equipment.

APC153 connected to an output load

The emitter of the NPN transistor is connected internally to the output power supply and acts as a reference if wanting to monitor the voltage of the output pulse. The voltage is close to 0 when the transistor is ON and rises to the voltage at terminal 8 when OFF.
If the receiving instrument is a current input only the connections at 8 and 10 are required.
If the receiving instrument is a voltage input then a pull up is required between 8 and 10 typically 1kohm is required and the input is connected to 9 and 10.

Connections will vary if pull up resistor and or pull up power is provided by the receiving instrument. It is important to remember that the output transistor cannot directly switch the full power supply current, a pull up is required. When using the APC153 the provided pull up power supply has a current limit lower than the raring of the output transistor making wiring mistakes more forgiving. When using a product it is important to look at the maximum current and voltage rating of the output transistor.

	APC153 Output Specification
Output frequency range	10Hz to 10kHz.
Pulse voltage level	1.0 up to 24Vdc (adjustable).
Output pulse drive	20mA maximum.
Output transistor	30V, 100mA.
Input/output isolation	1000Vrms/1500Vdc.

Relay Output

Relay outputs or contacts have the advantage of being mechanically isolated from the measuring device and usually have higher voltage ratings than solid state devices however the number of switching operations over a lifetime is lower.

APCS modules do not have a snubber circuit fitted to relay contact outputs and are specified with resistive loads. Appropriate measures must be made when using relay contact outputs. If switching an inductive load a snubber is required to reduce the sharp rise in voltage causing arching across the contacts. The snubber provides a short-term alternative current path so that the inductive element may be discharged. When using a product it is important to look at the maximum current and voltage rating of the relay contact and what type of load is is specified for (eg Relay contact: 8A/250V resistive, 3.5A/250V inductive).