Panasonic LC2H specifications Protective circuit for counter contact

PRECAUTIONS IN USING THE COUNTER

Cautions for circuits

1. Protective circuit for counter contact

In the circuit that switches an inductive load, a contact failure may occur at a contact point due to surge or inrush current resulting from that switching. Therefore, it is recommended that the following protective circuit be used to protect the contact point.

		CR circuit (r: resistor		c: capacitor)		Diode circuit		Varistor circuit
	Counter contact			Counter contact		Counter contact		Counter contact
Circuit			load	r	load		load		load
			Inductive		Inductive	Diode	Inductive		Inductive
	r	c		c				ZNRvaristor
AC	(see note.)			Available		Not available		Available
Application	Available			Available		Available		Available
DC

	If the load is a relay or solenoid, the release time lengthens.		The diode connected in parallel caus-	Using the rated voltage characteris-
			es the energy stored in the coil to	tics of the varistor, this circuit pre-
	Effective when connected to both contacts if the power supply voltage is
			flow to the coil in the form of current	vents excessively high voltages
	24 or 48 V and the voltage across the load is 100 to 200 V.
			and dissipates it as joule heat at the	from being applied across the con-
Features/Others	If the load is a timer, leakage current		resistance component of the induc-	tacts. This circuit also slightly
	flows through the CR circuit causing		tive load.	delays the release time.
	faulty operation.	—	This circuit further delays the release
	Note: If used with AC voltage, be sure		time compared to the CR circuit.
	the impedance of the load is sufficiently		(2 to 5 times the release time listed in
	smaller than that of the CR circuit.		the catalog)
	As a guide in selecting r and c,		Use a diode with a reverse break-
	c: 0.5 to 1 µF per 1 A contact current		down voltage at least 10 times the
	r: 0.5 to 1 Ω per 1 V contact voltage		circuit voltage and a forward cur-
	Values vary depending on the properties of the load and variations in counter charac-		rent at least as large as the load	—
Device Selection teristics.			current.
	Capacitor c acts to suppress the discharge the moment the contacts open. Resistor r		In electronic circuits where the cir-
	acts to limit the current when the power is turned on the next time. Test to confirm.		cuit voltages reverse breakdown
	Use a capacitor with a breakdown voltage of 200 to 300 V. Use AC type capacitors		voltage of about 2 to 3 times the
	(non-polarized) for AC circuits.		power supply voltage.

2.Type of Load and Inrush Current The type of load and its inrush current characteristics, together with the switch- ing frequency, are important factors which cause contact welding. Particularly for loads with inrush cur- rents, measure the steady state current and inrush current and use a relay or magnet switch which provides an ample margin of safety. The table below shows the relationship between typical loads and their inrush currents.

Type of load	Inrush current
Resistive load	Steady state current
Solenoid load	10 to 20 times the steady state current
Motor load	5 to 10 times the steady state current
Incandescent lamp load	10 to 15 times the steady state current
Mercury lamp load	1 to 3 times the steady state current
Sodium vapor lamp load	1 to 3 times the steady state current
Capacitive load	20 to 40 times the steady state current
Transformer load	5 to 15 times the steady state current

When you want large load and long life of the counter, do not control the load direct with a counter. When the counter is designed to use a relay or a magnet switch, you can acquire the longer life of the counter.

3.Connection of input (Except for LC4H-S/AC type)

The LC4H series use power supply with- out a transformer (power and input termi- nals are not insulated). In connecting

various kinds of input signals, therefore, use a power transformer in which the pri- mary side is separated from the ungrounded secondary side as shown in Fig. A, for the power supply for a sensor and other input devices so that short-cir- cuiting can be prevented.

(Fig. A) Good

Insurating transformer

(+)	Input equipment
(–)	(sensor, etc.)
	(–)

Counter

(Fig. B) No good
AC power supply
Insurating transformer
(+)	Input equipment
(–)	(sensor, etc.)
	(–)
Counter

AC power supply

Single coil transformer
(+)	Input equipment
(–)	(sensor, etc.)
	(–)
Counter	AC power
	routing

Do not use a single coil transformer (e.g., Sly-Duck). Otherwise, the internal circuit of the counter will be short-circuited as shown in Fig. B resulting in breakdown.

4.Long Continuous Current Flow Avoid keeping the counter on for a long period of time (over one month). Otherwise heat is generated and accu- mulated inside the counter, which may deteriorate its electronic parts. If the counter must be kept on for a long period of time, a relay is added. See the circuit diagram below.

R	C	R	R
	R Relay	C Counter	Receive output
			from contact
			at relay R

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