Fig. 3 

¡DPick-up voltage change due to coil temperature rise (hot start)
In DC relays, after continuous passage of current coil, if the current is turned OFF, then immediately turned ON again, due to the temperature rise in the coil, the pick-up voltage will become somewhat higher. Also, it will be the same as using it in a higher temperature atmosphere. The resistance/temperature relationship for copper wire is about 0.4% for 1^¢XC, and with ratio the coil resistance increase. That is, in order to cause operation of the relay, the current necessary becomes higher than the pick-up current, accompanying the rise in the resistance value.

¡DOperate time
In the case of AC operation, there is extensive variation in operate time depending upon the point in the phase at which the switch is turned ON for coil excitation, and it is expressed as a certain range, but for miniature types it is for the most part 1/2 cycle (about 10msec.). However, for the somewhat large type relay where bounce is large, the operate time is 7 to 16msec., with the release time in the order of 9 to 18msec. Also, in the case of DC operation, to the extent of large coil input, the operating time is rapid, but if it is too rapid, the ¡§A¡¨ contact bounce time is extended.

¡DStray circuit (bypass circuits)
In the case of sequence circuit construction, because of bypass flow or alternate routing, it is necessary to take care not to have erroneous operation or abnormal operation. To understand this condition while preparing sequence circuits, as shown in Fig. 4 with two lines written as the power source lines, the upper line is always (+) and lower line (-) (when the circuit is AC, the same thinking applies). Accordingly the (+) side is necessary the side for making contact connections (contacts for relays, timers, limit switches, etc.), and the (-) side is the load circuit side (relay coil, timer coil, magnet coil, solenoid coil, motor, lamp, etc.).
          
   Fig. 4 Example of a vertically written sequence circuit

Fig. 5 shows a example of stray circuit. In Fig. 5(a), with contacts A, B, and C closed, after relays R₁, R₂, and R₃ and the relays will hum and sometimes not be restored to the drop out condition.
The connection shown in Fig. 5(b) are correctly made. In addition, with regard to the DC circuit, because it is simple by means of a diode to prevent stray circuits, proper application should be made. 
             
     Fig. 5 Stray circuits

 ¡DGradual increase of coil impressed voltage and suicide circuit
When the voltage impressed on the coil is increased slowly, the relay transfering operation is unstable, the contact pressure drops contact bounce increases, and an unstable condition of contact occurs. This method of applying voltage to the coil should not be used, and consideration should be given to the method of impressing voltage on the coil (use of switching circuit). Also, in the case of latching relays, using self contacts ¡§B¡¨, the method of self coil circuit for complete interruption is used, but because of the possibility of trouble developing, care should be taken. The circuit shown in Fig. 6 causes a timing and sequential operation using a reed type relay, but it is not a good example with mixture of gradual increase of impressed voltage for the coil and a sucide circuit. In the timing portion for relay R₁, when the timing times out, chattering occurs causing trouble. In the initial test (trial production), it shows favorable operation, but as the number of operations increases, contact blackening (carbonization) plus the chattering of the relay creates instability in performance.
            
Fig. 6 A timing and sequential operation using a reed type relay

¡DPhase synchronization in AC load switching
If switching of the relay contacts is synchronized with the phase of the AC power, reduced electrical life, welded contacts, or a locking phenomenon (incomplete release) due to contact material transfer may occur. Therefore check the relay while it is operating in the actual system. However, if problem develop, control the relay using an appropriate phase. (Fig. 7).
           
 Fig. 7

¡DErroneous operation due to inductive interference
In situations where both control and load wiring are in close proximity, thought should be given to separating or shielding the conductors in order to prevent false relay operation. This becomes increasingly important with long wire runs, and can be achieved by using separate conduit for load control conductor. Inductive coupling can also be minimized of the load and control wiring.

¡DInfluence of external magnetic fields
Many modern electro-mechanical relays are of polarized, high sensitivity design. Care should be exercised in the placement of these devices when strong, external magnetic fields are prevent, such as in proximity to power transformers or permanent magnets (speaker, etc.).
Operational characteriatics may change under an external magnetic influence.

¡DLong tern current carrying
In application \s which involve lengthy duty cycles, the preferred configuration would be the use of the form B or N.C. contacts for long term duty. In those instances where the form A contact is held closed for extensive time period, coil heating will increase contact ¡§T¡¨ rise and may result in shorter than optimum life. Alternately, latching types may be considered for these applications, using a storage capacitor to ¡§Reset¡¨ the relay on power-down.

¡DRegarding electrolytic corrosion of coils
In the case of comparatively high voltage coil circuit (in particular above 48V DC), when such relays are used in high temperature and high humidity atmospheres or with continuous passage of current, the corrosion can be said to be the result of the occurrence of electrolytic corrosion. Because of the possibility of open circuit occurring, attention should be given to the following points.
The (+) side of the power source should be connected to the chassis. (Refer to Fig. 9) (Common to all relays)
In the case where unavoidably the (-) side is grounded, or in the case where grounding is not possible.
Insert the contacts (or switch) in the (+) side of the power source, and connect the start of the coil winding the (-) side. (Refer to Fig. 10) (Common to all relay)
When a grounding is not required, connect the ground terminal to the (+) side of the coil. (Refer to Fig. 11) (NF and NR with ground terminal).

¡DContact Protection Circuit Use of contact protective devices or protection circuits can suppress the counter emf to a low level. However, note thatincorrect use will result in an advance effect. Typical contact protection circuits are given in the table blow.

¡@

¡@

¡@

¡DAvoid using the protection circuits shown in the Fig. 8. Although DC inductive loads are usually more difficult to switch than resistive loads, use of the proper protection circuit will raise the characteristics to that for resistive loads.

Fig. 8

Although extremely effective in arc suppression as the contacts open, the contacts are susceptible to welding since energy is stored in C when the contacts open and discharge current flows from C when the contacts close.

Although extremely effective in arc suppression as the contacts open, the contacts are susceptible to welding since charging current flows from C when the contacts close.

¡@

¡DMounting the Protective Device
In the actual circuit, it is necessary to locate the protective device (diode, resistor, capacitor, varistor, etc.) in the immediate vicinity of the load or contact. If located too far away, the effectiveness of the protective device may diminish. As a guide, the distance should be within 50cm.

¡DAbnormal Corrosion During High Frequency Switching of DC loads (spark generation)
If, for example, a DC value or clutch is switched at s high frequency, a blue-green corrosion may develop. This occurs from the reaction with nitrogen in the air when sparks (arc discharge) are generated during switching. For relays with a case, the case must be removed or air holes drilled in the case. A similar phenomenon occurs in the presence of ammonia-based gas. Therefore, care is required in circuits where sparks are generated at a high frequency.

¡DType of load and Inrush Current
The type of load and its inrush current characteristics, together with the switching frequency are important factors which cause contact welding. Particularly for load with inrush currents, measure the steady state current and inrush current and select a relay which provides an ample margin of safety. The table on the right 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	Approx. 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 statecurrent
Transformer load	5 to 15 times the steady state current

¡@

Load Inrush Current Wave and Time
    
¡D
When Using Load Wires
If long wires (100 to 300m) are to be used in a relay contact circuit, inrush current maybe become a problem due to the stray capacitance exiting between wires. Add a resistor (approx. 10 to 50W) in series with the contacts. (fig. 9)
           
Fig.9

¡DPhase Synchronization in Switching AC Loads
If switching of the relay contacts is synchronized with the phase of the AC power, reduced electronic life, welded contacts, or a locking phenomenon (incomplete release) due to contact material transfer may occur. Therefore, check the relay while it is operating in the actual system. However, if problem develop, control the relay using an appropriate phase. (Fig. 10).
            
Fig.10

¡DConnection of load and contacts
Connect the load to the side of power supply as shown in Fig. 11(a). connect the contacts to the other side. This prevents high voltage from developing between contacts. If contacts connected to both side of the power supply as shown in (b), there is a risk of shorting the power supply when relatively close contacts short.             
Fig.11


RELAY DRIVE BY MEANS OF A TRANSISTOR

¡DConnection method
The voltage impressed on the relay is always full rated voltage, and in the OFF time, the voltage is complete zero for avoidance of trouble in use. (Fig. 1)

        
 Fig.1

¡DCountermeasures for surge voltage of relay control transistor
If the coil current is suddenly interrupted, a sudden high voltage pulse is developed in the coil. If this voltage exceeds the voltage resistance of the transistor, the transistor will be degraded, and this will lead to damage. It is absolutely necessary to connect a diode in the circuit as a means of preventing damage from the counter emf.
As suitable ratings for this diode, the current should be equivalent to the average rectified current to the coil, and the inverse blocking voltage should be about 3 times the value of the power source voltage. (Fig. 2)
        
Fig.2

¡DSnap action
(Characteristic of relays with voltage rise and fall of voltage)
Unlike the characteristic when voltage is impressed slowly on the relay coil, this is the case where it is necessary to impress the rated voltage in a short time and also to drop the voltage in a short time. (Fig. 3)
        
Fig. 3

¡DSchmitt circuit (Snap action circuit)
(Wave rectifying circuit)
When the input signal does not product a snap action, ordinarily a Schmitt trigger circuit is used to product safe snap action.
Characteristic points
1.  The common emitter resistor R_E must have a value sufficiently small compared with the resistance of the relay coil. (The voltage impressed on the relay must not be greater than the excitation voltsge.)
        
Fig. 4

2.  Due to the relay coil current, the difference in the voltage at point P when T₂ is conducting and at point P when T₁ is conducting creates hysteresis in the detection capability of the Schmitt circuit, and care must be taken in setting the values.
3.  When there is chattering in the input signal because of waveform oscillation, an RC time constant circuit should be inserted in the stage before the Schmitt trigger circuit. (However, the response speed drops.) (Fig. 4)

¡DAvoid Darlington circuit connection
(High amplification)
This circuit is a trap into which it is easy to fall when dealing with high circuit technology. This does not mean that it is immediately connected to the defect, but it is linked to troubles that occur after long periods of use and with many units in operation. (Fig.5)

        
Fig. 5

¡DResidual Coil Voltage
In switching applications where a semiconductor (transistor, UJT, etc.) is connected to the coil, a residual voltage is retained at the relay coil which mat cause incomplete restoration and faulty operation. By using DC coil, there may be a reduction in; the danger of the incomplete restoration, the contact pressure, and the vibration resistance. This is because the drop-out voltage is 10% or more of the rated voltage, a low value compared to that for AC coil, and also there is a tendency to increase the life by lowering the drop-out voltage. When the signal from the transistor¡¦s collector is taken and used to drive another circuit as shown in Fig. 6, a minute dark current flows to the relay even if the transistor is off. This may cause the problems described above.
        
Fig. 6 Connection to the next stage through collector

RELAY DRIVE BY MEANS OF SCR

¡DOrdinary drive method
For SCR drive, it is necessary to take particular care with regard to gate sensitivity and erroneous operation due to noise. (Fig. 7)
        
Fig.7

¡DCaution points regarding ON/OFF control circuit
(When used for temperature or similar control circuits)
When the relay contacts close simultaneously with an AC single phase power source, because the electrical life of the contacts suffers extreme shortening, care is necessary. (Fig. 8)
        
Fig. 8

1. When the relay is turned ON and OFF using a SRC, the SRC serves as a half wave power source as it is, and there are ample cases where the SRC is easily restored.
2. In this manner the relay operation and restoration timing are easily synchronized with the power source frequency, and the timing of the load switching also is easily synchronized.
3. When the load for the temperature control is a high current load such as a heater, the switching can occur only peak values and it can occur only at zero phase values as a phenomenon of this type of control. (Depending upon the sensitivity and response speed of the relay)
4.  Accordingly, either an extremely long life or a extremely short life results with wide variation, and it is necessary to take care with the initial device quality check.

RELAY DRIVE FROM EXTERNAL CONTACTS
Relays for PC board use have high sensitivity and high speed response characteristics, and because they respond sufficiently to chattering and bouncing, it is necessary to take care in their drive.
When the frequency of use is low, with the relay in response time caused by a condenser, it is possible to absorb the chattering and bouncing. (Fig. 9) (However, it is not possible to use only a condenser. A resistor should also be used with the capacitor.)
        
Fig. 9

ELECTRNIC CIRCUIT DRIVE BY MEANS OF A RELAY
¡DChatterless electronic circuit Even though a chatterless characteristic is a feature of relays, this is to the fullest extent a chatterless electrical circuit, much the same as a mercury relay. To meet the requirement for such circuits as the input to a binary counter, there is an electronic chatterless method in which chattering is absolutely not permissible. Even if chattering develops on one side, either the N.O. side contacts or the N.C. side contacts, the flip flop does not reverse, and the counter circuit can be fed pulsed without a miss. (However, bouncing from the N.O. side to the N.C. side must be absolutely avoided.) (Fig. 10)
        
Fig. 10
¡DTriac drive
With an electronic circuit using a direct drive from a triac, the electronic circuit will not be isolated from the power circuit, and because of this, troubles due to erroneous operation and damage can develop easily. The introduction of a relay drive is most economical and most effective solution. (Photo coupler and pulse transformer circuit are complicated.)
When a zero cross switching characteristic is necessary, a solid state relay (SSR) should be used. (Fig. 11)
        
Fig. 11

PC BOARD DESIGN CONSIDERATION
¡DPattern Layout for Relays
Since relays affect electronic circuits by generating noise, the following point should be noted.
Keep relay away from semiconductor devices. Design the pattern trances for shortest lengths. Place the surge arrester (diode, etc.) near the relay coil. Avoid routing pattern trances susceptible to noise (such as for audio signals) underneath the relay coil section. Avoid though-holes in places which cannot be seen from the top (e.g. at the base of the relay). Solder flowing up through such a hole may cause damage such as a broken seal. Even for the same circuit, pattern design considerations which minimize the influence of the ON/OFF operations of the relay coil and lamp on other electronic circuits are necessary. (Fig. 12)
        
Fig. 12

¡DWhen it is necessary to use hand soldering for one part of a component after dip soldering has been done
By providing a narrow slot in the circular part to the foil pattern, the slot will prevent the hole from being plugged with solder. (Fig. 13)
        
Fig. 13

¡DWhen the printed circuit board itself is used as a connector
The edge should be beveled. (This prevents peeling of the foil when the board is inserted into its socket.)
When only a single side is used as the connector blade, if there is a distortion in the circuit board, contact will be defective. Care should be taken. (Fig. 14)
        
Fig. 14

A Relay TTi Soldering and Cleaning Guide

Process	Guideline
1. Mounting of relay	¡DAvoid bending the terminals to make the relay self-clinching. ¡DRelay performance cannot be guaranteed if the terminal are bend.
2. Flux coating	¡DAdjust the position of the PC board so that flux does not overflow onto the top of it. ¡DUse rosin-based flux, which is non-corrosive and required no washing. ¡DDo not use Automatic Flux coating Method to dust-cover type relays. ¡DDo not overflow onto the top of PC Board, in which a case, the flux may even penetrate a flux-esistant type relay.
3. Preheating	¡DPreheating acts to improve solderability. ¡DPreheating according to the following conditions. Temperature 100¢XC 212¢XF or less Time Without approx. ¡@
4. Soldering ¡@	Automatic Soldering Hand Soldering ¡DFlow solder is the optimum method for soldering. ¡DAdjust the level of the solder so that it does not overflow onto the top of the PC board. ¡DUnless otherwise specified, solder under the following conditions depending on the type of relay. ?Keep the tip of the soldering iron clean. Soldering iron 30W to 60W Iron Tip Temperature Approx. 300¢XC 572¢XF Soldering Time Within approx. 3 seconds Solder H60 or H63 Solder Temperature Approx. 250¢XC 482¢XF ¡@ Soldering Time Within approx. 5 second Solder H60 or H63
5. Cooling	Automatic Soldering Hand Soldering ¡DImmediate air cooling is recommended to prevent deterioration of the relay and surrounding parts due of soldering heat. ¡DAlthough the environmentally sealed type relay (plastic sealed type, etc.) can be cleaned, avoid immersing the relay into cold liquid (such as cleaning solvent) immediately after soldering. Doing so many deteriorate the sealing performance.
6. Cleaning	¡DDo not clean dust-cover type relays and flux-resistanct type relays by immersion. Even if only the bottom surface of the PC board is cleaning (e.q. with a brush), careless cleaning may cause cleaning solvent to penetrate the relay. ¡DPlastic sealed type relays can be cleaning by immersion. Use a Freon-or alcohol-based cleaning solvent. Use of other cleaning solvent. (e.g. Trichlene, chloroethene, thinner, benzyl alcohol) may damage the relay case. However, some type of relays use materials which are chemical resistant. Select the suitable relay or solvent compatibility chart below. ¡DCleaning with the boiling method is recommended. Avoid ultrasonic cleaning on relays. Use of ultrasonic cleaning may cause breaks in the coil or slight sticking of the contacts due to the ultrasonic energy.¡@
7. coating	¡DDo not coat dust-cover type relays and flux-resistant type relays, since the coating material may penetrate the relay and cause contact failure. ¡DDo not use this coating method in TTi  relay.