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We stated earlier that the simplest control situation is when the device that we wish to control is electrically operated. A typical example might be a light or an electric motor. All we need is a control device that can be operated from a small low-voltage signal and will, in turn, control a much larger voltage or current. The most common electrical control devices are switches, relays, and solid- state devices.
Usually the control device simply turns a circuit on or off. In spite of its simplicity, the on-off control device is adequate for nearly every home control application. Of course, there are a few situations where an on-off control device is not adequate. One application that comes to mind is where we want to control the intensity of a light. Here we need a device that will turn the light on and off and control the amount of light that it emits. Another obvious application where on-off control is inadequate is where we wish to control the speed of a motor. We want a device that will not only turn the motor on but will also control how fast it turns.
In every case, one of the principal objectives of the control device is to isolate the control signal from the power line. This isolation is necessary to avoid potential electric shock or fire hazards.
The most obvious device to use for remote control is a simple relay. Here a low-voltage signal applied to the coil of the relay controls a much larger voltage that is switched by the relay contacts. In selecting a control relay, be sure that the contacts are capable of handling the voltage and current drawn by the device that you wish to control and that the resistance of the coil is suitable for the control signal you wish to use. it's a good idea to use enclosed relays so that the contacts will not become contaminated with dirt or dust.
The principal limitation of a relay is that the coil must be energized continuously to hold the relay in one of its two positions. This imposes rather strict conditions on the power supply of the control system. It must be able to supply enough current to energize all of the relays in the system when necessary. Usually it's much better if we only need to send control signals when we wish to change something. The rest of the time there is no signal on the control line. A system of this type has rather simple power supply requirements and has practically no risk of electric shock or fire.
The latching relay is an ideal component for any remote or automatic control system. A latching relay is one that will latch in either of its two positions. The only time that we have to apply a signal is when we wish to change the relay from one state to another. This permits the control signals to be in the form of pulses that are only transmitted when we want to change the state of whatever we are controlling. The rest of the time there is no voltage or current on any of the control lines.
Latching relays are available commercially from electronics supply houses or from electrical lighting houses. They are used commercially for low-voltage control of ordinary home lighting. Unfortunately many commercially available latching relays of this type are quite expensive. it's better, if possible, to find them in surplus houses where they are usually available for much less.
If you can’t buy a latching relay at a suitable price, you can always build your own. Figure 6-1 shows a sketch of a homemade latching relay. The heart of this relay is a reed switch that is operated magnetically. The contacts of the reed switch are normally open. When a magnet is brought near the switch, the contacts will close. In the arrangement of ill. 6-1 the reed switch is mounted in a small hollow cardboard tube. A small permanent magnet is also placed inside the tube and the ends of the tube are closed so that the magnet can’t fall out. If the magnet is moved directly under the reed switch the contacts will close. When the magnet is moved a short distance away, still inside the hollow tube, the reed switch will open.
On the outside of the cardboard tube we have wound two coils of fine wire. The magnetic fields from either of these coils cause the permanent magnet to slide back and forward inside the tube, opening and closing the reed switch. To close the switch we simply energize the coil at the left of the figure long enough for the magnet to move under the reed switch. The current can then be removed. To open the switch we energize the coil at the right of the figure.
The coils are only energized long enough to slide the magnet to the proper position. The limitations of this homemade latching relay are that it must be mounted horizontally and it may operate erratically if it's subjected to excessive vibration.
AN IMPROVED HOMEMADE LATCHING RELAY
Although the homemade latching relay shown in ill. 6-1 is fully adequate for many home applications, it has the limitation that the permanent magnet must move. This means that it must be mounted horizontally and may be subject to the influence of vibration.
If you are willing to do a little experimenting to find the correct dimensions and locations of the components, you can build a latching relay that is both simpler and more reliable. It has no moving parts except the reeds inside the reed switch and thus can be mounted in any position. it's no more subject to the influence of vibration than the reed switch itself.
The circuit of ill. 6-2 works on the principle that it takes less magnetic force to hold a reed switch closed than it does to actually close it when it's open. The trick is to find the proper position of the bias magnet shown in the figure. Usually this magnet can be taped directly to one of the leads of the reed switch. The magnet must be positioned so that it will not cause the reed switch to close, but will keep it closed if another magnetic field is used to actually close it.
The best way to find the proper position of the bias magnet is to use two small magnets. Tape one of the magnets loosely to one of the leads of the reed switch. Connect an ohmmeter across the reed switch so that you can easily tell whether the contacts are open or closed. Then bring the other magnet close to the reed as shown in ill. 6-3. Adjust the position of the bias magnet so that it will not cause the reed switch to close, but will keep it closed once the flux from the other magnet closes it. The relay should open and stay open when the other end of the magnet is brought close to it.
The coil is mounted so that its flux will add to or subtract from the flux from the bias magnet. It may be necessary to experiment a little to find the best position for the coil. Once a relay of this type has been designed experimentally, it should be possible to build others using the same type of coil, reed switch, and magnet with no trouble. This latching relay will require a little more design time than the one shown in ill. 6-1, but it will be much more reliable and easier to use.
The principle limitation of this type of relay is the current carrying capacity of its contacts. The reed shown in the figure can carry a current of about 1 A if the load is noninductive.
To use a reed switch to control larger currents, it's advisable to use an SCR to actually control the load, and use the reed relay to turn the SCR on.
The latching relay of ill. 6-2 only uses a single coil. it's latched by applying a current through the coil in one direction and is unlatched by running the current through the coil in the opposite direction. The relay of ill. 6-1 uses two separate coils, one for latching and one for unlatching the relay. Most commercial latching relays also use the two-coil arrangement.
CONTROL SYSTEM FOR LATCHING RELAYS
The latching relay has the advantage that it can be operated from a low-voltage dc power supply. Inasmuch as current only flows when a relay is being latched or unlatched, the power supply requirement is very small. It doesn’t hurt to overload a power supply for the second or so that is required to operate the relays.
The easiest way to latch and unlatch relays is to use a dual-polarity power supply. The same arrangement can be used with two coil relays by adding two diodes to the relay as shown in ill. 6-4. With this arrangement if the applied signal is positive with respect to ground, the current will flow through coil A. the latching coil. This will close the contacts of the relay. If the polarity of the applied signal is such that it's negative with respect to ground, the current will flow through coil B, the unlatching coil. This will open the contacts of the relay.
The same power supply can be used with the single coil relay because the direction of current through the coil will change with the polarity of the applied signal as shown in ill. 6-5. Thus it's possible with this power supply arrangement to use both single coil and dual coil latching relays on the same system. In either case, the relay will be opened by a signal of one polarity and closed by a signal of the opposite polarity. It doesn’t matter which type of relay happens to be on any particular circuit.
If commercial latching relays are used in a system, design of the power supply is very simple. It only has to supply the rated operating voltage of the relays. If homemade relays are used, however, it's advisable to determine the optimum operating voltage. The way to do this is to connect the relay in the circuit of ill. 6-6. Here the voltage is slowly increased until the relay latches. Then it's increased a little more. The coil of the relay is left connected to the supply for about 15 seconds to be sure that it will not heat up excessively. This is much longer than the relay will ever be energized in an actual control system, but it should be able to pass this test. Once the proper operating voltage has been found, the output of the power supply can be adjusted accordingly. Because of the intermittent nature of the latching relay arrangement, the power supply isn’t at all critical. Most relays can withstand a rather large surge of current for the one second or less that it takes to perform the latching or unlatching operation.
ill. 6-6. Circuit for finding relay voltage.
Figure 6-7 shows the schematic diagram of a dual-polarity power supply suitable for operating a latching relay system. It actually uses two separate half-wave rectifiers—one to supply the positive voltage and the other to furnish the negative voltage. The filter capacitors are not really necessary, but without them the relays may chatter annoyingly when the state is being changed. The supply shown can be set for either a 6 or a 12 volt output. Usually this will take care of any homemade latching relay. If the power supply is only being used to control latching relays, a transformer rating of one amp is fully adequate. If the supply will also be used to energize other circuits such as indicators, the transformer should be rated accordingly.
The fuse in the primary part of the circuit is a very important part of the supply and shouldn’t be left out. The supply of a control system is left energized at all times and should be protected against drawing excessive currents which might cause a fire. If the load is very heavy, it will be helpful to use a slow-blow type of fuse.
The power supply can be built right into the control box of the system, but this means that it will be necessary to run the power line into the control box. This is satisfactory, if precautions are taken to avoid the possibility of an electric shock in the event of a component failure. In many cases, it's advisable to mount the power supply in another location and run the operating voltages to the control box. In this way, the highest voltage in the control box will only be about 12 Vdc and the arrangement will be completely safe.
The control switching arrangement for a system using latching relays is extremely simple. Figure 6-8 shows a typical arrangement. There are three connections between the control box and the power supply—lines carrying +12 V, -12 V, and ground. One side of every control circuit is grounded. The switches are all momentary action push-button switches. When switch Si is closed, the positive supply line is momentarily connected to terminal A which goes to one of the control circuits. This positive pulse of current will cause the corresponding latching relay to latch, turning on whatever is connected to the circuit. When switch S2 is closed momentarily, the negative supply line will be connected to terminal A, unlatching the relay and turning the controlled device off.
ill. 6-7. Simple power supply for control system.
The diagram shows two additional circuits, but as many as desired can be added. Because only impulses are used for control signals, there is no limit to the number of devices that can be controlled by this simple switching arrangement.
A very small control box may be made by using a selector switch as shown in ill. 6-9. Here there is only one on switch and one off switch. Again they are momentary action push-button switches.
Closing switch S1 will momentarily apply a positive pulse to the wiper of the selector switch. The circuit that this pulse actually goes to is determined by the position of the selector switch. Thus if the switch is set to position A, the pulse will go to the latching relay corresponding to device A, turning it on. To use the system, simply set the selector switch to the position corresponding to the device that is to be controlled. Then closing switch Al will turn the device on and closing switch S2 will turn it off.
The switching arrangements of Figs. 6-9 and 6-10 can be used with the various indicator arrangements described in section 2. This will give an indication of the state of anything that is being controlled.
SOLID-STATE CONTROL DEVICES
Although the simple latching relays described in the preceding paragraphs are adequate for a great majority of home control functions, they are somewhat limited in the amount of current that the contacts can safely carry. Furthermore, if the controlled load is inductive, there may be sparking at the switch contacts causing deterioration of the contacts.
Increased current carrying capacity as well as freedom from contact problems can be obtained by using solid-state devices to switch the actual power.
Figure 6-10 shows the schematic diagram of a silicon con trolled rectifier (SCR). This device has three electrodes; an anode, a cathode, and a gate. The anode and cathode are quite similar to those in an ordinary diode. The difference is that no current will flow between the anode and the cathode of the SCR until a signal has been applied momentarily to the gate. Thus with the SCR connected as shown in ill. 6-10 no current will flow. It will look like a switch that is open. When a signal is applied to the gate for a very brief period, about 5 millionths of a second (5 microseconds) the SCR will “fire.” This can be done by momentarily closing switch Si. Current will now flow freely between the anode and the cathode as in an ordinary rectifier diode. The forward voltage drop across the SCR will be very small.
Note that once an SCR is turned on we no longer have any control over it by means of the signals applied to the gate. Removing the signal from the gate has no effect at all on the anode current. The only way we can turn the SCR off is to temporarily stop the anode current. This could be done by opening the circuit by means of switch S-2 or by momentarily short circuiting the SCR by closing switch S3 in the diagram. When this is done the anode current will stop and the SCR will be reset to its original nonconducting condition. Note that it takes a much longer period of time to shut the SCR off than it does to turn it on. When we interrupt the anode current we must wait at least 50 microseconds before reapplying forward voltage to be sure that the SCR will not retrigger.
The SCR has one rather unpleasant characteristic. It will turn on if the anode voltage is applied too suddenly. Thus sometimes an SCR will turn on when power is applied to the circuit even though no control signal is applied to the gate. This propensity of the SCR to turn on when voltage is applied is called the dv/dt effect. It can be minimized by the use of a snubber circuit. Details of snubber circuits will be given shortly.
Figure 6-11 shows a different type of SCR. Here the gate is not used for triggering. Instead, the SCR is turned on by applying a beam of light to a photosensitive area behind a glass window in the case. The beam of light has exactly the same effect as a signal applied to the gate of a normal SCR. Because this type of SCR is light activated, it's called a light activated SCR or simply LASCR. When the LASCR is dark no current will flow through it. As soon as light is applied to the window it will turn on. As with a regular SCR, the only way we can turn it off is to momentarily interrupt the anode current by opening the circuit or by shorting the anode and cathode leads together. Note that removing the light has no effect at all on the anode current.
Both of the SCRs that we have described so far conduct current in one direction only. In this respect they are very similar to the rectifiers used in ordinary power supplies. They are most suitable to use in dc circuits. If they are used in ac circuits, they will rectify the current and will only deliver power during one-half cycle of the applied voltage.
Figure 6-12 shows a device called a triac that is similar to an SCR except that it conducts current in both directions. The triac can be turned on in either direction by a small gate current of either polarity. Because the triac conducts in both directions it never has any high reverse voltage as is sometimes found in rectifiers and SCRs. If the voltage applied in either direction should become too high, the triac will simply be turned on. Most triacs are capable of conducting very high currents.
Figure 6-13 shows a typical triac circuit. Here when switch Si is closed a small gate current will be supplied to the triac. It will turn on and supply power to the lamp in the circuit. Note that Si must be kept closed for the triac to conduct. Otherwise the anode current will be interrupted every half cycle, and the triac will stay in the off state.
Figure 6-14 shows a two-terminal device called a diac, or bilateral trigger. It will not conduct current in either direction until the voltage across it reaches some specific breakdown voltage. Because there are two diodes in the unit, it can be turned on by an applied voltage of either polarity. Diacs are used to develop sharp pulses of current for turning on other switching semiconductor devices such as SCRs or triacs.
Finally there is the quadrac. This is a special device which contains both a triac and a diac in a single housing. Quadracs are sometimes referred to as triacs with trigger.
For any of these semiconductor switching devices, there are two important specifications that the circuit designer should be concerned with. These are peak voltage and power rating.
The peak voltage rating should never be exceeded. If a voltage higher than this is applied to an SCR, triac, or quadrac, the delicate semiconductor crystal can be destroyed. This specification is given as the peak voltage, not the rms voltage. You can find the peak voltage by multiplying the rms voltage times 1.4.
Vpp=Vrms x 1.4
e.g., 120 volts ac rms works out to 168 volts peak.
V = 120 x 1.4 = 168 volts peak
The peak voltage is always greater than the mis voltage. If you don’t exceed the peak voltage rating of the device with the m voltage you will leave yourself plenty of headroom for added safety.
The second important specification for an SCR, triac, or quadrac is the maximum power rating. Once again, this rating should not be exceeded by any signal applied to the device.
The power rating will usually be given in terms of current. e.g., 3 amps, 6 amps, or whatever. In a few cases, the specification may be given in wattage. Simply divide the maximum wattage by the maximum voltage to find the maximum acceptable current.
Wattage = Current x Voltage
The maximum power rating listed in the manufacturer’s spec sheet is usually made with the assumption that a heatsink will be used on the device. The heatsink may be omitted if the applied signal will always be well below the specified maximum ratings. Still, a heatsink is a good idea whenever possible. If the device will be operated near its maximum peak voltage and /or power rating, a good heatsink is essential.
We stated earlier that an SCR will turn on if the anode voltage is applied too fast. Each SCR has a critical time factor. If the anode voltage is applied in a shorter period than this critical time, the SCR will turn on regardless of whether or not a signal is applied to the gate.
Usually 60 Hz ac will not cause false triggering if the load is purely resistive, lithe load is inductive, however, the voltage rise may be fast enough to cause false triggering. To avoid this false triggering, snubber circuits are used.
Figure 6-15 shows a typical snubber circuit. It consists of a resistor and a capacitor in series directly across the SCR. The design of snubber circuits is rather involved and will not be treated in detail here. In the circuits described in the guide, the snubbers have been designed to give satisfactory performance. However, the reader will probably not use exactly the same load devices that the author used. e.g., a relay or a motor that happens to he available to the reader may have more or less inductance than those used in the original design. If spurious triggering is encountered, it can be cured by experimenting with the snubber circuit to deter mine proper component values.
RADIO and TELEVISION INTERFERENCE
SCR type controls switch very rapidly and hence tend to generate high frequency energy. This high frequency energy can cause annoying radio and television interference. If the motor is only used occasionally for short periods to perform an operation such as opening and closing draperies, the interference will probably not be very objectionable. If on the other hand, an SCR system is used to control the speed of the motor which will be running for a substantial period of time the resulting radio and television interference may be severe. To minimize interference from an SCR control, completely shield the control circuit. This will be all that is needed in some cases. In other cases it will be necessary to install filters in the leads running to and from the control system.
Figure 6-16 shows a typical radio interference filter. The capacitors are commercially available and the inductance can be made easily from available materials. The inductance in ill. 6-16 is made by winding ordinary enameled wire on a ferrite core of the type used on antenna transistor radios. By carefully shielding the control circuit and installing filters in the leads, interference can usually be reduced below the objectionable level.
TRIAC CONTROL OF AC LOADS
Figure 6-17 shows a circuit in which a triac is used to remotely switch ac circuits on and off. The gate of the triac is connected to the power line through transformer Ti at one end and through a variable resistor at the other end. The transformer T1 is an ordinary 120 V to 6 V transformer. The circuit is designed so that when the secondary of the transformer is open there will not be enough primary current to fire the triac. With some transformers, the leakage inductance of the transformer might cause enough primary current to fire the triac. Resistor R1 is variable and can be adjusted so that the triac will not fire when the secondary is open. When the secondary of transformer T1 is shorted by a switch at the control point, a large amount of primary current will flow causing the triac to fire.
Another way of looking at this is to consider the primary of the transformer as having a very high resistance when its secondary is open. This will prevent the triac from firing. When the secondary is shorted, the primary resistance will decrease drastically connecting the gate of the triac to one side of the line, thus causing it to fire.
When the secondary of the transformer is shorted a large amount of current will flow in the primary and would ordinarily burn it out. As soon as the large current starts to flow in the primary, the triac will fire. This will drop the voltage across the transformer with practically all of the line voltage appearing across the load. Thus the transformer current only flows during a small portion of each cycle of the ac line voltage. A half cycle later when the line voltage drops to zero, the triac will open and the whole process will be repeated. Thus we must keep the switch that is connected to the secondary of the transformer closed as long as we want power applied to the load.
This circuit is very practical for control of almost anything around the home. With the components shown in the figure it will control loads up to about 500 watts. About the only limitation of the arrangement is that a small pulsating current will flow in the control line all the time that the load is energized. This means that a momentary action switch can't be used for control. The pulsating current might also induce hum in intercom lines if they are run in the same cable as the control lines.
A LATCHING RELAY WITH A TRIAC
When we discussed the latching relay earlier in this section, we mentioned that its principal limitation was that the current carrying capacity of the contacts is somewhat limited, precluding its use in controlling heavy loads. Figure 6-18 shows a latching relay used in conjunction with a triac to control heavy loads. The load current that this arrangement can control is limited only by the rating of the triac.
In this circuit, the gate of the triac is normally open so it will not fire. When the latching relay is energized, the gate will be connected to one side of the line causing the triac to fire every half cycle applying power to the load. This circuit has all of the advantages of the latching relay without any of its limitations. Power is only applied to the circuit long enough to turn it on or off, so momentary action control switches can be used. The result is that there is no voltage or current on the control lines except when the state of the controlled device is being changed.
DIMMING OF LIGHTS
Figure 6-19 shows a circuit that will provide two levels of illumination. When full illumination is desired, the power line is connected directly to the lights. For reduced illumination, a half- wave diode of the type that is normally used as a power supply rectifier is connected in series with the lighting circuit.
The diode must be able to carry the entire lighting current in the forward direction, and the contacts of the latching relay must be able to carry the entire lighting current.
Although this circuit only gives two levels of illumination, it's fully adequate for many applications. The lighting power is cut in half when the diode is switched into the circuit. Because of the fact that incandescent lamps are not efficient at low currents, the amount of light will be considerably less than half.
There will be applications where two levels of illumination will not be suitable. If the lights involved are in the same room with the control center, it will probably be adequate to use a commercially available dimmer at the control point. This has the disadvantage that there is really no complete isolation of the control system from the power line. All of the other arrangements in this guide provide this isolation and any circuit that involves bringing something to the control box that is not fully isolated from the power line is definitely not recommended.
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Updated: Saturday, April 9, 2011 18:49 PST