Magnetic Timer Case Study

The Prototype Designer Timer Circuit

For the prototype electrical timer, the most widely used electronic circuit, both in the industry and in commercial circuitry, which is the time delay circuit or timer, within the category of timers, will be used. This is the most economical and is accurate as well, consisting of a resistor and a capacitor. When we need a timer, the first thing we must consider is the precision in the delay time, it is a very important basis to determine the elements that we are going to use in its conception and design. As mentioned above, a timer consists of an element that activates or deactivates a load after a more or less long pre-set time. In this way, we can determine the parameter related to the time that has elapsed so that the circuit susceptible to be programmed, is activated or deactivated or what is the same simply close or open a contact.

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The Simple Delay

The simplest of the delays and perhaps the least precise, requires a resistance of some value and a capacitor of considerable capacity, both will depend greatly on the voltage to which the device is connected. A delay in a cutting shear machine is required, which entails a certain danger and risk of an accident happening, to the operator that handles it. We must install a security system to avoid any accident as much as possible. We need a security system that meets, among others, at least the following requirements: That only when the operator is out of danger, the blade of the shear can lower.

Said safety system, must produce a minimum delay before lowering the blade.

It will be equipped with a sound and an intermittent warning light source that enhances safety.

The first step can be achieved with the combination of strategically placed switches, called limit switches and a pair of pushbuttons, located near the path of the blade or its access. For the second point, we can choose a rectifier diode D1, a resistor R1 and a capacitor C1. The extremely simple assembly is shown in figure 01 and below the description.

The diode D1 is responsible for rectifying the current provided by the secondary of a transformer to which the equipment that will be connected, for which the basic and elementary precautions must be observed when selecting the different mentioned elements. It is important to consider a margin of safety of the tension to which the components of the assembly will be subjected. Even if the DC voltage is present, the diode D1 is needed to avoid the return discharge, and then the resistor R1 is inserted, which will be directly responsible for the charging time of the electrolytic capacitor, that is, the higher the resistive value, the longer it corresponds. Charge the capacitor. In order not to enter into empirical calculations, it is a question of carrying out some tests or tests to find out the resistance that in principle must allow to pass a very low current, depending on the capacity of C1.

The next element, capacitor C1, must be chosen for a considerable capacity, which is very important, but without losing sight of the voltage, if we use the 220V mains voltage, the C1 voltage must be above 400V or greater than the voltage will be subjected, to avoid heat or puncture being permanently unusable. A condenser of those used in the motors of washing machines or refrigerators will be adequate. When choosing the capacitor, it is convenient to consider its size and whenever possible should be chosen as mentioned by an electrolytic model (hence the use of the diode) due essentially to greater capacity and smaller size. However, in some cases, this is not possible, using in this case one of the non-polarized industrial ones of about 8 to 12 mf and I repeat, for safety> 400V, for a mains voltage of 220V. There are some observations made when a voltage is applied to diode D1, of figure 01, the current is rectified half a wave when crossing it, this reduces it to approximately half, this tension is the resistance R1, which restricts its passage to a calculated value for a current pass of a few mA (milliamperes).

At the output of R1, the voltage is precipitated to charge the capacitor C1, which is the path that offers the least resistance and, that load time, is precisely the time it is intended to control, since during that time of loading, the Current will not flow beyond junction R1-C1. Keep in mind that the charging time (charge constant), does not represent more than two thirds (2/3) of the total capacity, exceeded which, the current will begin to flow to the next conductor element that finds, in our case may be a relay.

From the above, it can be assured that the current that crosses the circuit travels two paths; one represented by the dashed line (Ic) during the first 2/3 of loading, and another, that of the output (Id) in figure 01. The output can be connected to a relay that will be responsible for producing the desired effect connect / disconnect, as necessary. While making the assumption that it is the direct one, one can also use a more sophisticated form, the relay is connected in series with the resistor R1, which reduces its value, so that the relay is activated by the current of passage for the load. In both cases, it is perceived that the control is not such, since the constant charge of the capacitor is influenced by many parameters, therefore also unreliable. It needs more control and time range for better control and security. As it is clear from the assembly that although it is simple, its flexibility and precision are very compromised.

Astable Multivibrator

An astable multivibrator, is a modification of a bistable multivibrator, which in turn, is based on a monostable. An astable multivibrator consists of a few components and two transistors coupled by two capacitors. In the diagram of figure 03, it can be observed that an astable multivibrator, the values of the two capacitors of 100mf and those of R2 and R3 are both 15KW, at a working voltage of 5V, the cycle of the astable multivibrator is 1 second per each half cycle, the cycle time or total period, is approximately 2 seconds. The frequency at which the circuit operates is the reciprocal of the period, that is, 0.5 Hz. Observe that in the points marked as Q and / Q, a complementary signal is obtained, very useful in many projects, Q = 1 when / Q = 0 and vice versa.

The clock frequency and complete cycle can easily be adjusted by the values of the two coupling capacitors of the transistors or their load resistors. There is a limitation, however, in this: the frequency that operates is only approximate. It is not very accurate and is subject to dependence with the aging of the components over time.

The Intermittent as Delay

The solution can be in the transistors that allow a greater control of the different parameters, such as the tripping currents and voltages. See the intermittent formed by T1 and T2 in figure 04, the filp-flop or scale formed by these transistors T1 and T2 (universal NPN transistors), to which a third transistor T3 has been added, step separator to make the load independent of the relay at its output. By adjusting the times by the potentiometers P1 and P2, the flip-flop tilt is determined, obtaining better control and amplitude of the delay times. There is an explanation on the manner through which the assembly works. The key of this circuit is based on the fact that the two transistors T1 and T2 lead in opposition and in saturation. That is, T1 leads in saturation, the only condition is that meanwhile T2 does not have to drive and vice versa, when T2 goes into conduction it will saturate abruptly or as quickly as possible, cutting the transistor T1, this is the function of a scale, one goes up the other low. So, when we feed the circuit, consider initially that, the transistor T1 is in saturation, the voltage in its collector will be 0V, V1 = VCC, this will discharge C1, then the base of T2 through P1 and R2 will be positive. This causes the transistor T2 to tilt, which will go into saturation, taking the collector voltage from T2 to 0V, causing the system to restart at the same time.

While T1 is driving, C2 will be charging with the time constant R3 C2, this time is what determines the conduction of T1. Likewise, when...