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For many things, especially in mechanics, it is necessary to use some type of pulsating circuit that we can regulate to put signals to lamps, coils, injectors among other things. In this article I will tell you then how to make an oscillator with easy-to-acquire components and with few elements:
The first of all:
Or pulse generator to relay. It is very common to see this circuit on the internet:
The relay has two contacts, one normally open and the other normally closed. The load to be fed is usually connected to the NO contact of the relay, which gives the greatest amount of pulse time. The frequency is given by the values of C1 and the load that in this case is D1 and R1 in series. Increasing the values of C1 we obtain lower frequency.
The recommended values are between 100uF and 2000uF.Where:
In this case the circuit will operate as seen in the following image:
1.- When the circuit is energized by closing the button, the relay is activated and capacitor C1 is also charged.
2 .- At this time there is a slight delay framed in the time it takes the C1 to load, then most will go through C1 instead of the coil.
3.- When C1 is loaded, current flows through the relay coil and the contact is activated, leaving the relay coil again without a negative supply.
4.- At this moment the load of the C1 keeps the coil closed, consuming its energy.
5.- When the load of C1 is consumed, the relay changes state again and the cycle is restarted.
As you can see, the frequency is given by C1 and the resistance that we have connected to the normally open contact of the relay. So we do not have a stable frequency because it depends on what we connect there.
You can check the existence of high voltage transients by mounting the following assembly:
The additional thing is:
To solve this situation I propose the following circuit:
Like the previous circuit, it operates in the same way to generate the pulses, except that in this case a complete square wave will be produced in the output load, which will be handled by a secondary relay that will oscillate as a function of the frequency of C1 and the resistance value of Rele 2.
In this circuit add another relay in order to stabilize the pulse frequency for the load. For this, we can stabilize the + 12V voltage of relay 1 (RL1) with some regulator such as 7812, and independently power relay 2 (RL2). Then we will always have the same pulse frequency.
The other particular thing in this circuit, is the way of connection of the relay 2, placed in a double polarity arrangement, to the output we will obtain a complete square wave, which results in a more stable and better designed oscillator.
I added the diodes D2 and D3 to avoid the internal surges produced in the coils of the relays when the contacts operate. The surges if not handled well, shorten the useful life of the capacitor and inserts noise to all circuits that connect to that system. In some cases this noise is deadly. So you have to be very careful with these.
For this oscillator two relays are needed as the previous one that I proposed, except that with this a symmetrical pulse is produced; that is, our oscillator has the same operating cycles in both the off and the on.
Its operation is simple:
When we close the switch, the relay RL1 is off and its NC contact passes energy to the relay RL2 by actuating it.
When RL2 is activated, RL1 is activated, leaving RL2 consuming the energy of capacitor C2.
When C2 is completely discharged, RL2 is opened, it is at this moment that RL1 is activated with the load that C1 has. Leaving RL2 without tension and in the same resting position.
Once C1 is discharged, RL1 is deactivated and re-energized to RL2. Restarting the cycle in this way.
This design is much more stable than the previous one and also more economical. I show it below:
This circuit is a variant of the previous symmetric of two relays. The interesting thing about this proposal is that it uses a smaller and cheaper capacitor, just like the auxiliary relay is removed and replaced by an NPN transistor.
As always in my analysis I offer the advantages and disadvantages of the proposed circuits. Most of all for this one that is simple and quite popular, so I'll list them for you to see them.
1.- Frequency of operation limited and a little unstable.
2.- Pulses of irregular duration. The ignition cycle is much longer than the off cycle. (For the initial circuits)
3.- If you do not place the diodes in the relay coil, dangerous overvoltage and high frequency peaks are generated. It is not safe to connect it together with electronic equipment. Connecting it to the battery of the car with all the electronic elements of it would be very risky.
In the field of electronics:
There are many applications taking into account the diode in the coils of the relays.
In the automotive field:
It can be used as:
- Warning flashing light.
- Activation signal for windshield wipers.
- Coil tester.
- Injector tester, but only for small periods of time.
Remember that the opening pulse is very long and reheats it.
Video how i do it
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