How to design a voltage stabilizer using Relays and LM324 Op-Amp and NE555 Timer IC
A very common configuration of voltage stabilizer is using Op-Amp like LM324 and NE555 Timer IC. Before using any micro-controller this circuit configuration was used the most. Even still now, some companies are using this circuit where there is no digital display required. As because this design is easy and low cost, so still this circuit is popular. In this article, we’ll discuss voltage stabilizers using analog circuits. So let’s start!
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Before we start, we have to know how the voltage stabilizer works. To know about the working principle, you can read the previous article from here.
In an analog voltage stabilizer, the line voltage is stepped down to 12V or 18V to make a power supply for the operating circuit as well as the voltage sensing loop. Sometimes, a separate voltage loop is kept of 12V for sensing purposes. Then a voltage sensing circuit is used which is later used in the comparator circuits. A delay timer is kept to make an initial delay which helps in-line stabilization.
A set of Op-Amp comparator circuits are used for switching of different relays. These relays are used to change the transformer taps. Finally, the output is switched to the load. The whole process is not so complex at all. Only some blocks working at a time.
It is completely the designer’s choice that how many taps will be used for the voltage stabilizer. But a suitable range of the lowest taps is 5. For a 220V system, these 5 taps are usually kept as 150V, 180V, 220V, 280V,260V & 0V. 5 Relays are used for this type of voltage stabilizers. So based on the choice of taps, you should design your own transformer. The formula is very simple, practice is required for good performance. Here is a practical way to do the job.
- First, select the tap numbers then voltages
- Calculate the total power and wire size from the ampere rating keep a safety margin for wire size here.
- Calculate the core area from that power rating. Keep a safety margin here for overloading issue
- Find the bobbin size of the transformer
- Calculate the Turn per Voltage for your design
- Calculate total turns and mark each tap, like for first tap 150V, 405turns, then for the second tap of 180V 512 turns, and so on.
- Wind a sample transformer for testing. Check the voltages with and without load. A full load test should be run for several hours. This will generate heat and will help to remove the moisture inside the core and coils.
- Give a varnish bath in a container immediately, keep overnight [or at least 3hrs] and extract the excess varnish by hanging the transformer for hours.
- Once the transformer is dry again, fit the angles and other accessories. Then set the terminals and test again.
- Once it passes the whole process you can scale it up but if fails, calculate where the fault is, then correct it and remake.
Sounds easy? No? It depends on your practice. Those are the way to make a professional transformer. I’ll publish a different article on transformer calculation later.
Circuit diagram blocks:
There are several blocks in our circuit and you already know that. If each block is explained separately, it will be easier to understand. Let’s see in blocks.
Let’s use the 5 tap transformer for our design:
The voltage sensing and power supply circuit:
Here, using two separate winding works the best. because if only one winding is used, the sensing voltage will be changed due to the relay switches.
In the power supply circuit, two voltage regulators are used for better heat dissipation and division of headroom voltage. Diode D1 and D2 are used to protect the voltage regulators from reverse discharge. Sometimes, Transistors are used also LM317 can be used. The alternative circuits are:
Here LM317 voltage regulator is used. As the headroom voltage is high, so sufficient heatsink must be used for cooling. Another type of power supply is:
In this type, the Tip122 Darlington transistor is used with a Zener diode of 12V to form a voltage regulator. This type of voltage regulator is pretty common in voltage stabilizers or analog circuits.
Note: It’s totally the designer’s choice which type s/he will use. A combination of these circuits can be formed to get an optimum result.
An initial delay is a must for the voltage stabilizer. Otherwise, a spike can be pass to the load which is dangerous for the loads. NE555 IC is the first name that comes for this purpose. NE555 is configured in one shot or monostable mode for this timer purpose.
After the delay circuit, we need to design the comparator circuit which will actually control our relays for different voltage.
The most common Op-Amp ICs are LM324 and LM339. Depending on the designer’s choice s/he can use any one of these. While using LM339, you must keep in mind that it is open-collector type output and you need a pull-up resistor for that.
Here, R1 works as the hysteresis control resistor. Hysteresis is important in our circuit to protect the relays from sharp switching. Without hysteresis, the relay can be turned on/off at the same point of line voltage which will kill the relay as well as other connected contacts.
Relays are very common for isolated switching purposes. And for a voltage stabilizer, a relay is a common choice too. There are different types of relays in the market. We will use the very common one with one set of NO/COM/NC.
While using these types of relays, we must use a freewheeling diode across the coil. A transistor can be used for switching the relay coils. Also, a capacitor across the relay coil is preferable for good magnetic flux generation across the relay coil.
Now, we can combine all the parts into one to form the control circuit.
Complete circuit diagram:
The whole circuit diagram is pretty large. I tried to draw it all in a clear way. If I made any mistake kindly let me know, I’ll correct it later.
All the blocks are described earlier. In this circuit, all those blocks are attached to each other where they should be. There is a diode in between two consecutive comparator circuits to make them working one by one sequentially. Another two Op-Amp comparators are used to set the low and high voltage limit.
There is no other changes I think except a RC filter in sensing circuit.
Once the circuit is ready, you can fit this in a box like this with other accessories.
Besides the control circuit and transformer, you need some other accessories for a complete voltage stabilizer. Although it is not unknown to you if you come this far, here is the list for help:
- A box with suitable cutting holes for fittings
- AC cord for supply input
- Input circuit breaker/FUSE
- Output circuit breaker/FUSE
- Handle fitted with the box for handle
- Screws and washers
- Plastic supports for circuit placement
- Cable ties.
As you see, analog circuits are too complex and sometimes they become so hard to troubleshoot. But on the other hand, a micro-controller can solve many many complexities in any circuits. Analog circuits are cheap and once it is designed finally, can be used for a long time. It’s totally up to you what you’ll use. But I hope this article will help you make your own product.
Yes! I always post articles that can be turned into products or assist somehow. I think if you stay with me, one day your work will be productive and your business will grow. Thank you again. If you need any help kindly comment below.