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Transformer: Low Voltage Transformer

Author: Apogeeweb Date: 6 Jun 2020  1095

Electronic low voltage transformer

I Introduction

This article introduces the basics of low voltage transformer, including the definition of low voltage transformer, the principle of low voltage transformer, its installation method, malfunction, and how to repair a household low voltage transformer.

A transformer is an electronic instrument that we can use everywhere in our lives. For example, we are inseparable from the mobile phone chargers. The internal components also have transformers. For example, the power supply in our computers is also composed of transformers. Therefore, all electronic products are indispensable for transformers.

The transformers used in electronic components are all power transformers (included low voltage transformers). The power transformer is mainly used in electronic products to convert the power supply to the voltage required by the electronic circuit.



Ⅱ Basics of low voltage transformer

2.1 Definition of low voltage transformer

2.2 Types of low voltage transformers

2.3 How does low voltage transformer work?

2.4 Design requirements of low voltage transformer

2.5 Application environment

Ⅲ Differences between low&high voltage transformer

Ⅳ How to Estimate the Loss of Low Voltage Transformer

V Guide to What You May be Interested in

5.1 How to Install Low Voltage Transformer for Household?

5.2 How to Troubleshoot a Low-Voltage Transformer

5.3 Repairing a household transformer

5.4 Malfunction of factory low voltage transformer

VI One Question Related to Low Voltage Transformer

6.1 Question (Multiple choice questions)

6.2 Answer

II Basics of Low Voltage Transformer

2.1 Definition of Low Voltage Transformer

Transformers are important power equipment in our life, especially low-voltage transformer. Whether it is for large-scale power plants or home circuits, it is inseparable from the transformer. There are two kinds of transformers used in our life, one is the high-voltage transformer which used in the high-voltage or ultra-high-voltage circuit, the other is the low-voltage transformer which used in the family lighting circuit or small power circuit.

The so-called low-voltage transformer refers to the transformer with low load voltage. As long as the load is lower than 600V, it can be called a low-voltage transformer. The low-voltage transformer is mainly composed of primary coil, secondary coil, and a magnet iron circuit. Low voltage transformers are commonly used for low voltage lighting, which typically only uses 12 or 24 volts. 

2.2 Types of Low Voltage Transformers

There are two main types of low voltage transformers: electronic and magnetic.

(1)Electronic Low Voltage Transformers

Electronic transformers are much smaller, lighter, and less expensive than magnetic counterparts. But they only have about a 5-6 year lifespan. Also, electronic low voltage transformers are known for being noisy.

Electronic Low Voltage Transformers



Smaller and lighter, easier to hide

▪ Less expensive

▪ Shorter life span

▪ Noisy

▪ Heat-sensitive

(2)Magnetic Low Voltage Transformers

There are two types of magnetic transformers: stack laminated and toroidal.  

  • Stack laminated transformers have a longer lifespan, about 15 to 20 years. But they only operate at about 80 to 85 percent efficiency, also are known for being noisy.
  • Toroidal transformers are very quiet and more energy efficient. They can operate at about 90 to 95 percent efficiency. And they are long-lived, lasting 20 to 25 years.

Magnetic Low Voltage Transformers



▪ Long life span of over 15 years

▪ Energy efficient

▪ Quiet

▪ Heavy, big, and difficult to hide

▪ Expensive

2.3 How Does Low Voltage Transformer Work?

Electricity and magnetism are two kinds of energy that can mutually transform. Electricity can generate magnetism, and magnetism can also generate electricity. The low-voltage transformer uses this principle to adjust the voltage through the mutual conversion of electricity and magnetism.

A low voltage transformer consists of two electrical coils of wire, one of which is called primary winding and the other is called secondary winding. The primary side of the transformer collects power and the secondary side provides power.

low voltage transformer structureFigure 1. Low Voltage Transformer Structure

The two coils intertwine together on a magnet iron circuit core, but without electrically in contact with each other. The magnetic core is made of soft magnetic material, which consists of laminations connected together to help reduce core loss. The core allows power to be transferred from one coil to another. When the primary winding is connected to the power supply, the generated magnetic field transfers the voltage to the secondary winding.

How does low voltage transformer work?

Figure 2.  How Does Low Voltage Transformer Work?

In summary, the primary winding transforms the electrical power into magnet field when connected to the input voltage supply while secondary winding transforms alternating magnetic field into electrical power of required output.

2.4 Design Requirements of Low Voltage Transformer

The low-voltage transformer should be vacuum cast, molded resin encapsulated, 60Hz, Class F insulation, comply with the requirements of IEC726, and meet the requirements of capacity, voltage, phase number, and wiring shown. Each low-voltage transformer has separate primary and secondary windings, and there are two 2.5% normal voltage full-capacity taps above and below the rated voltage on the primary side. The low-voltage transformer should be installed on a base that can isolate, reduce vibration and noise, and the iron core and coil should be properly fixed to withstand the mechanical stress generated in the event of line failure and can withstand 16460 Lite 16460-6 V2. 0 2002/ 11/ 25 Vibration and impact during shipment. Unless otherwise specified, the impedance of low-voltage transformers shall be in accordance with IEC 726. The average noise level of the low-voltage transformer should not exceed the value specified in IEC 726. Each dry-type low-voltage transformer should have appropriate terminals to accommodate the required primary and secondary wiring connections. Low-voltage transformers can be reserved for cable entry from either side or bottom.

2.5 Application Environment

(1) Ambient air temperature - 5 ℃ to + 40 ℃, 24-hour

average value not more than + 35 ℃

(2) The altitude of the installation site shall not exceed 2000m

(3) The relative humidity of the atmosphere shall not exceed

50% when the ambient air temperature is + 40 ℃

(4) A place without violent shaking and impact vibration

III Differences Between Low&High Voltage Transformer


Low Voltage Transformer

High Voltage Transformer


continuous winding

Interleaved Windings


Oil-immersed cooling or air cooling

Oil-immersed cooling


oil tank structure

bell jar type oil tank

Ⅳ How to Estimate the Loss of Low Voltage Transformer

Transformer loss include copper loss and iron loss. The loss of iron loss is also called no-load loss, and the loss of copper loss is also called load loss. The open circuit of the secondary winding of the transformer applies the no-load current of rated frequency and rated voltage to the primary side, and the active loss caused by the transformer core is called the no-load loss of the transformer. The loss figure can be obtained by the unit loss of the iron core silicon steel sheet multiply the quality of the silicon steel sheet. In general, its loss is very small.

For example, the S9-100/10 distribution transformer has a no-load loss of 290W. The larger the transformer capacity, the smaller the no-load loss, generally between 0.3-0.15% of the rated capacity. However, the transformer works continuously for 24 hours, and this loss cannot be ignored.

The load loss: The short-circuit of the coil on the secondary side apply the rated current at the rated frequency on the primary side. The transformer loss at this time is the loss of the transformer coil and the iron core.

Let's take the S9 series transformer as an example and see its load loss value:

The load loss of S9-100/10000 transformer is 1500W.

The load loss of S9-1000/10000 transformer is 10300W.

The load loss of a transformer is generally between 0.9-1.8%. The larger the transformer, the smaller the load loss.

V Guide to What You May be Interested in

5.1 How to Install Low Voltage Transformer for Household? 

First, make sure you have an outlet available. Most family homes will have several outdoor power outlets installed. 

Second, think about the layout of the lighting. It is important to plan ahead when installing low voltage lighting so that you can avoid any potential problems, especially with the setup.

Third, choose a transformer.

①Magnetic or Electronic.

②Make sure to buy a transformer that can handle the lighting load you need it to.

Fourth, install the transformer. Transformers are installed simply by plugging them into your outdoor outlet.

5.2 How to Troubleshoot a Low-Voltage Transformer

(1) First, check whether there are obvious abnormalities by observing the appearance of the transformer: such as whether the coil lead is broken, whether the insulating material has scorch marks, whether the fastening screw of the iron core is loose, whether the silicon steel sheet is rusted, and whether the winding coil is exposed Wait.

(2) Test a transformer with a digital multimeter

open all secondary windings, put the multimeter in the AC block (500mA, in series into the primary winding). When the plug of the primary winding is inserted into the 220V AC mains supply, the multimeter indicates the no-load current value. This value shall not be greater than 10% - 20% of the full load current of the transformer. Generally, the normal no-load current of power transformer of common electronic equipment should be about 100mA. If too much is exceeded, the transformer has a short-circuit fault.


Low Voltage Electronic Transformer Troubleshooting Guide

5.3 Repairing a Transformer

(1) Understand the reasons for the problem. Generally, a transformer has failed due to some fault in the electrical circuit.

(2) Check the replacement transformer. If the short circuit is caused by component failure, the new transformer may still burn out. If you replace the transformer, examine it to make sure that the incident will not occur again.

(3) Check the condition of the external fuse. If the transformer has an internal fuse, there may not be a fuse on the power line. On the contrary, the device must be protected by a fuse mounted on the power circuit. Check whether the fuse is intact and replace the faulty fuse.

(4) Check the secondary power consumption. Sometimes, this kind of consumption is very high, which leads to equipment failure. If the transformer has multiple ratings and the multimeter shows the value "OL" during the measurement, it may be a short circuit in the secondary winding.

5.4 Malfunction of Factory Low Voltage Transformer

(1) Abnormal sound in transformer

The abnormal sound inside the transformer may be caused by the following reasons:

①Heavy overload causes buzzing sound inside the transformer; 

② Due to poor internal contact or breakdown point, the transformer occurs crackle sound

③ Some parts of the connection shaft and the core of the transformer are loose, which causes the silicon steel sheet to vibrate

④ When there is a grounding or short-circuit fault in the power grid, a large current flows through the winding, which will produce strong noise

⑤ The iron core, the winding discharges to the shell, or the core ground wire is disconnected. All these can make the transformer discharge sound.

(2) The oil level of the transformer is too high or too low

Under normal circumstances, the changes in oil temperature can cause a change in oil level. As the oil temperature changes, the oil level also changes accordingly. However, under abnormal conditions, abnormal oil levels can also be caused by faults such as oil seepage and water seepage and other accidents. Second, the change in oil temperature is related to load conditions, ambient temperature and other conditions. When the change of oil level is inconsistent with these elements, it may be a false oil level.

(3) Transformer oil quality has deteriorated or oil temperature has suddenly increased

In working condition, the main function of transformer oil is cooling and insulation. When running overheated for a long time or if water enters the casing and absorbs moisture, the oil quality will deteriorate. Through the oil mark, we can found  that the oil color is abnormally deepened or blackened. Through sampling and analysis, it can be detected that the oil contains carbon particles and moisture, the acid value increases, the flashpoint decreases, and the insulation strength decreases. This situation can easily lead to serious accidents.

A sudden rise in oil temperature when the transformer is operating normally is often related to the overheating inside the transformer. The iron core catches fire, the internal screws are loose, the cooling device is faulty, the transformer is heavily overloaded, all may cause the oil temperature to rise suddenly.

(4) Transformer on fire

When the transformer is out of order and it is not handled in time, it may catch fire. When the transformer is on fire, the insulating oil burns and turns into gas, which makes the oil tank burst. The burning insulating oil sprays out of the transformer, which will cause equipment damage and property loss. The short circuit inside or outside the transformer wire, severe overload, a lightning strike may cause the transformer to catch fire.

6.1 Question (Multiple choice questions)

The winding of wire around a core which is connected to a source of energy is called the ______ coil.

  1. primary
  2. secondary
  3. tertiary

6.2 Answer


Ordering & Quality

Photo Mfr. Part # Company Description Package PDF Qty Pricing
HX5004ENLT HX5004ENLT Company:Pulse Electronics Network Remark:PULSE XFMR 1 CT:1CT TX/RX 350UH Package:300
In Stock:On Order
300+: $6.45780
H5120NL H5120NL Company:Pulse Electronics Network Remark:Pulse Transformer 1CT:1CT Surface Mount Package:N/A
In Stock:On Order
1+: $8.59000
10+: $6.67900
25+: $6.49800
50+: $5.59560
100+: $5.34280
250+: $5.23452
500+: $5.14426
1000+: $5.05400
5000+: $4.96375
H1164NLT H1164NLT Company:Pulse Electronics Network Remark:XFRMR MODUL AUTO MDI/MDIX 10/100 Package:250
In Stock:1750
250+: $6.97952
500+: $6.88050
1250+: $6.80625
6250+: $6.68250
12500+: $6.58350
HX5084NL HX5084NL Company:Pulse Electronics Network Remark:330µH Pulse Transformer Surface Mount Package:N/A
In Stock:On Order
1+: $6.50000
10+: $4.94000
25+: $4.81000
50+: $4.42000
100+: $4.16000
250+: $4.03000
500+: $3.90000
1000+: $3.77000
5000+: $3.64000
H1102 H1102 Company:Pulse Electronics Network Remark:LAN Pulse Transformer 1CT:1CT Surface Mount Package:N/A
In Stock:On Order
G501 G501 Company:Tamura Remark:2mH Isolation and Data Interface (Encapsulated) Pulse Transformer 1:1:1 Ter Through Hole Package:N/A
In Stock:On Order
H5007 H5007 Company:Pulse Electronics Network Remark:Isolation and Data Interface (Encapsulated) Pulse Transformer 1CT:1CT Surface Mount Package:N/A
In Stock:On Order
G512 G512 Company:Tamura Remark:2mH Isolation and Data Interface (Encapsulated) Pulse Transformer 2CT:1CT Through Hole Package:N/A
In Stock:On Order
1+: $11.16000
10+: $8.55000
25+: $7.24360
50+: $7.12500
100+: $6.76880
250+: $6.69752
500+: $6.60250
1000+: $6.53125
5000+: $6.41250

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