Step-Down Voltage Transformer

Pictured above is an iPhone 5s and Oscium’s iMSO-204L digital oscilloscope measuring voltage waveforms on a voltage transformer. The two channel oscilloscope gives users more capability than two single channel oscilloscopes. The iMSO App includes advanced mathematical functionality for comparing one channel against the other. Users can now compare and analyze the amplitude and phase of the two signals.

Transformers, Voltage Levels And Phase Shift

Transformers are AC devices that rely on the coupling of electric fields between adjacent coils to transfer energy from the primary to the secondary winding. The magnetic core between the two windings increases the effect of coupling, and the efficiency of the transformer. The figure below shows the AC voltage waveforms of the primary (green) and secondary (blue) windings. The two waveforms appear 180 degrees out of phase with the zero crossings perfectly aligned.

The phase shift between windings is a result of the opposite direction of the coil windings. Schematic symbols for transformers frequently include “dot notation”, to indicate the relative winding direction of each coil. The dot convention used in the transformer symbol below emphasizes the opposite winding direction between windings that results in the observed 180 degree shift.

Example: Desktop Lamp With Voltage Transformer

Pictured here is a desktop lamp that is powered through a voltage transformer. The lamp runs off of 110 VAC stepped down to 40 VAC through the transformer in the center of the image. Similar transformers are used in countless home appliances where it’s necessary to convert voltage levels from the source either up or down to be used in the load.

The diagram below depicts the major components in the lamp’s circuit.

The AC Source represents the household 110 VAC from the wall outlet.

The bulb on the far right of the diagram represents the lamp. The actual bulb in the lamp itself is a low voltage (40 VAC) device that still produces high intensity bright light. The relatively lower voltage and apparent high power consumption implies that the bulb draws significant current.

The dotted line represents the plastic case around the transformer. This lamp uses a single winding step down transformer to change 110 VAC into the 40 VAC used by the bulb.

The last element in the lamp circuit is the switch. Mechanically, the switch style is a single pole-single throw (SPST); there is a single electrical contact and that moves to make or break the circuit when the switch is actuated.

In the figure below Oscium’s iMSO-204L measures the voltages across the primary and secondary windings. Oscium’s new oscilloscope tool allows users to measure two voltages simultaneously.

Perhaps the most common application for two channel oscilloscopes might be measuring the gain of voltage amplifier circuits, where Gain (V/V) = Output_Voltage / Input_Voltage. In this case, however, the oscilloscope measures the input and output voltages of a single phase step down transformer. With this type of electrical component, the output voltage is purposefully lower than the input. The voltage reduction between input and output is directly proportional to the windings ration between the input and output coils.

The iMSO2 oscilloscope app works with iOS devices to display the iMSO-204L oscilloscope measurements on an intuitive touch screen interface. Users can use their two fingers to pinch-zoom to expand or contract the vertical and horizontal axes.

Learn More About Transformers

A discussion about transformers is outside the scope of this blog post. However, there are several online resurces available that provide above average treatment of the topic. One such website is electronic-tutorials.ws. Follow the link below to navigate into the website’s discussion about transformers.

http://www.electronics-tutorials.ws/transformer/transformer-basics.html

Pictured here is a functional diagram of a simple transformer, with introductory text from the electronics-tutorial website. Read up and enjoy!

Notice that the two coil windings are not electrically connected but are only linked magnetically. A single-phase transformer can operate to either increase or decrease the voltage applied to the primary winding. When a transformer is used to “increase” the voltage on its secondary winding with respect to the primary, it is called a Step-up transformer. When it is used to “decrease” the voltage on the secondary winding with respect to the primary it is called a Step-down transformer.