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Software for transformer power supplies


This piece of Open-Source software can be used to simulate transformer power supplies and to select components for that.

Power Supply Software

The software is written in Lazarus Pascal. The two versions differ. The version 2 Source code for Lazarus and compiled versions for 64-Bit operating systems are provided here:
VersionWindowsLinux
Source codeExecutableSource codeExecutable
2V2 Win Source V2 Win64 Executable (not provided)
1V1 Win Source V1 Win64 Executable V1 Lin Source V1 Lin Executable

Use of the software

  1. Selecting type
  2. Transformer properties
  3. Diodes and rectifier bridges
  4. Capacitor, DC current
  5. Simulation
  6. Results

1 Selecting type

In the drop down field in the upper left the type of power supply can be selected.
Single diode Rectification with one single diode, one transformer coil, half wave rectification
Bridge rectifier Rectification with a rectifier bridge of the Graetz type with four diodes, a single transformer coil, full wave rectification
Double coil Rectification with two transformer coils and two diodes, full wave rectification
Changing the type comes immediately into effect:

One way In half wave rectification only the positive halfwave is active, the duration over which the capacitor is loaded is shorter, the duration over which the consumed DC current has to be supplied from the capacitor alone is longer.

Double coil When using a two coil transformer with two diodes both half waves are actively loading the capacitor. As only one diode with its conducting state voltage drop is required, the voltages are at their maximum.


Top of page Type Transformer Diodes Capacitor Simulation Results

2 Selecting transformer properties

Transformer properties Transformer equations Central parameters for the transformer are its nominal power (VA), the mathmatical product of nominal voltage Unom and nominal current Inom.

The other parameters are optional, but are required for a realistic modelling. The transformer coil(s) consist of a wire with a resistance (Ohm's law). The coil resistance in Ω can be measured on the transformer. To account for the voltage losses over this inner resistance Ri the transformer is designed for a higher voltage. This is called the no load voltage, V. Only if the design current flows the nominal voltage is reached, the over voltage is consumed by the inner resistance of the coil(s).

Some transformer sellers publish the no load voltage or the no load voltage factor, UTr = fnlv * Unom, others find those information not worth publishing, even though it influences voltages a lot (especially in small power transformers.

An additional effect that occurs in rectification is that all voltages are effective values Ueff. The effective voltage is the voltage that determines power, not the peak voltage of the alternating current. The mean voltage, with negative half waves changing their sign, averages the peaks by filling the valleys of the AC, thereby yielding an average power. With a sine wave AC the difference between the peak voltage and the effective voltage is of a factor of √2 or the 1.414-fold. With a 9 V transformer the no load voltage behaves as follows:

No load Voltage 9Veff Rectifying this voltage with a diode and loading a capacitor with that yields approximately 12 V and not 9 V!

Additionally considering the compensation of the inner resistance yields even higher voltages following rectification (no load):

Voltage with compensating the inner resistance The maximum voltage of the 9 V transformer now is higher than 17 V. A 16 V capacitor would be overcharged with that. This effect is even more dramatic with very small transformers with less than 1 VA.

If you change the power of the transformer in the software, the no load voltage factor is estimated from the size of the transformer and changes occur in the factor and in the no load voltage. If the nominal current (in mA) is changed the transformer power and the other parameters are changed, too. So better first select nominal current and/or power first, no load voltage, no load voltage factor and coil resistance later on. With double coil transformers the parameters voltage, current and inner resistance apply for one coil each, except for the power of the transformer.

Top of page Type Transformer Diodes Capacitor Simulation Results

3 Selecting the diodes/bridges

Diode dynamic Diode fixed The software offers two options for the diodes: a fixed voltage (click into the selection field Dynamic) or a linear approximation of the diode voltage drop. Select the voltage drop of the diode at a current of 0.1 A and 1 A, determined either from the specification or measured in practice.

In version 2 of the software a logarithmic function is used to calcul÷ate the current-depending forward voltage.

4 Selecting the load capacitor and consumed current

Capacity, current, frequency The load capacity in µF is written into the edit field Capacitor C1. If you select a too large capacity, the load voltage reaches its saturation only after very long times, e.g. here:

Large capacity Caused by the high inner resistance of the transformer the capacitor is charged slowly and reaches saturation only after several hundred half waves. If a microcontroller needs a high slew rate of the operating voltage then a large capacitor is not a good idea.

Large current The consumed current in mA is written to the next edit field. If this current nears the nominal current of the transformer the voltage on the load capacitor drops and the ribble increases, like here.

Switching between 50 and 60 Hz frequency can be done in the respective selection field. It changes the times of the displayed waves.

Top of page Type Transformer Diodes Capacitor Simulation Results

5 Selecting simulation parameters

The number of waves to be displayed can be selected in the two edit fields. The picture that is displayed can be saved as a graphics file, either in PNG or in BMP format.

6 Result field

The picture displays relevant parameters and results in the yellow field: Maximum and minimum voltage of the capacitor, the resulting ribble and the maximum current through the diodes during the last displayed wave.

Top of page Type Transformer Diodes Capacitor Simulation Results

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