3250 Series AC & DC Electronic Loads - Application Description

The 3250 Series consists of part numbers 3250 (60V/20A/300W), 3251 (150V/8A/300W), and 3252 (300V/4A/300W). These AC & DC electronic loads are suitable for endless DC, AC, or DC and AC power source testing purposes; for example, testing AC to DC power supplies, AC to DC voltage adapters, DC to DC power supplies, AC transformers, Intercom System ring generators. (See figures 1 through 5)

This article will discuss AC operation, DC operation and other functions in detail.


Figure 1 Example of AC & DC Power Supply Testing


Figure 2 Example of DC & DC converter testing 


Figure 3 Example of DC & AC Inverter testing 


Figure 4 Example of Power Transformer testing


Figure 5 Example of Power Transformer and Rectifier testing

1. AC Operation 

When testing an AC either constant current or constant resistance modes may be used.
1.1 Constant Current Operation Mode 
1.1.1 When selecting the constant current mode, the load current will remain the same whether the AC source voltage fluctuates or not. (See figure 6)


Figure 6 AC Equivalent circuit of CC mode operation

1.1.2 The constant current mode may be used for AC source load adjustment or source capacitance testing. These functions may be programmed via the GPIB, RS-232C interface.

1.1.3 With a constant current, the loads needs to be set to match the input AC source frequency in order to assure load current and voltage are in the same phase. The 3250 series loads have the capability of detecting zero crossing and input load wave form. There are five different Crest Factor (peak value) current wave forms suitable for testing input voltage peak current capability: 1.414, 2, 2.5, 3.0, and 3.5. Please see the following block diagram.


Figure 7 Block diagram of CC mode operation

1.2 Constant Resistant Operation Mode 

1.2.1 When a 3250 series load is connected to an AC source (as shown in figure 8) in the Constant Resistant (CR) mode, its load current setting is directly proportional to the voltage, which means load current and voltage increase at the same time.


Figure 8 AC Equivalent circuit of CR mode operation

1.2.2 The constant resistance operation mode may be used for over current start testing or AC power start up. While it can also be used for testing load adjustment rates or power capacity the power source output voltage will decrease as the current increases. Voltage and current should be recorded at the same time in order to calculate the change rate, which is different from the constant current mode previously discussed as having a stable load current where it is only necessary to record the voltage for the change rate.

1.2.3 There is no need to set these units in the constant current mode because the electronic load current wave form reference source is a voltage wave form. (Please see diagram below)


Figure 9 Block diagram of CR mode operation

2. DC Operation 

The 3250 Series has two DC operating modes, constant current and constant resistance
2.1 Constant Current Mode 
Refer to 1.2.1 substituting 'DC source' for 'AC source'. With a DC voltage input the frequency should be set at 0 Hz for straight load current wave form; if set at 50 or 60 Hz the load current wave form will resemble a 50 or 60 Hz voltage wave form. Please refer to figure 10.


Figure 10 DC Equivalent circuit of CC mode operation

2.2 Constant Resistant Mode 
Refer to 1.2.1 substituting 'DC source' for 'AC source'. Please refer to figure 11.


Figure 11 DC Equivalent circuit of CR mode operation

 3. Other Operation Features

 3.1 Electronic Meter 

This series comes equipped with 3 AC & DC RMS electronic meters: voltage, current and power meters. A VA meter displays capability (by multiplying voltage by current). The data provided by these four meters provides accurate and complete AC & DC measurements, there is no need for additional switch settings. New data will be provided every 0.5 seconds. As these units display two pieces of data at a time, the V/A/W button will display either V A or W VA. See figure 12.


Figure 12 Function diagram of Electronic Meter

3.2 GO/NG Comparison 

This series has upper limit and lower limit measurement comparison function capability. When the measured value is between upper and lower limits it is a GO or pass. Outside these limits the reading is a NG or fail and the NG L.E.D. will light up to indicate there is a failing condition. Measuring values include voltage, current, watt and VA; each has its own upper and lower limit and all values must be within the proper limits for the tested unit to pass. This series' displays only show an NG L.E.D. When using logic symbols, results will be as follows:


When using a block diagram, the NG function will appear as follows : 


Figure 13 Block diagram of GO/NG function

3.3 Load ON/OFF 

A front panel button is used to control the load ON/OFF function. When the LED is illuminated the load on function is activated. CC or CR modes may be preset and will absorb the power through the loads. Changing load setting will change the current wave form. When the LOAD button is OFF the loads will stop loading work and the current meter will show 0 ampere. Regardless of whether the load is ON or OFF, source voltage can still be read.

3.4 V Sense ON/OFF

Voltage sense on is activated with a front panel switch and on status is indicated by its illuminated LED. The 3250 V Sense connector is connected to the unit under test for measurement control. V Sense internal block diagram is shown in Figure 12. V Sense ON/OFF control are from load input, and related measurements may be read on the front panel volt meter when in the constant current mode. Constant current zero cross sensors are not used in a DC voltage application.


3.5 Setting for Constant Current Operation Mode 

When in the constant current mode it is necessary to press the Frequency button in order that the wave form bank will synchronize with the signal source (SYNC ON/OFF).

3.5.1 Frequency 
In the constant current mode for an AC application, synchronization of the internal wave forms generated is required to obtain the relative positions of voltage and current. This series'specification is 0 Hz (DC), 10 Hz - 400 Hz (AC); actual setting ranges are 0.0 Hz - 99.9 Hz, and 100 Hz - 999 Hz. Adjustments can be made at 0.1 Hz or 1 Hz intervals.
3.5.2 Wave Form Bank 
In the constant current mode, there are five peak current options: 1.414, 2.0, 2.5, 3.0, and 3.5 (see figure 7). These are stored in 1 of 12 internal wave form banks. 55 additional wave forms can be set. EPROM memory is used to store this information, it is stored digitally and a D/A converter is used to convert to an analog wave form.
If the preset wave forms in the constant current or the constant resistance modes do not meet specified requirements, unique wave forms had been created. Refer to the 3250 Series Operation Manual for more Crest Factor, Square, and Step wave forms.
3.5.3 Synchronize Signal Source 
AC operation in the constant current mode requires synchronized signals in order to synchronize voltage and current wave forms. The synchronize source has internal Off, or external On options.
This series has an internal self synchronizing mode. When set in the SYNC Off position, the internal zero crossing sensor will recognize a wave form zero crossing signal and convert it to a square wave. This signals the internal wave form generator to force synchronization. As a result, load current wave form is related to voltage wave form.
If the voltage wave form is incorrect, this internal synchronization will not occur. In this case, external synchronization should be used otherwise only constant resistance can be used to synchronize load current and voltage wave forms. User should provide a square wave that synchronizes with the voltage to the mainframe BNC input, and set module SYNC to On in order for the constant wave form generator to synchronize with the external signal source.
4. Making A Selection
4.1 Selecting A Load Module
The 3250 Series consists of part numbers 3250 (60V/20A/300W), 3251 (150V/8A/300W), and 3252 (300V/4A/300W). Select the load that meets your voltage and current requirements. Higher accuracy is achieved when the module selected has a current rating closest to the application. If the power rating exceeds the power of a single module it is possible to parallel additional modules to achieve the power requirements.
4.2 Selecting a 3300 Series Mainframe
4.2.1 When a single module will satisfy the testing requirements, the 3302 electronic load mainframe would be the best choice. If requirements are expected to expand, a 3300A should be considered as up to three additional modules may be added as required. Blank panels may be purchased in the interim. Both the 3302 and 3300A can be purchased with or without a GPIB interface.
4.2.2 The 3300A mainframe accepts up to four 3310 and/or 3250 electronic load modules, in any combination. For a multi or mixed load requirement this is the best choice.
4.2.3 The 3300A and 3302 mainframes can be paralleled. There is no limit on the number of mainframes/modules that can be connected in parallel.