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 

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.

1.2 Constant Resistant Operation Mode 

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 

Figure 10 DC Equivalent circuit of CC mode operation

Figure 11 DC Equivalent circuit of CR mode operation

 3. Other Operation Features

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

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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 

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).