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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
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.
