- Input voltage
- 0 to 360 V on all four models
- Input current
- 1 mA to 30 A as printed in the parameter table; 0.1 mA on the model cards
- Input power
- 150 W on CH9720B and CH9720BU; 300 W on CH9720C and CH9720CU
- Load modes
- Constant voltage, constant current, constant power, constant resistance and short circuit
- Accuracy and resolution
- ±(0.05% + 0.03% FS) on voltage and ±(0.05% + 0.05% FS) on current; 0.1 mV and 0.1 mA on the lowest ranges
- Sampling
- 500 kHz, with voltage and current ripple measured as Vpp and Ipp
- Battery test
- 0.8 to 360 V, capacity to 999 AH, discharge time 1 to 60000 s
- Interfaces
- RS232C with SCPI, handler connector and USB host
- Body
- 230 × 100 × 350 mm, about 6 kg or 6.5 kg
A bench supply is judged by what happens when it is asked for current it did not plan to give. An electronic load is the instrument that asks. This series puts a 360 V, 30 A sink in a body the size of a bench meter, in two power levels and with or without local storage.
Product overview
The CH9720X series is a programmable DC electronic load in four models. All four take 0 to 360 V and up to 30 A; two deliver 150 W and two deliver 300 W. The models with a U suffix add USB storage and screen capture, so readings and screen images can be saved without a computer.
The load has five modes: constant voltage, constant current, constant power, constant resistance and short circuit. A battery discharge function, a dynamic test with two timing values, ripple measurement and a 60-step list test sit alongside them. The instrument samples at 500 kHz and displays voltage, current and power against their limits on a 4.3-inch colour screen.
The front panel carries the display at the left, a numeric keypad and two rotary knobs at the right, and soft keys along the lower edge. The rear carries the sense connector, the serial port, the handler connector, the mains inlet and the voltage selector. The body is 230 mm wide at about 6 kg, or 6.5 kg on the 300 W versions.
How the programmable DC load works
The load is a power stage that sinks current from the source under test. In constant current mode it holds the current at the set value and the voltage at the terminals is whatever the source produces; in constant voltage mode it holds the voltage and lets the current follow. Constant resistance and constant power do the same job against a different reference, and short circuit applies the lowest resistance the stage can present.
Every mode is a way of asking the source a question. A supply that regulates well into a constant current load may still ring when the load steps, which is what the dynamic mode is for: it switches between two levels with timings from 0.05 ms to 999 s and records how the output responds.
The measurement is taken at the terminals rather than at the source, so the voltage read is the voltage the load sees, after the lead drop. Sampling at 500 kHz is what allows the ripple to be reported as a peak-to-peak figure with separate positive and negative peaks, rather than as an average that hides them.
The battery function runs the same stage in a controlled discharge. It starts at the pack voltage, holds a current, accumulates capacity and stops at a set time or cut-off, which is a different job from holding a fixed load.
How to select the right programmable DC load
The first question is the voltage. A 360 V input covers non-isolated supplies and power factor corrected front ends, which is where most of this class of work sits. A source that delivers more than 360 V is outside the series.
The second is the power. A 150 W model holds 30 A only up to 5 V, because power is the product of the two; at 48 V the same model reaches about 3 A. The 300 W models double that. Reading the current range without the power limit is the most common way to specify the wrong model.
The third is the mode. A resistive load is not the same test as a current sink, and a supply specified into a resistance needs the constant resistance mode rather than constant current. Short circuit is a fault test, not an operating point, and should be chosen deliberately.
The fourth is the record. The serial link with SCPI and the handler connector are standard on every model; USB storage is what the U suffix adds. A bench that logs through a computer does not need it, and a bench that collects results by hand does.
Send the output voltage and current of the source, the power it delivers continuously, and the tests the drawing requires, and a model can be proposed against them.
Measurement limitations and selection considerations
The load sinks current; it cannot source it. A product that needs a source and a sink at the same time, such as a battery that has to be charged and discharged in one cycle, needs two instruments.
The power rating is a continuous figure and the current rating is a maximum at low voltage. A test that draws 30 A at 48 V asks for 1440 W from a 150 W instrument and will trip its protection rather than run. The protection threshold is set at 5% above the rated condition, so the instrument protects itself before it is damaged, but the test does not happen.
Lead resistance sits between the source and the load. At 30 A a 10 mΩ lead drops 0.3 V, which is a real error on a 5 V supply. The remote sense connection at the rear exists for this and should be used whenever the current is high.
Ripple figures depend on the bandwidth of the measurement and on where it is taken. Two loads with different filter settings will report different ripple on the same supply, so the setting belongs in the test record along with the reading.
Finally, the material does not state the mains supply or the operating temperature range. Both should be confirmed before the instrument is written into a test procedure, particularly on a bench that runs continuously.
Every value below is taken from source documentation. Where a parameter is not stated in the available material it is marked as information required rather than estimated.
