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Frequently Asked Questions

Hardware

For which applications are Electronic Loads (DC Loads) suitable?

- Power supply test
- Control unit test
- Check of automobile generators
- Check of fuel cells
- Check of battery chargers
- Simulation of electronic consumers

For which applications are AC Loads suitable?

- Transformer test
- Safety test

What can happen at overvoltage or reverse polarity?

If a voltage higher than the maximum admissible input voltage is applied to an Electronic Load a short-circuit may be caused which may lead to undefined current flow, arcs and even fire. The same can happen if the unit under test is applied in reverse polarity. Therefore we strongly recommend to connect an external fuse between Input+ of the Electronic Load and the DUT! Dimensioning the fuse depends on the demands of the DUT.

How shall an external fuse be dimensioned?

The load circuit of the Electronic Load does not have an internal fuse. The reason for this is that no fuse would be the right one for the large number of devices under test. Therefore, the user must dimension the external fuse in a way that his respective device under test is protected. The fuse has to be switched between the positive pole of the device under test and the positive load input.

When shall the Zero Volt Option be used?

The Zero Volt Option extends the operating range of the Electronic Load to very low input voltages, when otherwise the load could not control the full current. The Zero Volt Option is only available for DC loads.

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The Zero Volt Option compensates for voltage losses and allows the operating of the device down to input voltages of a few millivolts. With this option the device allows the test of current limitation characteristics down to the total short-circuit. If the device is set to operating mode current and a higher load current is adjusted than the UUT can provide, the voltage of the unit under test collapses to 0V and the amperemeter shows the short-circuit current. For the operating mode voltage the load voltage can be adjusted down to 0V.
For operating mode resistance the resistance range is extended from the lower limiting value of any range to 0 Ohm. When the sense terminals are connected to the output terminals of the UUT, the short-circuit will be controlled up to the position where the sense terminals are connected with the output terminals of the unit under test. That means, even the resistance of the load cables is taken into account and compensated. To do so, the load terminals can become even negative (max. -0.5 V) to compensate the voltage loss on the load cables. Therefore a short-circuit over longer cables can be adjusted, which is not possible with power switches. The in-built watching of the polarity won’t produce a reversed polarity of the UUT.

Power reduction caused by Zero Volt Option When a device is extended with the Zero Volt Option, a power reduction of the device has to be taken into account. This power reduction depends on the adjusted load current and can be calculated according to the following formula:

P = Pn - (I × 4.5V)

P: remaining device power
Pn: nominal power of the device
I: load current in A

 

After the installation of the Zero Volt Option there may occour static voltages at the load input up to approximately 4 V.
Displaying this negative voltage is permitted and doesn’t restrict the functionality of the device.

How can I prevent stability problems caused by an oscillating system?

When power supplies or other circuit arrangements stabilizing an output parameter by a control loop are tested, two regualtors are connected together when applying the electronic load to the UUT. When a phase shift of more than 180° and an amplification >1 are reached by the system, the oscillation condition is true and the system starts to oscillate.
This state is no fault of the electronic load but is unwanted in tests. The oscillating system can be stabilized by interrupting the conditions for the oscillating system.
A capacitor may be connected parallel in series with an resistor to the load input. Sometimes a small MKT capacitor of about 1µF with 1 Ohm of resistance in series is already enough to stabilize the system.
In addition, at the DS and ZS series of electronic loads a slower regulation speed may be set (see chapter Analog I/O connector).

How can I avoid input coupling by current drawing lines?

Especially in Constant Resistance mode you have to note that an input coupling from the input lines to the voltage measurement may occur if sense lines are used.
Since in constant resistance mode the exact acquisition of the input voltage is important to make the correct current setting a magnetic coupling into the sense lines causing a positive feedback makes the system instable.
At first, you have to reduce the coupling.
That means:
Separate the sense lines from the input lines. Also separate the sense lines from all other lines drawing current, e.g. mains supply.
The sense lines should be twisted to eliminate the induced voltage.
Never twist the sense lines with any of the current drawing lines!
The load input lines should be twisted or at least run in parallel to compensate the magnetic fields.
And of course:
Keep all lines as short as possible!
If all these steps don't bring the desired success, a capacitor may be connected between the sense lines.

How can I prevent a distorted slew rate in dynamic mode?

To reach the best possible current slew rate in dynamic mode the following conditions have to be fulfilled:

  • The dynamic input resistance of the voltage supply has to be very low. The electronic load is not able to compensate voltage variations at the moment of the fastest possible current variation.
  • The resistance of the input lines must be very low (same reason as in a) )
  • The input lines must be non-inductive. Inductive lines (all cables have got an inductive component) in addition with its ohmic resistance result in a limitation of the maximum possible current slew rate. The electronic load can not perform a fast current slew rate if the slew rate is limited by the connecting lines. Furthermore, the connecting lines behave like an energy store (self-induction) and deliver current into load and UUT when being unloaded.
How can I measure the current slew rate?

The current slew rate measurement must be made with a clip-on probe which is fast enough. (E.g. Tektronix Current Measurement )
Current measurements by measurement shunts mostly deliver faulty results since most shunts are inductive. Such measurements deliver false slew rates with oscillations.

How can I avoid distorted analog outputs?

Especially when clocked power supplies are tested, situations may occur in which the Monitor outputs for current, voltage etc. (Iprop, Uprop, Pprop) at the analog I/O connector are distorted.
The causation has to be searched in the way of test assembly.
Clocked power supplies have got filters in the output circuit and capacitors from the output to protective earth.
Also the electronic load and other instruments include filters because of EMV reasons.
The common mode noise voltage (voltage between each output terminal and protective earth) causes a differential current through the anti-interference capacitor and the electronic load (or other instruments) back to the load output.
This differential current often generates high-frequent superpositions at the measurement signals.
At dynamic tests very high interference voltage may occur.
To solve this problem, you can supply the electronic load and/or the other instruments by insulating transformers with low coupling capacity.
Thereby the interference circuit is interrupted and the measurement quality is made better.

How do ground loops arise and how can I prevent them?
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Ground Loops can be prevented by using galvanically isolated analog inputs at the data acquisition system or electronic load. For this case we can offer you for series ZS a "galvanically isolated analog interface". The standard Analog I/O card can be exchanged against an insulated version. All measuring and controlling signals are connected via insulating amplifiers and optocouplers.

Software

USB: USB interface not visible at WIN7

What can I do if if the USB interface of the ZS/NL Load is not visible as Virtual COM Port anymore in the Windows Device Manager after the system was ported from an XP to a Windows 7 or Windows 10 operating system?

  • For NL and ZS series electronic loads delivered before June 2014 the USB driver of H&H's homepage has to be used. In other cases, the most up-to-date driver from FTDI has to be used (http://www.ftdichip.com/Drivers/VCP.htm).
  • The H&H USB driver is not signed and only tested with Windows 7 (64  Bit) and Windows 10. Please contact the H&H support if there are still problems with the USB driver.)
  • The date code of delivery is coded in the last four characters of the load's serial number (MMYY).

 

GPIB: What can I do if a stand-alone device cannot be programmed?

Check if the LEDs at the back panel light up for a short time at power-on.
Also the Remote LED at the front panel must light up shortly at power-on (only DS Series).
After power-on, wait at least 5s until the first command is sent to the device.
After the first command has been sent to the device the back Remote LED must light up permanently. If not, the following reasons are possible: a wrong/invalid GPIB Address is set at the device.
In a system of several GPIB devices two or more devices have set the same GPIB address. the controlling program uses an invalid termination character (see manual). There is a hardware error.
The Remote LED at the front panel must also light up permanently after the first command is given to the device. If not, a wrong sub-address is addressed in the command string (see manual) or there is a hardware error.

GPIB: What can I do if i don´t get responses to queries from a stand-alone device?

Check DIP switches CR, LF, EOI at the back panel: at least one of them must be "ON" (ex factory: all ON).
The control software must use the same termination combination in Read mode as set at the device (see manual).
If the device accepts commands but doesn't respond to queries it is possible that you address it with sub address 0 (see manual) although a sub address between 1 and 999 is set in the device.
See manual for how to find out a device's sub address.

GPIB/RS232: What can I do if one or more devices in a system cannot be programmed?

- Check system bus cable (see manual)
- Check sub addresses (see manual). If devices have been ordered as system the increasing sub addresses beginning from 1 are set ex factory.

GPIB/RS232: What can I do if one or more devices in a system don't respond to queries?

- Check system bus cable.
- Check sub addresses (see manual).

RS232: What can I do if a stand-alone device cannot be programmed?

Check if the LEDs at the back panel light up for a short time at power-on.
Also the Remote LED at the front panel must light up shortly at power-on.
After power-on, wait at least 5s until the first command is sent to the device.
After the first command has been sent to the device the back Remote LED must light up permanently. If not, the following reasons are possible:

  • The RS 232 parameters (baud rate, data length, stop bits, parity) at the device don't match with those in the controlling program (see manual).
  • The controlling program uses an invalid termination charakter (see manual). there is a hardware error.
  • The Remote LED at the front panel must also light up permanently after the first command is given to the device. If not, a wrong sub-address is addressed in the command string (see manual) or there is a hardware error.
RS232: What can I do if i don´t get answers from a stand-alone device to queries?

There must be a delay between sending the query command and reading the respond (see manual).
The control program must use the same termination combination as fixed in the device (see manual).
If the device accepts commands but doesn't respond to queries it is possible that you address it with sub address 0 (system address, see manual) although a sub address between 1 and 999 is set in the device.
See manual for how to find out a device's sub address.

ZS Series: What can I do if there are inconsistencies with Trigger Input/Trigger Voltage (Pin Assignment Analog I/O)?

Starting from manual version ZS_HW_7C the pin assignment is described correctly (Pin 23,25). Please download the newest manual version!