In the previous sections, I’ve taken a look at how the R&S HMP4040.04 compares to the rest of the products on the market and completed the unboxing process. In this blog, I will look at the usability aspects of the power supply, with some basic testing of functionality and special features.


Things to Note

If you have not read the unboxing chapter, it is important to note that this HMP4040.04 arrived in a damaged condition. Rather than delay releasing a RoadTest review significantly, I have decided to release this review of the damaged unit with an understanding that it would be updated once an undamaged unit has been supplied. As a result, testing has been performed with some limitations and some of the observed issues may not be a result of the unit’s design. Readers are asked to keep this in mind as they read the review.

HMP4040.04 Channel Failure Powered OffHMP4040.04 Channel Failure Powered On

One particular failure that does affect my ability to fully evaluate all of its functionality is the loss of Channel 4. This channel was initially operative, but after about 40 hours of use, ceased to function. The output voltage is always slightly negative regardless of commanded voltage/current and persists through a full instrument reset. Switching the channel does result in relay clicks but only subtle changes in output voltage. I have run a short “exerciser” script to command the output on/off over 1000 times with no change in condition and have declared the channel lost. This failure, however, does not appear to impact on the performance of the remainder of the unit and basic testing continued utilising the three remaining channels, which is why some tests use four channels and others use three.

HMP4040.04 Version

Testing of the HMP4040.04 was undertaken with the current firmware at time of review, version 2.62.


Only basic testing has been detailed in this chapter - further testing of the instrument to verify its specifications is undertaken in Ch5: Instrument Performance Testing.


Support & First Power-Up

The HMP4040.04 comes with a printed quick start guide which is shared with some other members of the HMP series. However, a full manual and support software can be downloaded from their website. Thoroughly reading through the dedicated manual, it seems that around a third of the content is common to the printed quick start guide – you’d really want to obtain the dedicated manual mainly to have the programming command reference. The manual itself is not too thick and only has a few minor typographical errors which I’ve pointed out to Rohde & Schwarz for correction.


When it comes to support, I’ve had no difficulties with Rohde & Schwarz when it comes to contacting them for technical enquiries or for repair quotes, which is something that can’t be easily said about some other companies. I found their responses to be clear, precise and timely which is fantastic.


Getting started with a power supply is fairly simple and this unit is no exception. After placing the unit on a sturdy surface, checking the fuses and voltage selector match your mains supply, selecting the appropriate mains cable, plugging it in and turning it on, you’re ready to go.


The HMP4040.04 features a power button coloured red in the top left of the front panel. This actually feels to be a long plastic rod which reaches into the rear corner of the unit, actuating a mechanical hardware power switch that gives a reassuring “click” and ensuring proper isolation when powered down.


Upon powering up, the LCD displays the Rohde & Schwarz logo while the unit does a self-test, cycling the keypad LEDs through their respective colours and running the fan at full speed before ramping down. At the completion of the self-test, the LCD displays the “home” screen which lists the channel settings (assuming the outputs are powered down).

HMP4040.04 LCD Home Screen


User Interface

HMP4040.04 Front Panel On

When it comes to the user interface, the most noticeable part is the generously sized LCD screen. While it is only monochrome, with a resolution of 240x128, it is backlit and used to display each channel’s voltage/current/power status and used to display the operating menus. By default, the LCD was slightly flickery with a low contrast, but this is easily improved through the settings menu. Unfortunately, due to the limited display space, the HMP4040.04 suffers a similar flaw to many other supplies in that it is not capable of displaying both the channel set-point and metered readings simultaneously. The LCD seems to update rather frequently, I estimate at a rate of about 4Hz.


Aside from the LCD, user feedback is provided through the use of LEDs which shine through the translucent silicone keypad buttons. Each channel lights green to indicates it has been “enabled”, whereas the global output button lights white to indicate the outputs are switched on. The voltage/current/track/fuse/remote buttons light white to indicate the mode has been selected, whereas the directional buttons light white to indicate when they can be used.


Adjusting the voltage for a channel is a multi-step process – select the Voltage button, then click the channel you wish to change (if not already selected) which would light up blue. Then, you can either use the knob, direction buttons or keypad to enter in a new value and either press enter or wait until the fall-back time has elapsed and the setting is applied. To change multiple channels at once, you can use the Track button, prior to selecting the channels you want the change to apply to. This seems to be a fairly standard process for multi-channel supplies. Settings can be saved and recalled from internal memory.


However, the HMP4040.04 does offer the possibility for direct entry which can make it much quicker to enter a specific value, which is a feature many smaller units cannot afford to have.


The layout of the buttons seems fairly sensible, with the master output button at the top right and the master power button at the top left, allowing for easy identification by touch in case of emergency. During operation, the channel lights indicate CC (red) or CV (green) operation. The LEDs are quite bright but can also be configured with the LCD brightness through the menus.


Aural feedback is provided by an onboard beeper that beeps to acknowledge every key-press and in case of error conditions. This can be switched off in the menus as well.


Power output is sent through a set of shrouded banana connectors on the front panel. The choice of shrouded connectors can improve safety when used with compatible connectors, preventing accidental contact with voltages that can exceed SELV when used in series. The downside is that no protection is offered with traditional banana plugs and it makes connection of heavier gauge wires or bare wires more difficult as it can only be achieved through the connections at the rear. Unfortunately, without the possibility to configure series/parallel operation internally, external connections are required which can increase the wire clutter.


The terminal connection block at the rear allows for connection to a fixed wiring harness with the use of a small flat-bladed screwdriver. Each terminal is recessed, although the aperture is somewhat small, making connection of very thick wire impractical.


Sense connections are provided on the front and on the rear, and unlike some other power supplies, does not need to be manually configured for use. If they are connected, then the voltage drop along the wires will be compensated, otherwise the supply operates based on the voltage sensed at the supply-end. This makes it easier for users as they don’t need to remember to set the mode (two-wire or four-wire) and thus reduces the possibility of error.


Menu Layout

Configuration of the power supply and its features can be done through the nested menus which are displayed on the LCD.

HMP4040.04 Menu 1HMP4040.04 Menu 2

The menu features a number of selections which can be navigated using the four directional arrow buttons and rotary knob with push-to-select. The menu has a relatively logical layout with sub-sections for each major configuration category. Some of these sub-menus will be explored in their respective sections later in this review.

HMP4040.04 Interface MenuHMP4040.04 Interface Submenu

Selecting a sub-menu such as the Interface menu can result in further sub-menus. Pressing the left arrow key brings you up a level on the menu.

HMP4040.04 IP SettingsHMP4040.04 IP Information Menu

Unfortunately, while the unit does have a keypad which enables direct entry for most numeric fields, it seems that it is not possible to do this while configuring a static IP address, instead requiring copious use of the rotary input knob to set the IP address. Luckily this is not something you would have to do frequently.

HMP4040.04 Display and Key MenuHMP4040.04 Beeper Menu

It is good that the HMP4040.04 allows for configuration of the display and key brightness, as by default, it is almost too bright in my opinion. Likewise, the LCD came pre-configured with too low of a contrast, resulting in faded text and noticeable flickering. The fall-back time configuration allows for configuring the delay to auto-dimming or auto-exiting of a current/voltage/track/fuse setting mode. The beeper is something I often disable on my devices to reduce annoyance and I’m glad to see that catered for as well.


Acoustic Noise & Load Test

To assess the noise of the unit required putting the unit under some stress. Already, from the power-on self-test, the fans are capable of fairly high-speed operation which results in a noticeable amount of wind noise with a small amount of motor whine.


At room temperature operation, I series all four rails and put the output into a bank of incandescent mains bulbs and a two-bar electric heater. As it turns out, with this load, it was possible to run almost exactly at the full load (with a slight drift in power dissipation as the heater warms up). I was able to run the supply for an hour at >360W of load with the fans only slightly audible over my computer fans. It was surprisingly quiet for the load – I assume it is over-engineered so that it can still deliver the full rated load in case of rack-mounting, dust accumulation and higher ambient temperatures.


Another thing that was greatly appreciated was the fan speed was regulated almost continuously, with the fans coming on almost imperceptibly and ramping speeds in a step-less fashion. The lack of motor whine also means that most of the noise is the sound of the air moving, which is “whiter” in spectral content.


This is in stark contrast to the Keysight E36103A I reviewed previously that uses one small fan and creates a rather annoying high-pitched whine whenever the supply is even approaching three-quarter load. The greater annoyance was the fan speed would oscillate between several discrete steps, creating a “hee-hawww-hee-hawww” sound.

HMP4040.04 Full Load TestHMP4040.04 Overload Test

When the supply is overloaded, the unit beeps and shuts down all outputs and displays a “Power limit exceeded!” message on the screen.


Generally speaking, during operation, the power supply is mostly quiet, without any noticeable switching converter coil whine or noise, which was subtly noticeable on the E36103A. The loudest part of the unit, however, are the relays which control each channel which switch with a noticeable “clunk”. This can be slightly distracting in a quiet room, but it does mean that a channel powered off is truly disconnected and allows you to audibly confirm when a channel has tripped out without even looking at the display.


Special Features: Electronic Fuse & FuseLink

The electronic fuse capability of the HMP4040.04 is a form of overcurrent protection (OCP). Enabling the fuse on a given channel converts the current setting to a maximum current setting rather than a “constant current” operation. Where the electronic fuse is enabled on a channel, the indicator “FUSE” appears under the corresponding current display of the channel.

HMP4040.04 Fuse Delay MenuHMP4040.04 Fuse Linking Menu

A fuse delay option is provided in 10ms steps, up to a maximum of 250ms. Once tripped, the supply beeps and turns off the channel – unless a fuse-link has also been set, in which other channels will also be turned off. The Fuse Linking menu allows you to set the Fuse Link hierarchy. This allows for a fuse trip on any channel to turn off linked channels as well, preventing the risk of damage to multi-voltage devices which might have strict power sequencing requirements. In the menu above, I have linked CH1 with CH2 and CH3 – so a trip on CH1 will turn off CH2 and CH3. However, no other channels are linked, thus if a trip occurs on any other channel, only that channel will be powered down. This is the flexibility of being able to configure the linking for each channel independently.

BK8600 Fuse Testing SettingsT4A Fuse Characteristic at 10A

I initially thought the electronic fuse emulated the I2t characteristic of a real fuse – on the right is a 3AG T4A fuse subjected to 10A which took about 11.5s to blow. As a result, I decided to generate a load pulse train using my B&K Precision Model 8600 DC Electronic Load which alternated between 0.5A and 1A for increasingly lengthy periods. Setting the FuseLink to link CH1 with CH2 and CH3, the Fuse Delay to 250ms and the current limit to 1A resulted in this unexpected result.

HMP4040.04 Fuse Test - Fuses OffHMP4040.04 Fuse Test - Fuse 1A

Without fuses on, the supply current limits at 1A and the output voltage on CH1 falls to near zero. Upon activating fuses, rather unexpectedly, the unit shuts down as soon as we hit 1A for a few milliseconds.

HMP4040.04 Fuse Test - Fuse 1.095AHMP4040.04 Fuse Test - Fuse 1.100A

Increasing the supply limit to 1.095A results in longer load pulses tolerated and at 1.100A, the whole train does not trip the supply.

HMP4040.04 Fuse Test - Link Time

The associated channel trip time is about 69ms, but this behaviour was unexpected so I contacted R&S for clarification. As it turns out, the fuse delay is only for channel start-up and the fuse current limit is a hard limit.

HMP4040.04 Fuse Test - 0msHMP4040.04 Fuse Test - 10ms

Re-testing with a T4A AG3 fuse as the load and 10A as the limit, we can see at 0ms delay, the supply comes on, overshoots briefly, supplies for about 4ms before shutting down. At 10ms, this is about 14ms to shutdown.

HMP4040.04 Fuse Test - 100msHMP4040.04 Fuse Test - 250ms

Increasing the delay to 100ms and 250ms increases the run time correspondingly, thus the fuse delay is useful to prevent initial tripping at power-on due to the load of charging capacitors, but does not affect the fuse performance after this time has passed. As a result, the electronic fuse does not behave quite as a traditional fuse does – it is a hard limit.


Special Features: Overvoltage Protection (OVP)

The Over Voltage Protection feature allows you to protect the device under test from excessive voltages by triggering a shut-down of the output once exceeding a given voltage.

HMP4040.04 OVP Settings Menu

The OVP setting dialog has the threshold for the channel and a mode. In the “measured” mode, the OVP trips based on the metered voltage on that channel. In the “protected” mode, the OVP is tripped whenever the channel is set to a greater voltage than the configured limit.


Once tripped, the corresponding channel turns off and the power supply “beeps”. The letters OVP appear and flash underneath the voltage for the given channel where the OVP is triggered. Remaining channels continue to operate and the output master switch remains on. It is possible to reactivate the tripped channel by pushing the corresponding channel button.


It is possible to configure OVP to a level lower than the output voltage for a given channel, in which case the channel trips practically immediately after powering on, confirming operation.

HMP4040.04 OVP-Fuse-ARB Display


Special Features: EasyArb

Another feature of the HMP4040.04 is the “EasyArb” feature, which allows the power supply to generate an arbitrary waveform. Unlike a function generator, however, the length of the waveform is limited to 128 points of defined voltage/current/delay with a set number of repetitions (or infinite). The minimum delay time is 10ms for 100 points per second, although in practice, this will also depend on the power supply’s transient performance.

HMP4040.04 EasyArb Menu

The submenu allows you to transfer the waveform to each channel memory, start/stop running arbitrary function on each of the channels and edit/clear/save/recall the waveform. Channel-to-channel alignment of the arbitrary function is not guaranteed and synchronisation is not possible.

HMP4040.04 EasyArb Edit MenuHMP4040.04 EasyArb Save Menu

The arbitrary editor allows you to directly edit the points that define the generated waveform. The save feature allows you to store the waveform into a number of on-board memory locations for later recall without the assistance of a computer.

With this feature, it is possible to generate automated sequences of voltages/currents for testing.



Using the Rohde & Schwarz HMP4040.04 in a standalone fashion reveals that the supply is the product of very sensible design decisions. While it may appear somewhat old due to the use of a monochrome display, it is bright and large for easy reading with an acceptable viewing angle. The use of a direct-entry keypad allows for more rapid entry of numeric values, while directional keys and a rotary knob allow for alternative ways to adjust values. Colour-coded LEDs are used to denote channel selection, operating status and input modes which can be understood at a glance. The method of operation is intuitive and easy to get acquainted with, and the menu set-up is relatively straightforward and logical.


More importantly, the layout of the power button in the top left and the master output button in the top right makes it easy to identify by feel in case of an emergency to allow for rapid de-energising of the load. The presence of the sense terminals that require no manual configuration eliminates the potential for misconfiguration and saves hassle. The sure-fire isolation provided by relay-controlled outputs provides an added level of reassurance.


I was especially impressed by the low noise of the cooling system even under full load for an hour. Using an unconventional cross-flow design, the fans start imperceptibly and run at a continuously variable speed. The fans are powerful and capable, demonstrated by the self-test sequence on power-up. However, in a regular open room with a full load, the fans ran nowhere near their maximum and were only barely audible above my computer fans. The amount of margin available should ensure reliable full-load delivery even in demanding situations such as rack-mounted configurations with elevated ambient temperatures.


The downsides identified included the omission of rail sequencing on the device (e.g. power up delays between rails), limitations with monochrome LCD display data (no display of output set-points and metering simultaneously), the use of shrouded banana connectors that may not be ideal for use with thicker wire or bare wire, the small shrouded terminal blocks on the rear which make thicker wire difficult to use, the lack of internal series/paralleling which can result in external wire clutter and inconsistencies in the IP address entry menu which requires a lot of knob-spinning to enter an address.


It is also worth noting that the HMP4040.04 also offers an integrated electronic fuse for protecting devices under test (DUT) with configurable delay for capacitor charging in-rush currents. While this doesn’t emulate a real fuse (as it is a hard current limit), it is useful where a current-trip (OCP) behaviour is desired. FuseLink functionality allows for multiple outputs to trip “nearly” simultaneously, thus avoiding damage in the case of multi-rail DUTs. The HMP4040.04 also offers an over-voltage protection (OVP) based on measured or configured voltage that also protects the DUT against over-voltage. Finally, the EasyArb functionality allows for sequences of pre-defined voltages, currents and delays to be generated from the power supply without the assistance of a computer.


In all, I found the HMP4040.04 to be featureful, highly performant, easy to use on the bench and with a sensible traditional design. The only drawbacks identified are relatively minor in my opinion, some of them are personal preferences and many of them are common to other supplies on the market as well. As a result, I’d have to say that the HMP4040.04 does a good job when it comes to user experience.



This blog is a part of the Rohde & Schwarz HMP4040.04 Programmable Power Supply RoadTest Review.