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Power supply for home lab - do I really need a R&S?
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rf-messkopf:

--- Quote from: mhsprang on October 04, 2024, 04:21:57 pm ---If you go back and take a look at reply #37, second image, you see the current spike first, then drop from 20 mA to 10 mA (setpoint) in several steps over a period of 12 seconds. That has nothing to do with any capacitor discharge but everything with something that goes terrible wrong in the firmware.

--- End quote ---

Agreed, that should definitely not happen and is out of the setting accuracy spec. R&S should fix that.

I have not seen something similar on the HMP4040 or any Agilent supply, though.
nctnico:
I think you need to define it differently. An ideal current limited power supply has two control loops running at the same time: one loop regulates the output so the set current limit isn't exceeded, the other loop regulates the output so the set output voltage is not exceeded. And this should be an AND function. Set current AND set voltage should not be exceeded.

From the graphs it looks like that in some power supplies the current limit control loop is disabled for a short while causing the output to be regulated to the set voltage but ignoring the requirement for the current limit.

I don't think this is a matter of specification but the very basic behaviour of a lab power supply. The set current limit and set voltage should never be exceeded under any circumstance as this will cause damage to devices connected to the power supply; hence there is no need to specify this.  :)
mhsprang:
@pdenisowski: Although I am an electronics engineer, I am also a "user". So, if I purchase a power supply with CV and CC capablities, then to my circuit I design I am the engineer but to the power supply I am "merely" the user. In my opinion, I do not need to get to understand how my power supply works, based on assumptions and experiments. Instead, I rely on its specifications in the datasheet.

If I set it to be used as a current source with a certain max. voltage, I may expect, as a user, that those setpoints are honoured. I don't care about control loops and arguments about how the V and I loops interact, I only care that if I set the max current to be 10 mA, that the current will not exceed 10 mA. I will accept that there is a large capacitor at the output terminals, and that is why I use the "output enable" button to enable the power, instead of plugging in the cables to my delicate circuit, assuming the output cap is after the switch. Even this should be in the specifications, IMO.

To come back to my example in reply #37: I connected the 50 Ohm resistor to the power supply, which was set to 8V and 10 mA. I enabled the power to the resistor by pressing the Output button on the NGE100 while the resitor was connected already. So from that point onwards, it is the power supply's responsibility to deliver what I asked it to: 10 mA of current with a max. voltage of 8V. Then I hit the output button, as a user.

Seeing the current overshoot to 140 mA for 30 ms is one thing, althoug it exceeds the 200 us in the specs, but seeing the current drop from 20 mA to the setpoint of 10 mA over 12 seconds is a blatant violation of the specifications of this power supply. One could also say: I did not change the load. I merely enabled the output to a constant load.
blackdog:
Hi pdenisowski, :-)

I have a suspicion that you don't understand what test was done....

Preparation
Set a LAB Power Supply to 8V and set the current to max 10mA.

Now connect a 50 Ohm resistor across the output terminals of the channel you set.
Connect a scope across the 50 Ohm resistor.

Set the scope to say 100ms time base time and single shot and the trigger level to say 30% of the expected value.
If you want to do it nicely, set the scope for your used channel to current measurement and then adjust the sensitivity in the probe settings to 50V/A.
Now the peak current is neatly readable on the scope screen.
Connect the scope with a BNC-Crock cable 1:1 low cost probe, as in my previous photo.

If everything is set up correctly and you turn on the LAB Power Supply via the enable switch, you will see the behavior of the Power Supply you are testing.
The maximum value of the current through the 50 Ohm resistor should not exceed 10mA.
This above is a good test for the behavior of the I-Loop control.

This test is slightly different from the test for the dynamic Voltage behavior of a LAB Power Supply.
In this, you vary the load current over a short period of time and see how well the LAB power supply responds in terms of the U-Loop.
Many manufacturers specify from 50 to 90 or 100% load, and there are some that test 10% to 100% as a test in this dynamic U-Loop test for e.g. within 50uSec within a few mV of the set voltage value.

By the way, these are just two of the tests you can do on LAB Power supplies dynamic behavior.
Like this measurement which I also think is important, i have never seen a manufacturer specify how much commonmode interference comes out of a LAB Power supply output.
Especially the ones that have switching behavior, linear post-regulation usually doesn't help with that.

But let me not digress, the current test at issue here shows nicely what a manufacturer thinks is important,
my measurements of a few LAB Power Supply's shows that fast current control at the GW Instek Power Supply is well taken care of.

I hope things are more clear now.

Kind regards,
Bram
mawyatt:
Think most users work with a Lab Supply the way Bram describes, and this is also how we generally utilize a Lab Supply, especially when sensitive circuits are involved.

The Power Supply is Mains Powered On, Voltage and Current Limits are set, the DUT is connected either before or after the Limits set, then the PS is Enabled turning ON the Output(s).

In this use case the Power Supply should not overshoot the Voltage or Current Limits set by the user IMO.

Some Lab Supplies like the GPP4323 and SPD3303X have an ALL ON/OFF Single Button to Toggle the Outputs ON and OFF which hints at the above described use case. This is a nice feature when one is utilizing multiple outputs which is common with complex DUT circuits/systems.

Best,
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