Products > Test Equipment
Power supply for home lab - do I really need a R&S?
thm_w:
--- Quote from: blackdog on October 03, 2024, 09:11:19 pm ---Hi,
As promised some measurements I made on LAB power supplies I own that have an enable key that are here on the test bench.
--- End quote ---
Its good that you measured current, probably should standardize on that.
Peak of the first one was 25mA, R&S was 120mA, so not nearly as bad, IMO, but still not good.
edit: Measured on DP832 and initial peak was only ever 18mA maximum with 50 ohm, regardless of current setting. So its only an issue if your current limit is sub ~15mA.
blackdog:
Hi thm_w , :D
I measured the voltage across a 50 Ohm resistor and set the scope channel to indicate the current value.
I think it is all about not exceeding the set current and then it is more obvious that you indicate the current value on the scope image.
That an LED can handle 2 or 4 times the maximum current for time “X” is not so important.
Every LAB power supply is used for all kinds of other applications.
My moral from almost 60 years of experience.
I always test an LED with a series resistor.
If I ever want to push some power into a battery, I always do it via a series diode.
I don't like “fast”, I like to be careful and make sure that I don't break the d.u.t. I connect to a LAB power supply.
Kind regards,
Bram
thm_w:
--- Quote from: blackdog on October 03, 2024, 09:38:52 pm ---I think it is all about not exceeding the set current and then it is more obvious that you indicate the current value on the scope image
--- End quote ---
Agreed, updated my post.
Martin72:
Fast current control is a good thing for sensitive components if you want to connect them directly to the power supply.
However, there are situations where this is rather counterproductive.
Certain DC/DC converter circuits have an inrush current that can easily be many times the normal input current.
If this is not taken into account when switching on, it can, at best, lead to non-functioning without side effects, but in some cases it can lead to permanent current flow, causing the circuit to “hang”.
As always, the dose makes the poison.
However, I also prefer series resistors for LEDs, it's just in my nature... ;)
rf-messkopf:
--- Quote from: Martin72 on October 03, 2024, 10:32:45 pm ---Fast current control is a good thing for sensitive components if you want to connect them directly to the power supply.
However, there are situations where this is rather counterproductive.
--- End quote ---
The delayed switchover from CV to CC mode, where the supply sits there for a few milliseconds before an exponential ramp down is seen (as expected for a capacitor discharged through a resistor), might actually be implemented on purpose. This delay might also be responsible for the overshoot that is seen when the output is switched on with the enable button with a load connected.
The current control loop itself seems to be much faster. Attached is the result of the following experiment: Supply (R&S HMP4040) is set to 8 V, 150 mA, output on, and loaded with a 50 ohms resistor (i.e., it is in CC mode with 7.5 V at the output). Then an additional 4.7 ohms resistor is switched in parallel to the 50 ohms (i.e., output voltage should jump down to 0.64 V). The transition takes about 1.7 ms (from 90% to 10%) and looks exponential as expected for a cap discharged through a resistor, with no obvious delay before the exponential decrease.
Maybe R&S can comment on this behavior.
If it is intentional, it would be nice if the user had control over it, though.
Edit: Repeated the same measurement, but with the 4.7 ohms resistor switched in by a MOSFET in order to demonstrate that there is no appreciable delay between switching and supply response. Lower trace is the gate drive pulse.
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