Electronics > Projects, Designs, and Technical Stuff
Lab Power Supply - The Lost Current
<< < (11/26) > >>
John Heath:
I am sensing unknown variables as part of the problem. Measuring the voltage drop across a resistor relies on that resistor being what it says it is to know the current. 200 m ohms is not easy to measure. A hall effect DC current probe will measure the current in no uncertain terms and it would be a nice addition to your test equipment. My DC current probe cost 35 bucks. Cheap. Make sure it is a DC current probe not the other type that is limited to AC only.

 Failing this you can use your cell phone as a current probe to know how much current. It requires a little calibration as cell phones were not intended to be used this way. Google magnetic app for android and you will find lots of apps for free. Find a known to be true 1 amp source that can be trusted. Run 1 amp through a wire and find the sweet spot on the back of your cell phone where the magnetic reading is the highest. Mark that spot on your phone and mark what the magnetic strength is for 1 amp. That's it , you now own a free hall effect current probe calibrated to 1 amp and as a side benefit you can make a phone call. :-+
radoczi94:

--- Quote from: John Heath on February 10, 2018, 02:46:08 am ---I am sensing unknown variables as part of the problem. Measuring the voltage drop across a resistor relies on that resistor being what it says it is to know the current. 200 m ohms is not easy to measure. A hall effect DC current probe will measure the current in no uncertain terms and it would be a nice addition to your test equipment. My DC current probe cost 35 bucks. Cheap. Make sure it is a DC current probe not the other type that is limited to AC only.

--- End quote ---
The resistor was pretty close to the nominal value. It could not have vary that much, it was +-5% 300ppm and it was just a little bit above the ambient temperature. The problem was: I used inadequate equipment. The best theory is that there were some serious oscillation and it tricked the Wun-Hung-Lo multimeters,so they just showed an average.

I did not even knew, that such a thing exist, will do some research on it, thanks.


--- Quote from: xavier60 on February 09, 2018, 12:59:34 am ---Just because a PSU has CC, doesn't really mean that it's short circuit protected. Even my Agilent U8002A will supply over 20 amps into a sub ohm load for 100us before it begins to sluggishly current limit.
I mocked up a CV/CC regulator to experiment with the CC response. The cause of the delayed CC response was because the output of the CC op-amp normally sits at close to its full + rail voltage. When the PSU is suddenly overloaded, it then takes a long time for the CC op-amp's output to slew down to the point where it takes control of the Base.
One of the reasons is that the loop compensation capacitor is usually connected directly between the op-amp's output and inverting input. I have connected the capacitor to the other side of the ORing diode so that the op-amp's output can slew at its maximum rate until the ORing diode conducts.
Because the LM358 that I have used doesn't slew that fast anyway, 0.3V/us, I have put a diode and LED in its feedback path that keeps its output at 2.2v so that it doesn't have far to swing down to take control of the Base. I am ordering some faster NE5532 op-amps to see if I can omit this extra complication.

Extra: I have used an LED for the CC ORing diode to give CC indication.

--- End quote ---
That design is almost identical to the one formed in my head. My concern is, that I want to use fast transistors, around 30MHz.The opamps should be order of magnitude slower to prevent loop oscillation without compensation. But the slew rate of the opamp should be high enough to be able to handle the switch on-off events, with an acceptable waveform on the output. Guess, I should learn to use a simulator software before I start soldering.
John Heath:
Speaking of oscillations I noted a 10 uf condenser on the output of your power supply. Who ordered that? 10 uf in CC mode means much higher current than the setting for the CC mode in the first 1 m second. It defeats the purpose of current limiting for sudden current demands in the 1 m second range. That is fishy. Did they put it there as an after thought to compensate for an oscillation problem in CC mode? Try a 100 uf quality condenser and see if things improve. It's a long shot but you never know. If it does improve then the current limiting is only for average not sudden current demands. This is not good as one could burn out an IC with a sudden short even if the current limiter is set to 100 m amps. A raw 100 uf quality condenser will be more than happy to provide the destructive current to destroy this IC .
xavier60:

--- Quote from: radoczi94 on February 10, 2018, 01:12:39 pm ---That design is almost identical to the one formed in my head. My concern is, that I want to use fast transistors, around 30MHz.The opamps should be order of magnitude slower to prevent loop oscillation without compensation. But the slew rate of the opamp should be high enough to be able to handle the switch on-off events, with an acceptable waveform on the output. Guess, I should learn to use a simulator software before I start soldering.

--- End quote ---
Oscillation is a big concern. My experiment did oscillate for various reasons. One time it seemed to be Darlington that was the cause. That's why I put the 220 ohm in series with the Base.
One of my suppliers has some 150V 73A MOSFETS(PSMN020-150W) selling  cheap, so I bought some to experiment with in my regulator.
C:
Many ways to stop or limit Oscillation.

Rate of change positive could be different from negative.

Multi-stage feedback.

Areas of circuit where frequency response is different.

Positive feedback to get large change with negative feedback for small change.

For an analog or you could have two. A min OR to control output & a MAX OR to prevent swing to rail.

Many ways other then just slow.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod