EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Terry Bites on February 05, 2021, 10:12:54 am
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What. No.
Let stop this once and for all, constant current and current limiting are not the same thing.
So CC is really CL. When you are in "CC mode" your PSU is not a constant current source or sink.
Obvious- we all know that right. But a CCS is a different animal.
:palm:
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On good power supplies CC is a constant current generator, but it usually have a capacitor across, i.e. it cannot response fast to changing load resistance.
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Power supplies intended for good constant current response use a different frequency compensation scheme which does not require large output capacitance much like an audio power amplifier.
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The main problem with CC mode is when you use OCP limit and CC for low current.
PSU has internal capacitor on the output, so when you press enable output, the capacitor consume a lot of current and OCP triggers.
You can avoid that with setup PSU setting, enable output and then connect the load.
In such way you can avoid unwanted over-current protection at power on. :)
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It is a constant current source with a maximum voltage limit. :-//
But the output capacitor in bad PSUs makes it a little difficult to do it well. :)
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It is a constant current source with a maximum voltage limit. :-//
As opposed to the constant current source with unlimited maximum voltage in the new ACME FloobyDust 2000? :-DD Sorry, couldn't resist.
Seems OP is striving for perfection? Or did he find a trend of PSU not meeting their specifications? Surely most commercial PSU perform better than a simple BJT CCS, no?
EDIT: to pin numbers (or at least graphs) at the issue some alluded to, I measured the voltage across a beefy 10Ohm resistor of various PSUs I had easy access to, all which were set to CC mode (100mA if not otherwise noted) when enabling the output (or connecting the load for PSU which lack that feature). In order to not distract from cerebus' pertinent closing words, I squeeze those in here.
Leader LPS 152:
[attach=1]
Ouch. Don't do that.
HP6611C:
[attach=2]
HP6623:
[attach=3]
(note: here the current was set to 200mA. If you ever had to lift one of those, you wouldn't wonder why it doesn't sweat the small stuff)
A wee bit slow, methinks.
HP6227B:
[attach=4]
(it's old and not in perfect shape, so one can't expect it to work still within specifications)
GWinstek GPS-1850D:
[attach=5]
(fleamarket special and a bit funky)
Keithley 225 current source:
[attach=6]
showing how it's supposed to be done, no overshoot. Note the scale.
EDIT2: hmmh, inline image attachments seems to be broken?
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Output capacitor can be death trap for components like LEDs of young players :)
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Output capacitor can be death trap for components like LEDs of young players :)
In the case of a laboratory source, when the LEDs are connected to the hotswap, yes the LEDs will be difficult. But experienced people do not switch the load when the current source is turned on. :)
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When I need a power supply with true constant current limiting I use a Souce-Measure-Unit (SMU) instead of a standard power supply, usually an E5270B (https://www.keysight.com/en/pd-502208-pn-E5270B/8-slot-precision-measurement-mainframe?cc=US&lc=eng) or Keithley (https://www.tek.com/keithley-source-measure-units).
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Surely most commercial PSU perform better than a simple BJT CCS, no?
Yeah, it performs better at exploding newbie circuits >:D
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What. No.
Let stop this once and for all, constant current and current limiting are not the same thing.
So CC is really CL. When you are in "CC mode" your PSU is not a constant current source or sink.
Obvious- we all know that right. But a CCS is a different animal.
:palm:
Why is it that this comes up again and again? A CC/CV power supply is a constant current source whenever the load resistance is less than \$\frac{V_{set}}{I_{set}}\$ and a constant voltage source when it's higher. This is exactly the same behaviour as a practical constant current source. That's it plain and simple. If you're trying to claim that they are not constant current sources because they are not ideal constant current sources with infinite compliance voltages then I suspect that you're just trying to make fools of us.