Shorting swtich for cheapo power supply CC setting

[Jeff_Birt]

Wow, I was just posting about a junk box switch adding a little convenience

Arcing when opening the contacts will most probably not happen, as the current is limited per definition. What I mean is the following situation: switch is open. You set the output voltage to a high value, lets say 30V. This charges up the output capacitor of the supply to 30V. As this is a switch-mode supply, the capacitance will be maybe several 100uF and low ESR. Now you close the switch. This will short circuit the capacitor. The current at this moment is only limited by the ESR and other parasitics, and can be hundreds of amps for a short time, damaging the switch contacts.

Damage from over current is due to heat. Given the relatively small amount of energy stored in the caps I don't think any appreciable amount of heat will be generated (there might be more heat dissipated by the 20AWG lead wires even though they are only 30mm long). If you want to consider the nitty gritty details there would be things like arcing during the switch contact bounce, the switch contact material, etc. You'll often find silver contacts on contacts for high DC currents and other alloys for lower currents. The contact material is chosen to match the switch load rating so that the contacts will be self cleaning. A set of contacts designed to handle 100A would do a poor job with a microamp signal. Years ago when I did welding automation I worked with power supplies that would deliver upwards of 600A. At those current levels things get interesting 

[tatus1969]

tatus1969, I do understand how the Current Limit circuit works  , and it is a Current Limit circuit not Constant Current circuit.

I expected that, I just wanted to illustrate the similarity of CV and CC.

"In electronics, a constant current system is one that varies the voltage across a load to maintain a constant electric current."

in which way is a lab PSU in CC mode doing anything different?

Even with your battery charging examples, when the battery charges up the PSU drops out of current limit mode and goes back into Constant Voltage mode.
A true CC PSU cannot do this, it will maintain the set current no matter what the load.

Sorry, I again disagree. Also a true CC PSU has a maximum voltage that it can deliver, I still see no difference. Checked the Keithley's spec out of curiosity: voltage limit is adjustable from 0.1V to 105V. What would it do when that is reached? I presume it will keep the voltage constant at this value. So we could start the entire discussion over here, I would jump to the position that that voltage limit could also be called CV operation 

[MosherIV]

    "In electronics, a constant current system is one that varies the voltage across a load to maintain a constant electric current."

in which way is a lab PSU in CC mode doing anything different?

With a Lab/Bench PSU, if the load drops, the PSU will come out of Current Limit mode.
A CC PSU will not change the current no matter how low the load goes, this is why Lab/Bench PSU are NOT (striclty) CC PSU.

They can be used in a way which can control the current to a fixed level but it is highly dependent on the load.

a true CC PSU has a maximum voltage that it can deliver

Agreed

a true CC PSU has a maximum voltage that it can deliver, I still see no difference. Checked the Keithley's spec out of curiosity: voltage limit is adjustable from 0.1V to 105V. What would it do when that is reached? I presume it will keep the voltage constant at this value.

A CC PSU goes the other way round, if the load resistance goes too high, the PSU may not be able to maintain the current and will hit a Voltage Limit.

The difference is that they have a primary function - Lab/Bench PSU are designed to mainly control voltage and have a secondary function to limit current.
CC PSU are mainly to supply a fixed (maybe adjustable) amount of current no matter the load but are subject to a Voltage Limit/Ceiling.

[madires]

A CV PSU goes into current limiting mode, if the current limit is exceeded. And it goes back again to CV mode, when the current is less than the limit. A CC PSU goes into voltage limiting mode, if the voltage limit is exceeded. And it goes back again to CC mode, when the voltage is less than the limit. They are not the same, different control logic. But I understand that most people call current limiting on a CV PSU "CC mode", because the PSU acts like a CC source in that specific situation.

[tatus1969]

looks like we can continue this endlessly... But I have the impression that we have started repeating arguments. I enjoyed it very much folks, thanks for the discussion! 

[MosherIV]

looks like we can continue this endlessly... But I have the impression that we have started repeating arguments.

Agreed.

I thought I would not get a chance to use this emojicon

 

 

[tatus1969]

Wow, I was just posting about a junk box switch adding a little convenience

That's how things can develop easily among techies like us 

Damage from over current is due to heat. Given the relatively small amount of energy stored in the caps I don't think any appreciable amount of heat will be generated (there might be more heat dissipated by the 20AWG lead wires even though they are only 30mm long). If you want to consider the nitty gritty details there would be things like arcing during the switch contact bounce, the switch contact material, etc. You'll often find silver contacts on contacts for high DC currents and other alloys for lower currents. The contact material is chosen to match the switch load rating so that the contacts will be self cleaning. A set of contacts designed to handle 100A would do a poor job with a microamp signal. Years ago when I did welding automation I worked with power supplies that would deliver upwards of 600A. At those current levels things get interesting 

I pointed this out from personal experience. It depends on your PSU and your switch, I cross my fingers for a long live for yours

Short video from my lab PSU (by the way, I can highly recommend that model despite of being cheapish/Chinese):

https://www.dropbox.com/sh/h944q58nf4vagv3/AACIyhjlNYUYNFEii4KDtVefa?dl=0

[tatus1969]

looks like we can continue this endlessly... But I have the impression that we have started repeating arguments.

Agreed.

I thought I would not get a chance to use this emojicon

 

 

 

[Fungus]

CC = The voltage will rise until the preset current flows through the device.

CL = The voltage will drop until only the preset current flows through the device.

I know you all know that but being pedantic about the difference is important.

[Fungus]

Did HP get it wrong?

It sure looks that way.

(Argument by authority? Really???)

[Jeff_Birt]

I pointed this out from personal experience. It depends on your PSU and your switch, I cross my fingers for a long live for yours

Short video from my lab PSU (by the way, I can highly recommend that model despite of being cheapish/Chinese):

https://www.dropbox.com/sh/h944q58nf4vagv3/AACIyhjlNYUYNFEii4KDtVefa?dl=0

The link comes up with "folder empty".

As a side note. I set the supply to deliver maximum current, 10.5A in this case, and flipped the switch. After 5 minutes the switch was about 95F. It will probably last as long as the cheap power supply

[tatus1969]

I pointed this out from personal experience. It depends on your PSU and your switch, I cross my fingers for a long live for yours

Short video from my lab PSU (by the way, I can highly recommend that model despite of being cheapish/Chinese):

https://www.dropbox.com/sh/h944q58nf4vagv3/AACIyhjlNYUYNFEii4KDtVefa?dl=0

The link comes up with "folder empty".

As a side note. I set the supply to deliver maximum current, 10.5A in this case, and flipped the switch. After 5 minutes the switch was about 95F. It will probably last as long as the cheap power supply

posted too quickly, was still uploading. Now it is there.

[FlyingHacker]

Did HP get it wrong?

It sure looks that way.

(Argument by authority? Really???)

It was a question not an argument you troll.

I don't think he was trolling here. He is referring to the logical fallacy known as "Argument from Authority," or as I learned it, "Appeal to Authority."

https://en.m.wikipedia.org/wiki/Argument_from_authority

[Jeff_Birt]

Can we all agree this topic has been covered as covered can be? Let's all move on...

[tatus1969]

Can we all agree this topic has been covered as covered can be? Let's all move on...

got a chance to watch my video?