Power supply CC and CL

[TiTanerCZE]

Hi, I have a silly question about constant current and constant limit on lab power supply.

When power supply is set in constant voltage mode and current limit is set, I would expect from power supply to disable the output when CL value is reach.
Why would power supply switch from CV to CC mode, when CL is reach? 

[Kleinstein]

The extra CL is not standard with lab supplies. So it can be different between models. One possibility would be that the CC limit can only be adjusted up to the CL level, as a kind of safety to avoid accidentally to high a current. So it should be explained in the instructions.

[ocset]

TiTanerCZE...What you say sounds normal……..you want to set a certain voltage, but you dont want to be supplying more than a certain current, so you set the CL….then, if your load trys to draw more than the CL value,  the power supply just clamps the current at the CL value, and of course, if the load stays heavy, the output voltage will fall, ….it falls because the load is now too heavy for keeping the vout at that level given the CL value  that you have set.

It works by there being two error amplifiers……one is the output voltage error amplifier..and this works under normal case….but your current sense is always in there, and when that current trips the current limit threshold, then the current error amplifier kicks in, and as long as the too_heavy load stays on, the current is clamped to the CL level…………..the voltage must fall because you have  limited the current………..there isn’t enough current to keep the supplies output capacitors topped up and supply the load…so the voltage must fall….what actually happens is the voltage error amplifier gets railed, and is out of the picture, at least until the load gets lighter.

[xavier60]

My Agilent U8002A has Current Limit and an Over Current Protection mode. The CL is the same thing as Constant Current.
If the OCP mode is active and the current draw reaches the setting, the LCD displays "trip" and the output is switched off.

[TiTanerCZE]

xavier60 That is what would i expect.

treez But why would you want that? Isnt better to switch output off? It is simple to make circuit like that?

Kleinstein: Yes, but why would you go from CV to CC, why not to just put output to groud? (When you are in a CV mode)

[rstofer]

xavier60 That is what would i expect.

treez But why would you want that? Isnt better to switch output off? It is simple to make circuit like that?

Kleinstein: Yes, but why would you go from CV to CC, why not to just put output to groud? (When you are in a CV mode)

In general, CC is really current limiting and it is usually implemented as voltage foldback.  If I actually want a current of 100 mA and I don't care what voltage it takes to get it, I can set the PS to its max voltage and then set the CL to 100 mA.  The PS will then attempt to deliver a constant current regardless of the voltage it takes.  If the load suddenly becomes an open circuit, the PS will raise the output to max.

Tripping isn't really implemented in power supplies.  You can make a good argument for it but that's not the way current limiting is usually implemented.

Current limiting is likely to occur on a new project at initial power-up.  You are probably watching the panel so you will see the voltage foldback almost instantly.  Internally, it was instantly.  The PS is very unlikely to damage the project if the current limit is set quite low.

Another point:  You really want to be able to see the current limit setting.  Older power supplies just had a potentiometer and you had to short the output to see the setting on the ammeter.  Newer digital supplies will display the setting with no need to short the output.  This is a great feature of the modern bench PS.