constant current vs current limit
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 2nd August 2012, 10:38 AM #1 bengaltiger diyAudio Member   Join Date: Jul 2012 constant current vs current limit Hi, I am looking for more light on this subject. Please help me to understand. Regards, BT
 2nd August 2012, 10:48 AM #2 DF96   diyAudio Member   Join Date: May 2007 Constant current means that you get an almost constant current whatever voltage you apply (within reason). Current limit means you get an almost constant voltage until you try to draw too much current, then you either get a constant current or a foldback to a lower current.
 2nd August 2012, 11:20 AM #3 sofaspud   diyAudio Member     Join Date: Nov 2005 Location: San Antonio You can use a LED as demonstration: A 20mA constant current means that whatever voltage is applied, 20mA will flow through the LED. It's directly analogous to "constant voltage" regulation. A 20mA current limit is when various voltages may be applied, but the current is set for a maximum of 20mA. It can be less, but it can't be more. A fuse is the most common current limiter, but perhaps the least sophisticated. If you have a load that changes resistance, then as DF96 says, the more sophisticated circuit will change to a foldback action, or shunt current, or will reduce voltage. Ohm's Law is pretty relentless. __________________ It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
 5th August 2012, 08:05 AM #4 jitter   diyAudio Member     Join Date: Aug 2004 Location: Eindhoven, The Netherlands An elaboration: Theoretically, a constant voltage (CV) source delivers the same voltage regardless of the load. As you might expect, a theoretical constant current (CC) source does the same, but with current instead. As sofaspud already mentioned, this is dictated by Ohm's Law U = I * R. U = tension or voltage; I = current R = resistance. In order for a CV source to work with a varying load (R), the only variable that can be changed in order to keep the voltage constant is the current. E.g. if you halve the resistance, there's no other option than to double the current to keep the voltage the same. Similarly for a CC source, if you halve the resistance the only action the CC source can take is to halve the voltage to keep the current constant. Now, in the real world this works reasonably well within the limits of the source. Most regulated PSUs act as a CV source within design limits. With respect to the title of this thread: "constant current vs current limit": CC is as desribed above, but current limit (CL) means the limit of the amount of current that can flow. Of course there's a difference between the maximum current a source is able to supply and allowed to supply. In a simple PSU with e.g. a 78xx regulator the voltage is constant until the limit of current that the 78xx can supply is reached. It's not able to supply more, it's reached the design limit. You can also have a more sophisticated PSU that has adjustable current limit, in this case the user can limit the current to below the design limit and is probably what OP meant. Exceeding the current limit might result in an action depending on the design. Some sources may fold back, as DF wrote, or trip a total shutdown, or go into CC mode. This latter is probably what goes for the (vast) majority of PSUs with adjustable CL. The laboratory PSUs with adjustable CL I use at work change from CV mode to CC mode when exceeding the (adjustable) current limit. In this case, increasing the load above the limit results in a dropping voltage as the current can no longer increase to keep the voltage constant. Since from the current limit on the current is fixed, it automatically becomes a CC source. Once the load decreases below the CL, the source switches back to CV mode. A CV- or CC-LED lights to inform the user of the status. Mostly we use this feature to limit currents flowing into boards that have never been tested to a level that would prevent damage in case of a short cicruit or reversed polarity. Indeed, to limit the current to a safe level rather than to act as a CC source. Last edited by jitter; 5th August 2012 at 08:14 AM.

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