Constant voltage and constant current

[Wizpic]

is there a curcuit that can tell between constant current and constant voltage ?

What I want to do is build a power supply and have 2 LEDS one to show when it's set in constant voltage amd the ohter constant current

any ideas or links

wizpic

[otter]

My first thought would be to have two small load resistors that you can switch in and measure the output voltage under different load conditions; if a change in load does not alter the output voltage, then it must be operating in constant voltage mode.
Not very elegant. somebody may have a better solution.
Regards
Otter

[FriskyFerret]

Hint: If the power supply is in CC mode then it isn't in CV mode. If the power supply is in CV mode then it isn't in CC mode.

The mode the power supply is in is determined from the state of the current-limit circuit. If the CC circuit is active, illuminate the CC LED and extinguish the CV LED.

You're designing the most trivial part of the circuit first. You need to put in some book-time. Start by reading and re-reading the power supply chapters of The Art of Electronics. There is a section on fold-back current limiting. Though this is not CC, it will assist you in understanding the basics.

You should study the schematic and theory of operation of a very good power supply, one of the HP bench power supplies like the E3615A. It doesn't have to be that model, any older HP power supply manual will do as long as it has schematics and Theory of Operation section. You may be able to download some from HP (Agilent, now) directly or you can find them at internet manual dumps run by the amateur radio guys.
 

[bird777]

Here is an simple constant supply current generator:

http://rapidshare.com/files/77410003/08di41.pdf.html

[twonuts]

Check the datasheets for 78xx 3 leg fixed voltage regulators.
These can be used for either fixed voltage output, or constant current output depending on your circuit configuration. All the data on how to do this is usually included in the datasheet for the regulator used, with a number of schematics also.

As FriskyFerret says, you will need to switch between modes. The changeover switch can also be used to switch between leds. Simple to do and no need for any complicated circuits.