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Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
how do power supplies know if they're in Constant Voltage or Constant Current mode? is it done by detecting voltage sag? if the output voltage sags by X amount then its in CC mode? and if it doesn't sag then its in CV mode?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Watching the voltage sag wont be useful in a regulated linear or SMPS.
It has a known Low ohm-resistance in series with the output, when a voltage appears too low across the Rs it stays in CV, when the Voltage across the series resistor rises to a trip point, the PS starts to lower the V to keep the current at the set level. Foldback current limiting can also use this method.
When you turn the Current set knob (on the front of a PS) your really changing the Op-amp that watches that series resistor, thus you can have variable voltage and variable current limit on some of the nicer full featured PS's.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Yes back in 2001 when i was doing analog/digital breadboard stuff at Ohlone College, i had a knob for CV and A knob for CC, when the load coditions changed you could hear a relay click on and off, and there was a Green and red LED to tell you which mode the PS was in, these SMPS's (the nice ones $$$) have an active load that cuts in at low load to stop high voltage pulsing and surging.
(and yeah some idiots shorted them out and wondered why they had 1.2 A at only 0.3 V. )
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Yep. I have a PSU like that. You can specify a voltage and a current. If the current draw goes above the limit, it switches into CC and reduces the voltage until that current is drawn.
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CV and CC mode?
