constant current vs current limit - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 2nd August 2012, 11:38 AM   #1
diyAudio Member
 
Join Date: Jul 2012
Default constant current vs current limit

Hi, I am looking for more light on this subject. Please help me to understand. Regards, BT
  Reply With Quote
Old 2nd August 2012, 11:48 AM   #2
DF96 is offline DF96  England
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.
  Reply With Quote
Old 2nd August 2012, 12:20 PM   #3
diyAudio Member
 
sofaspud's Avatar
 
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
  Reply With Quote
Old 5th August 2012, 09:05 AM   #4
jitter is offline jitter  Netherlands
diyAudio Member
 
jitter's Avatar
 
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 09:14 AM.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
7809 with bypass transistor and current limit Blacknight Power Supplies 0 14th December 2011 10:37 PM
Determining current limit Neutrality Solid State 1 31st August 2011 07:12 AM
ALW Regulator Current Limit? fmak Solid State 7 27th January 2006 09:38 AM
Which NTC to limit inrush current Mlaen Solid State 24 10th February 2005 01:52 AM
Current limit of EL84 grid? G Tubes / Valves 10 22nd February 2003 08:08 PM


New To Site? Need Help?

All times are GMT. The time now is 10:50 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2