Is there a difference in current limiting vs adjusting - Page 1
Forum section: electronics / beginners

author: mikael1985
author_rank: Contributor
timestamp: October 21, 2015, 12:30:35 pm
content: Hi,I looking at a variable buck module to which I can use to convert an old laptop transformer to a variable power supply - nothing fancy, but I would like it to keep the voltage floating when the current is set and vice versa, but some of the buck modules I looked at seems to have adjusteable current limiting, and I'm very unsure if this is the same as adjusteable current.Anyway I have already bought the unit, but still curious if I have misunderstood anything.Thank youMikaelThe unit with current limiting is this onehttp://www.ebay.com/itm/5A-Lithium-Charger-CV-CC-buck-Step-down-Power-Supply-Module-LED-Driver/281684189429?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D33873%26meid%3Df58cc777be9a4a56a2d515f80fe5740e%26pid%3D100005%26rk%3D3%26rkt%3D6%26sd%3D231723145418And this is the unit I boughthttp://www.ebay.com/itm/DC-DC-CC-CV-Buck-Converter-Step-down-Power-Supply-Module-7-32V-to-0-8-28V-12A-SY-/331395517418?hash=item4d28b64bea:g:4B0AAOSw0vBUdd4P

author: Rick Law
author_rank: Super Contributor
timestamp: October 21, 2015, 02:49:44 pm
content: I am a relative newbie too, but I will share what I know.First, V & I are related. V=IR by ohms law.If you adjust V, then the load's characteristic controls what current it takes. With these buck/boost boards, you don't so much adjust either but you put in limits. You "set" it to 5V, it goes from 0V to 5V and not exceeding 5V (within the board's tolerance). You "set" it to 200mA, it goes from 0mA to 200mA. So your limit is 5V 200mA -- whichever limit is reached first as voltage is increased.Continuing with the example above (5V 200mA), if the load is very low in resistance (say 1ohm), it goes to 200mA when it reach 0.2V (as evaluated by V=IR). While the limit is 5V, it can't go higher than 0.2V as it is limited at 200mA. So, you are current-limited.If the load resistance is very high, say 1Kohm, it will need 200V to get to 200mA. The board will stop at 5V. This is the limit it reached first, and now you are voltage-limited.When your load is resistive, it is pure/smooth V=IR. You turn the knob up (voltage up), it go smoothly as the knob turns and the resistor gets hotter as it "burn away" the power. When your load is say an LED (current-limited), once voltage reach certain threshold, the LED is "fully lit". While V=IR, driving either up will not give you more light but just burn your LED out. Similar to charging a battery (since the board you link to claim to be a charger), as the battery gets full, it will take in less current.Staying on charging since that board is a "charger" board, If for example you are charging a near empty LiIon (3.7V type), and for example you set it at 4.2V and 500mA current limit. When it is near empty, it will take in a good bit of current. So by the time it reaches 3.8V (for example), it could reach that 500mA current limit already, so now you are current limited. As it gets fuller, it wont take as much current at 3.8V. Since V limit is set to 4.2V, so it can increase the voltage until it reaches the 500mA limit. So, it is increasing the voltage to keep the current constant. That is the CC part of things. It can keep increasing V to keep CC until it reaches the 4.2V limit you set. At that time, the voltage stays at the 4.2V limit you set -- so this is the CV part as the battery continue to soak up what it can at 4.2V. The current it can take will keep dropping till it can take no more at that voltage.Bare in mind, LiIon use CC/CV (constant current, constant voltage) so the above would work nicely. Other battery types such as NiMH doesn't use CC/CV. NiMH battery shows a slight voltage drop when full (aka negative-delta-V). So, CC/CV charging will not work so well for NiMH (what voltage limit do you set?)Hope this helps.Rick(EDIT: adding this:)By the way, since you want to use it as your PSU (as I do, laptop power brick), your problem will be noise and stability.The voltage limit say is set to 5V, but there is some tolerance there. As load change, it will make V changes a little also, so it is near 5V but not at 5V. Volt controls the current, so your current varies as well.Choose your board with "noise" in mind as well. Some regulators are nosier than others. After a varying mix of the above types, I now use the B3603 - a digitally controlled one with a laptop power brick:https://www.eevblog.com/forum/reviews/b3603-dcdc-buck-converter-mini-review-and-how-the-set-key-could-be-fatal/