How do you tell what "type" of power a DC power supply is? - Page 1
Forum section: electronics / projects

author: DW1961
author_rank: Frequent Contributor
timestamp: July 22, 2020, 05:36:48 am
content: I ordered some LED strips that came with the best power supplies I could find on Amazon. Those were 12V 5 Amp models. There is one I am ordering tomorrow that is 24V 2.5 Amps.The 24V models says, "Only for LED lighting." I know some of you have mentioned different types of DC power and supplies. I know I'm eventual going to mix them up, so what's the difference and how can I test them--or can I--to designate which ones are which type of DC power?

author: bob91343
author_rank: Super Contributor
timestamp: July 22, 2020, 05:46:36 am
content: I can't imagine a power supply that only can be used for LED lighting. Perhaps it's constant current or something. Read the specs.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 22, 2020, 06:06:44 am
content: Quote from: bob91343 on July 22, 2020, 05:46:36 amYep, something like that. Just not sure what.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 22, 2020, 04:38:49 pm
content: Quote from: bob91343 on July 22, 2020, 05:46:36 amPossibly, it can be non-isolated from mains too, and have very weak or almost absent EMC filters.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 22, 2020, 06:10:37 pm
content: Constant current vs constant voltage?Switching vs linear?

author: coromonadalix
author_rank: Super Contributor
timestamp: July 22, 2020, 08:22:47 pm
content: Some led strips will need a led driver, the leds are controlled in currentSome led strip have the limitting current resistors integrated in the led voltages paths inside it, if its a clear led strip like many, youll be able to se them and the leds arrangements, and normally a simple 24vdc psu will be okay. I used some psus with a small adjustable voltage trimpot, dropped them at 22 vdc, just to drop the intensity a little.Example: a led driver have a voltage range say between 18 to 32 volts (seen some like this in the past) for a 24vdc strip, but the current is limitted, the voltage will adjust itself with the load applied / given by the led strip.Normally a led driver are most of the time directly connected to the led, no resistors needed ... like the 100watts leds 10x10 matrix, they work at least at 32-36vdc and NEED a led driver to work reliably.

author: John B
author_rank: Frequent Contributor
timestamp: July 22, 2020, 10:46:13 pm
content: Do you have a link to the 24V 2.5A power supply? It would be explicitly labelled as constant current if that's the type of LED driver that it is. I've seen power supplies labelled as "LED only" power due to the fact that they have horrible noise and ripple, that's all.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 23, 2020, 06:35:29 am
content: Quote from: John B on July 22, 2020, 10:46:13 pmSure, it says (CV) on it so I assume that means constant voltage, not current?I don't even know why I wanted that model. I can't use it for computers and and I can only use it with 24V PSU, so I don't know what I was thinking.

author: magic
author_rank: Super Contributor
timestamp: July 23, 2020, 08:29:08 am
content: Maybe it's constant voltage but not stable with capacitive loadsIf you are curious, order it, test it, write an Amazon review. Isn't this why they are offering it to you for free in the first place?editI'm kidding, I have no idea what it is and if there is anything special about it. Externally, looks like any other switcher. Certainly not linear.

author: Zero999
author_rank: Super Contributor
timestamp: July 23, 2020, 08:44:25 am
content: It's just an ordinary switched mode power supply. Any special claims made about it will be purely marking.

author: John B
author_rank: Frequent Contributor
timestamp: July 23, 2020, 10:09:28 am
content: It looks sketchy. I'm not sure I'd trust a cheapo AC/DC pack off ebay/ali-whatever anymore, certainly not one that is "UL LIGTED"I've had too many bad experiences, fake specs, bad reliability, even completely random modules shoved inside the plastic pack you see. Not to mention the safety risk you take in isolation and potentially starting fires.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 23, 2020, 05:03:39 pm
content: Quote from: John B on July 23, 2020, 10:09:28 amUL Ligted? HAHAHAHA, NICE catch!So, back to the original question, what is the difference in DC PSU? Does it matter what "type" they are regarding what you can run with them?I'll start another thread about portable DC power supplies and how to tell if they are any good or not. I guess my question is can you use any quality 12V portable supply with any 12V application, regardless of type of power it delivers?PS--I'm not plugging in anything to my "good" electronics before I know it's a good supply.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 23, 2020, 05:08:48 pm
content: Might be just for regulation and applied standards reasons.Lighting is a different kind of equipment than e.g. a general purpose power brick, so it's tested and declared conforming to different standards.I bet the load device (wether it's actual LED lighting or something else that can cope with the provided output) can't read that fine print and will just work

author: DW1961
author_rank: Frequent Contributor
timestamp: July 23, 2020, 06:10:26 pm
content: Quote from: capt bullshot on July 23, 2020, 05:08:48 pmYeah, I think you are just right on that too. Reading this page really clarified things. Most all LED light strips or multi-light LED system us CV drivers. One poster above said it might be because ripple and some other undesirable artifact is present becasue it is a 'dirty' supply, but LEDs don't care about ripple (maybe). Constant Voltage is the LED standard.https://www.ledsupply.com/blog/constant-current-led-drivers-vs-constant-voltage-led-drivers/Incidentally, my SONY laptop power supply (made by Light-on) says CC/CV on it so I guess it can do both. That makes me wonder if laptops need both? IF so, we know for sure one reason that power supply says for "LEDs only."

author: capt bullshot
author_rank: Super Contributor
timestamp: July 23, 2020, 06:31:21 pm
content: Quote from: DW1961 on July 23, 2020, 06:10:26 pmThat'd be rather uncommon. Usual laptop bricks have CV output, and some kind of overload limiting / shutdown mechanism (which of course might be some kind of CC operation). I've never seen a laptop that relied on CC operation of its supply.

author: TimFox
author_rank: Super Contributor
timestamp: July 23, 2020, 07:52:32 pm
content: Reading the label, it appears to be a constant-current power supply for 2.5 A (2500 mA), with a compliance (maximum voltage across load) of 24 V. However, I don't think that is correct: instead it is probably a constant-voltage supply ("CV") at 24 VDC, with a maximum current of 2.5 A. A switch-mode constant-current supply is great for driving LEDs in series without extra loss from a series resistor, but it looks like that strip includes SMD resistors.

author: John B
author_rank: Frequent Contributor
timestamp: July 23, 2020, 09:35:19 pm
content: Quote from: DW1961 on July 23, 2020, 06:10:26 pmCV/CC power supplies can be thought of as either maximum condition being met. So for a maximum of 24V and 2.5A, it will attempt to push 2.5A through the load, up to a maximum output voltage of 24V, or it puts out 24V up to a maximum load of 2.5A, after which the output voltage reduces to keep the current at 2.5A.This would describe supplies designed for lighting, however I don't see why a laptop charger would function that way. CV power supplies generally have a hiccup mode when the max output current is exceeded. They don't gradually reduce voltage, it cuts out completely for a second or two then reattempts power.I also wouldn't overlook the noise and ripple. LED installations involve long lengths of wire, which in conjunction with the power supplies inherent conducted interference can contribute to a noisy environment. In the past that has given me problems with magnetic pickups and noise on my scopesYou can always get a quality CV driver then use a second step down CC module to run LEDs. That's a more flexible way to implement dimming. I usually buy Meanwell drivers from the usual electronic parts suppliers. There's nothing particularly special about Meanwell, but they have a large range of voltages, power levels, form factors, CC/CV supplies etc generally at a decent price.Also, avoid PWM dimming methods for controlling lights. They produce the most interference.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 24, 2020, 12:28:21 am
content: Quote from: capt bullshot on July 23, 2020, 06:31:21 pmWell, that's Sony for you. This laptop is one of the last Sony manufactured, and it's 6 years old and I've had zero problems with it, including the battery pack.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 24, 2020, 12:29:18 am
content: Quote from: TimFox on July 23, 2020, 07:52:32 pmIt says right on it, "(CV)."

author: DW1961
author_rank: Frequent Contributor
timestamp: July 24, 2020, 01:14:15 am
content: So the question remains, how do you know what DC supply to plug into a device? Will the device have that information on it? Or domostapplication use CV?As long as LED strips have resistors, CV is the supply to use. According to that source I posted, CV power is virtually all applications for premade LED strips.But what about other applications?

author: John B
author_rank: Frequent Contributor
timestamp: July 24, 2020, 03:02:55 am
content: Yes most applications utilise a CV power supply. When there's a single load circuit, like a string of LEDs, then it makes more sense to implement CC power supplies.In this case the current is limited by the resistors in the LED strips, within certain margins. The current goes up and down with temperature, but is kept to an acceptable limit due to the resistors.

author: james_s
author_rank: Super Contributor
timestamp: July 24, 2020, 04:20:37 am
content: Any decent equipment will have the power requirements listed right on it, or at the very least in a datasheet.I haven't played with any of those power supplies that say they're for LED use only but I suspect they skimp on output filtering if they have any at all. It doesn't really matter if your LED strip is flickering at 20kHz, you're not gonna see the flicker, but if you try to power an electronic device from a power supply that has a load of ripple in the output it will wreak havoc.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 24, 2020, 06:09:45 am
content: Quote from: james_s on July 24, 2020, 04:20:37 amNo doubt. I should get a analyzer. I could just TEST EVERYTHING! I should also get a variable power supply, too. I see them all over Amazon for 100 bucks.

author: james_s
author_rank: Super Contributor
timestamp: July 24, 2020, 06:53:30 am
content: Anyone who does anything at all with electronics should have a multimeter, even if it's just a basic $20 model. A bench power supply with CV and CC modes is very handy but there are substitutes you can use in a pinch to accomplish most things.

author: AndyC_772
author_rank: Super Contributor
timestamp: July 24, 2020, 08:16:44 am
content: Quote from: capt bullshot on July 23, 2020, 05:08:48 pm^^^ This.If a power supply is sold with a CE mark, then it's sold as being compliant with allapplicablestandards.If the set of standards which apply is different - say, because it's being used with computing, communications or other equipment, instead of the type of equipment for which its testing and certification applies - then it's no longer compliant.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 24, 2020, 07:38:29 pm
content: Quote from: james_s on July 24, 2020, 06:53:30 amI have three DMMs. All capable of doing current test. What could I use for a power source instead of getting a bench supply? I mean, they're like $120.00 bucks on Amazon.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 24, 2020, 07:40:54 pm
content: Quote from: AndyC_772 on July 24, 2020, 08:16:44 amGood information, but why does that power supply I linked to above have a CE mark on it and say "LED use only?" Also, how could you test it to make sure it is compliant? I'd love to know that. Does complaint mean ripple and etc are in spec?

author: John B
author_rank: Frequent Contributor
timestamp: July 24, 2020, 08:35:29 pm
content: For the same reason it says UL LIGTED, all it proves is that they have a silkscreen printer. Hence why I try to stick to industrial brand name supplies from legit suppliers.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 24, 2020, 09:04:27 pm
content: Quote from: John B on July 24, 2020, 08:35:29 pmlol, yep. I guess the only way to test it is to put it under load and xx heat and scope it. I don't have that ability, sadly. It would be interesting to see exactly what it's doing.I know I'm going to have to get a scope and learn how to use it to test stuff like this. I just want to KNOW!

author: james_s
author_rank: Super Contributor
timestamp: July 25, 2020, 04:57:07 am
content: If you have any serious interest in electronics you should get a scope, they've gotten really cheap, relatively speaking. A decent 4 channel scope costs less than most smartphones these days.

author: AndyC_772
author_rank: Super Contributor
timestamp: July 25, 2020, 06:55:42 am
content: Quote from: DW1961 on July 24, 2020, 07:40:54 pmThe standards in question relate to EMC (radio interference) and safety, not functionality. The ripple voltage, for example, could be atrocious, but that wouldn't make it non-compliant from a legal standpoint.To test it properly, you'd need to take it to an approvals lab where they'd put it in a screened enclosure, connect it to a load (which in this case is an LED lighting strip) and measure the radio emissions from it. It's also tested for its conducted emissions back up the mains cable, immunity to spikes, surges and drop-outs in the mains supply, and immunity to radio interference generated by other nearby equipment.In terms of safety they'll look at things like electrical isolation between the mains and the low voltage output, the temperature of the case when it's under full load, flammability, current leakage to earth, and whether or not it's correctly marked with voltage and current ratings, type of fuse and so on. A lot of the process is about paperwork.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 25, 2020, 05:04:35 pm
content: Quote from: james_s on July 25, 2020, 04:57:07 amI've noticed bench power supplies (adjustable) are really cheap on Amazon. Any suggestions under 110.00 bucks? Same for scopes, but they seems like they start around 400+.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 25, 2020, 05:08:52 pm
content: Quote from: AndyC_772 on July 25, 2020, 06:55:42 amGot it. I would hope those PSU would at least be safe. A scope could test for voltage and ripple and other undesirable, though, correct? I'm less worried about burning to death than protecting my equipment

author: james_s
author_rank: Super Contributor
timestamp: July 25, 2020, 05:41:43 pm
content: Quote from: DW1961 on July 25, 2020, 05:04:35 pmThe cheapest decent new scope I'm aware of is the Rigol DS1054z which is trivial to hack to 100MHz and costs $349 from Tequipment, that's dirt cheap for a scope.On a tighter budget there are some 2 channel models from Rigol and Siglent, or if you have space for a "boat anchor" and don't mind a gamble on used gear, I've seen 1990s CRT digital scopes from A-list brands like Tek and HP go for as little as $100. Then there are old analog scopes, you might get lucky and score one of those for free if you're in the right place at the right time, but typically working ones fetch about $1/MHz.

author: TimNJ
author_rank: Super Contributor
timestamp: July 26, 2020, 06:48:19 am
content: Quote from: DW1961 on July 25, 2020, 05:08:52 pmJust be aware that there is a lot more sketchy stuff on Amazon than you might think. I don’t know the vetting process for items like power supplies, but there are certainly many which have slipped through the cracks. Some may have questionable safety and/or lack relevant safety documentation. Others might be safe but do not comply with mandated efficiency or EMC requirements. As expected , these items tend to be cheap and thus appealing to consumers, but in reality may be ignoring some regulations intended to help the consumer and protect society as a whole. Not to mention, undercutting legitimate manufacturers who ARE abiding by the laws.By the way, technically power supplies for LED lighting should be IEC61000-3-2 “Class C” compliant for line frequency harmonic current. This is a measure of current drawn by the power supply at harmonics of the line frequency (50/60Hz). Drawing large currents at the harmonics can cause line voltage distortion, which in turn can mess up other systems on the same power distribution network.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 26, 2020, 08:33:01 pm
content: Quote from: TimNJ on July 26, 2020, 06:48:19 amWell, let's hope they test enough of them to keep them relatively in compliance, but I have my doubts too. The only way to tell is to scope them under load. It would be interesting to do.

author: james_s
author_rank: Super Contributor
timestamp: July 27, 2020, 05:22:25 pm
content: Quote from: DW1961 on July 24, 2020, 07:38:29 pmI didn't see this earlier.I used various salvaged "wall wart" type adapters for years, batteries, power supplies salvaged out of scrapped equipment. Now there are a lot of cheap modules you can get from China to build a bench power supply, just add a surplus laptop charger or other similar PSU and put it in a box and you're good to go, I think Dave did a review on one of those a while back. If you have a decent thrift store around those can be a good place to get power adapters, used to be one down near our cabin that had a whole bin of cast off AC adapters and power cords for 99c each, I got several good ones from there. Those all in one scanner/printer/fax things get junked all the time and often have an adapter that puts out around 32V at several amps.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 27, 2020, 10:43:25 pm
content: Quote from: james_s on July 27, 2020, 05:22:25 pmThat's a good idea, but those bench supplies on amazon are like 50-70 bucks. Also, I want t make sure the power is clean. Who knows what the ripple is in those other types of supplies is, for instance? a bench supply would make things so much easier.On the other hand, if I had a decent scope, I could test the supply under load and see how it performs - lol.

author: cdev
author_rank: Super Contributor
timestamp: July 27, 2020, 10:48:50 pm
content: Quote from: DW1961 on July 26, 2020, 08:33:01 pmSome of the designers of the cheap power supplies should go to jail for cutting corners the way they do. I say that because they kill people, usually poor young people, all of the time.

author: james_s
author_rank: Super Contributor
timestamp: July 28, 2020, 03:46:18 am
content: Quote from: DW1961 on July 27, 2020, 10:43:25 pmIf you have the budget then a proper bench supply is better, I'm just trying to give you options.Don't forget there are also a lot of good quality used power supplies available from places like ebay. Others may have some specific recommendations.