Want to start using some electrical equipment, but what a circus.

[Decoman]

So I have come around to consider working with electronics, in order to install led's into scale models. These would be 2-3V, 20mA lights and such, and afaik powered by some 9V power supply (I would have to rig that, which I've done before with 12V).

Now.. what a circus and horror show this seems, buying cheaper electronics equipment. My ambition to work with electronics would have to be to not ending up killing myself, and there is the appeal in acquiring an more affordable digital variable power supply and working with a multimeter.

I've already ordered the 'UNI-T UT61E' digital multimeter, and I think I will be happy with that. Seems safe enough for smaller things, but ofc I am fairly new to working with electronics and I am just used to wire basic lighting in my apartment (which is allowed legally where I live in Northern Europe). My mains are afaik AC 230V where I live.

What is more troublesome is buying the digital variable power supply. The only model that I am currently considering, is this older KORAD KA3005P, and I like the design for the buttons compared to similar products which has a different front side set of buttons. I know there is a D version afaik without a USB port on the back, but I don't know if the P or D version would be better than the other.

Trawling through this forum thread: https://www.eevblog.com/forum/blog/eevblog-315-korad-ka3005p-reviewfail/ I have the impression that I might perhaps get to have a decent power supply, but one that won't end up killing me because it was designed badly. Yet, the unit might fail, because it is a low priced product with questionable merit with regard to choice and the quality of components and design.

I guess I would like to know, what a customer ought to know in 2017, about this over five year old power supply, which I can buy off eBay and advertised as being "new". As seen here:
https://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=Korad+KA3005P&_sop=15

Also, I am wondering if the KORAD KA3005P is suitable for working with small led's, that is designed to work with 2-3V and rated for about 20mA.
The appeal, is to both toy around with the led's ramping up the brightness, test their longevity on a breadboard with some basic fixed 9V power supply), and generally try to get work with led's to have a working lighting setup on models should I decide to do all the work og rigging and designing a setup myself.

If you have anything to say about the choice of products here, feel free to chime in. I am curious to learn if there are better products for this price range of about $150 (USA dollars), and if the build quality is improved in 2017, as opposed to some years ago.


Also, perhaps most important, in a way. What dangers are there using such basic equipment as a digital variable power supply?

How can I safely open up the unit for inspecting the insides for obvious flaws? The only thing I know is that certain electrical components might have a hazardous/lethal current stored in them, even when the unit is not plugged to the regular power grid.

Presumably, it would be most important to check to see if the unit is properly grounded.

[bd139]

If you’re in Northern Europe you may be able to pick up a TTi/Thurlby/Thandar supply for the same amount on eBay as a new Korad. They are far better supplies and likely safer. The Korads have a history of blowing up, display failures, all sorts. The better brand ones last 20-30 years (!)

UT61E is fine. Don’t stick it in the mains though.

[Decoman]

Interesting, I will certainly check those brands out. I am willing to spend a little more money, but I do not want to end up buying a $1000 power supply.


Btw, I am looking at this youtube video claiming to be calibrating the Korad KA3005P unit. Maybe this is not some true hardware calibration, but just operating with a programmed offset? If anyone has a clue as to what this guy is doing, feel free to chime in.
(English subtitling)

[ChrisLX200]

You might consider one of the cheap DC-DC buck convertors, powered by (say..) a 24v DC wall wart it will do what you have described easily. It provides CC and CV controled outputs, and mine is capable of outputting 60w. They work well.

[Decoman]

Would I be correct that one can simply use digital multimeter to get precise output values off the power supply? Because you can just read off the output, and adjust up or down until the output is equal to some precise value on the multimeter? Presumably, the multimeter is more accurate than the power supply reading, but heh maybe I am in error assuming this.

[Decoman]

You might consider one of the cheap DC-DC buck convertors, powered by (say..) a 24v DC wall wart it will do what you have described easily. It provides CC and CV controled outputs, and mine is capable of outputting 60w. They work well.

I am sorry but I am not familiar with that type of product, and I don't know how it works. Looking at the photo, it doesn't seem to be many buttons on it.

[Decoman]

I see there is a 'Siglent SPD3303C'  (Max 32V/3,2A) digital power supply on sale locally, as 'new' for the equivalent of $350. This unit looks to have a triple output unless I am mistaken. I haven't bothered checking out any reviews for this, and the price is double of what I would initially pay, but still I will consider a $350 power supply. Looking at the photos, the unit has a selector on the back for both 220V and 230V.

[iainwhite]

I am sorry but I am not familiar with that type of product, and I don't know how it works. Looking at the photo, it doesn't seem to be many buttons on it.

Dave talks about this power supply in EEVblog #1030
Most adjustments are made with the rotary encoder knob
Link: Here

[rhb]

The appeal, is to both toy around with the led's ramping up the brightness, test their longevity on a breadboard with some basic fixed 9V power supply), and generally try to get work with led's to have a working lighting setup on models should I decide to do all the work og rigging and designing a setup myself.

The usual way to control the brightness of LEDs is to use PWM (pulse width modulation) from something like an Arduino.  You flash the LED at,  for example, 1000 times per second and vary the length of the pulses.

For your modest needs, you might want to simply build an adjustable linear supply using an LM317 and the transformer and possibly other parts from a junked piece of consumer audio gear.  It's just a handful of parts and there are circuit diagrams all over the place.  Building a PS is an excellent beginner's project.  I made a 5 V fixed supply back when I started playing with TTL logic.  For a long time my other supply was a commercial 12 V supply built around an LM317 and sold to power CB radios.  I bought it dead for $1 and spent $0.50 for a new LM317 to fix it.

FWIW I just bought a Instek GPE-4323 linear supply which has two 0-30 3A outputs and  0-5 & 0-15 1A outputs for around  $275 IIRC.  It's fiddly to set because they used pots instead of rotary encoders, but it's nice clean power.  The built in meters are as accurate as my best DMM.   

Setting the output voltage of a DIY PS using a DMM is perfectly fine.  Just make sure you can't accidentally reset the output voltage.  I have some fancy surplus units that use a screwdriver adjustment with a locknut.

[Old Printer]

Unless you are looking to power a mile long string of LEDs, a power supply the size of what you are looking at seems a bit much, though I do like my toys as much as anybody. Ben Eater has some excellent videos on learning the ins and outs of what you are looking at doing, and with a lot less gear. This is a link to the first in a several part series that would give you an excellent foundation for what you are interested in doing. All that he uses are some simple components and an Apple phone charger to start with. No safety concerns here, and the charger could be replaced with a few flashlight batteries to stay away from mains power all together. Dave

[Decoman]

@Old Printer
Thanks for linking to those videos. I learned from that, that you can get 5V off usb chargers.

Lets say for example, 20 led's, with 20 mA each, maximum.
I would expect to have a parallel setup, to better be able to figure out what led fails, as opposed to having a serial setup where one failing led have all led's fail to light up as well.

Uuuuh, so because I presently lack the knowledge of electornics (hey, it's an upcoming projekt ok ).
I can't tell how many amps 20 led's will use. I would guess 0.4 amps with a 9V power supply, as if multiplying 20 mA with 20 leds. I guess 0.4 A with a 9V power supply isn't much. Hm, though, the one 9V powersupply I found for sale the other day, has an on/off switch on it, but a limit of 500 mA which doesn't seem much.

Anyway, I can probably use the power supply for some other things as well, like powering computer fans in creative ways. I already built a spray booth with heh 7 fans. The wiring work was a nightmare. One mistake cutting one wire too short, and I would have found myself having to do a lot of work to fix it as I remember it.

Btw, I need a good and inexpensive wire stripper if anyone knows of a nice model. I gave up after buying this terrible wire stripper that wasn't accurate enough for small wires. I have since seen an other type of model, which also rips off the excess plastic from the wire. I'll go have a look on eBay later this evening, but I just don't like the aspect of getting a wire stripper that aren't any good.

[Decoman]

Btw, I am finding it hard to understand what a 'transistor' is. Heh, I never understood it. Never had any use for such knowledge though, until maybe now (I should probably understand what such a component is and how it works).

As I understand it, a 'transistor' is supposed some kind of switch for amplifying the power by some factor, but how am I supposed to understand where this power comes from, as the power can't come from nothing.

Would a 'transistor' be simply a splicing of two power circuits? One circuit that has a voltage drop with whatever the electric power in the circuit up till then has been used for, and which has to be fed more power at the end?

So sorry if this is a silly question, or if I am making silly assumptions here.

Also, I wonder if the current though a wire would be affected, if the wire had a pear shape bulge of the same material as the conducting material in the rest of the wire. Hm, maybe that part of the wire would act like a resistor?

Also, if a resistor is rated for max 1/4 watts, how am I best to understand that concept? I thought watts were power used over time, as if the power you used was accounted for, in that way. I've heard so much about volts, amps and resistance, that this watts concept seem more  alien to me. I must be eh ignorant of something here I think.

[rstofer]

You are way off on transistors.  I recommend Google...

In short, you control the amount of collector current in a transistor by controlling the base current.  There is a DC Gain figure (Hfe) that gives you an approximation for the ratio of base current to collector current.  An Hfe of 100 means a 1 mA of base current can control 100 mA of collector current if the transistor is capable of handling such current and the Hfe holds up at higher current.

You would be using the transistor as a switch

https://learn.sparkfun.com/tutorials/transistors/applications-i-switches

Running the LEDs in parallel implies a LOT of current.  More often than not, you see several in series and several parallel strings.  It depends on how much voltage you have to begin with.  20 LEDs would take more than 40V is they were all in series but 5 strings of 4 would only need somewhere around 10V (I'm guessing at LED data) and just 100 mA of current.  Remember, you absolutely need the resistor and it needs to drop some amount of voltage if it is to limit current.

[Old Printer]

If you are interested in transistors, and if you are interested in electronics at all you should be, go back to Ben Eaters YT page and watch some of the other videos in the group with the one I linked. He is a great teacher. Basic transistors are the building blocks of modern day circuits. They are switches and amplifiers along with many other things. Many of the discrete transistors from circuits of the 60's & 70's have been placed inside IC's, along with resistors and other components, and now replaced with chips like FPGA's etc. Anyway, if you are going to ever move beyond simply lighting some LEDs then a basic understanding of transistors and their allied components would serve you well. There is a huge store of information on YT alone, so dig in if it interests you. Dave

[Decoman]

Sure, I understand that, it's just that, so far, I just don't get it, and the concept of how to best understand how a transistor works, is eluding me, and I even watched some videos already.

If a transistor is just a switch, I get that (I think). Though it is unclear atm if that is a switch for voltage V or current A.

But, I'll go now and watch this video titled "How a transistor works" by Ben Eater.

[Decoman]

Another question to anyone that can answer it: If you add soldering to a flimsy piece of wiring, to stick that new tip into a socket that clamps the screw tight onto the connector beneath the wire, would the soldering crack and spill all over as the screw clamps down on the soldered wire tip, or would the soldered tip just be squeezed and deformed, without cracking the soldering tin?

And: Does the thickness of any wire-to-ground matter in a circuit? I would think so, but I want to ask anyway. I guess it makes sense that the input wire and the earth wire must be the same type and having the same diameter.

And: Does grounding serve more than one purpose of having safety? Presumably, grounding electrical equipment will have surplus current flow into ground and away from other fragile things.

And: What is the preferred way to discharge a capacitor that you suspect is charged? I've heard/read that working with electronics can be dangerous when unplugged because of charged capacitors.

And: Are there other components on electronics like capacitors that also would be hazardous to come in contact with, because they too store an electrical charge?

[glarsson]

If you add soldering to a flimsy piece of wiring, to stick that new tip into a socket that clamps the screw tight onto the connector beneath the wire, would the soldering crack and spill all over as the screw clamps down on the soldered wire tip, or would the soldered tip just be squeezed and deformed, without cracking the soldering tin?

The solder is soft and will deform, not crack. However, as the solder is soft the clamping force on the screw will loosen after a while (as the sole is soft) and the connection will be bad. Do not solder wires before inserting them in a screw terminal or crimping them to a crimp connector. Crimp on a ferrule if the wire strands are too unruly.

[rstofer]

Another question to anyone that can answer it: If you add soldering to a flimsy piece of wiring, to stick that new tip into a socket that clamps the screw tight onto the connector beneath the wire, would the soldering crack and spill all over as the screw clamps down on the soldered wire tip, or would the soldered tip just be squeezed and deformed, without cracking the soldering tin?

Answered above

And: Does the thickness of any wire-to-ground matter in a circuit? I would think so, but I want to ask anyway. I guess it makes sense that the input wire and the earth wire must be the same type and having the same diameter.

Of course!  At some point diameter is related to resistance and resistance is related to voltage drop and, generally, we don't want voltage drop in wire.  I'm not going to go into the math bit just yet...

And: Does grounding serve more than one purpose of having safety? Presumably, grounding electrical equipment will have surplus current flow into ground and away from other fragile things.

There are a couple of 'grounds' and you seem to be talking about safety ground - Earth ground.  No current should ever flow into the Earth ground.  If it does, there is something wrong with the system.  It is there to shunt short circuit current to ground and prevent the system from becoming 'hot' with respect to a grounded user.  There is also 'circuit ground', a topic for another time...

And: What is the preferred way to discharge a capacitor that you suspect is charged? I've heard/read that working with electronics can be dangerous when unplugged because of charged capacitors.

First we need to know the voltage.  Then we size a resistor to deplete the voltage over some period of time.  Low voltages can be depleted in a second or so without generating a lot of heat.  Higher voltage need more time.  If you have a resistor, use Ohm's Power Law - P = V^2 / R (Power in watts equals Voltage squared divided by Resistance in Ohms.  For 100V with 1/2W resistor, 1/2 = 100^2 / R so the resistor should be on the order of 20,000 Ohms.  Then you need to work out the resistor-capacitor time constant T = R*C (resistance in Ohms times Capacitance in Farads.  Say a 1000 ufd capacitor at 100V with that 20k ohm resistor.  The time constant is 0.001 * 20000 = 20 seconds.  You need to hold the connection for 6 time constants or 2 minutes.

Actually, 2/3 of the voltage will be gone in the first time constant but I don't want to get into the math behind the discharge curve.

You really need to be up on things like Ohm's Law and time constants before you try this.  It's pretty easy to choose too small a value and things burn or too large a value and it takes a long time to discharge the caps.

I work at 3.3V or 5V so this isn't such a problem.  The circuit itself will discharge the capacitors soon enough.

And: Are there other components on electronics like capacitors that also would be hazardous to come in contact with, because they too store an electrical charge?

Inductors store a charge in the magnetic field.  When the current is interrupted, the field collapses and generates a voltage.

This isn't much of a problem for entry level electronics where we're working at 12V or so.  It CAN be a problem if the capacitors are VERY large - like 47,000 ufd.  In my work, 1000 ufd is LARGE and more typical values are 100 ufd.

Stay away from mains powered circuits until you have a LOT of experience.

[rstofer]

For high voltage or high capacitance values, it is not uncommon to permanently install high value resistors across the capacitor.  Maybe 1Mohm or so.  This won't discharge the capacitor any time soon but at least it won't be charged hours later.

[Decoman]

For high voltage or high capacitance values, it is not uncommon to permanently install high value resistors across the capacitor.  Maybe 1Mohm or so.  This won't discharge the capacitor any time soon but at least it won't be charged hours later.

What does "across the capacitor" mean? Would that maybe mean inside the capacitor? Hm, I guess you maybe mean on each side of a capacitor.

Thanks for the input btw!

[Old Printer]

For high voltage or high capacitance values, it is not uncommon to permanently install high value resistors across the capacitor.  Maybe 1Mohm or so.  This won't discharge the capacitor any time soon but at least it won't be charged hours later.

What does "across the capacitor" mean? Would that maybe mean inside the capacitor? Hm, I guess you maybe mean on each side of a capacitor.

Thanks for the input btw!

Yes, one resistor lead to one side of the capacitor, then the other lead to the other side of the capacitor, as in a dead short except the correct sized resistor will only allow the stored current to pass slowly so the damage usually associated with a dead short will not happen.

[rhb]

Take a look at EEVblog #1030.  Add a very basic DC supply, transformer, rectifier and capacitors scavenged out of a dead piece of consumer entertainment kit and you're all set.

[Decoman]

A question: Lacking advanced knowledge of electronics, I am bewildered looking at videos showing the use of a digital variable power supply, seeing how you can seemingly adjust both voltage and amps.

What happens if you set your output power to 2V and eh lets say 100mA? Or, what is going on? I guess I am wondering, why does the variable power supply have a knob for adjusting the current value?

I guess power goes into the circuit you have set up with 2V of power, but what does the 100mA setting do? Does the power supply simply ensure that the circuit is eh saturated with a set amount of electrical power, for a circuit that has a certain amount of resistance in it? So, if your circuit suddenly added resistance, then maybe the power supply would compensate to match the programmed 100mA value?

I find it hard to wrap my head around the concept of adjusting the amps on a variable power supply, as I don't find that intuitive. I guess I don't know enough about amps, or I am missing some other basic knowledge about electronics. My current level of understanding of amps is that amps is a limit of power as configured by the hardware in my circuit box for my apartment. So, that, if I were to plug in numerous power sucking electronic devices, presumably a circuit would blow, or something starts burning.

Edit: Hmm, now that I think about it. I guess maybe the adjustable amp value on a variable power supply, simply sets a limit to the power that can be drawn by the electronics. Eh, otherwise the electronics might draw unlimited power, or just (maybe) drawing maximum power from the mains voltage if it wasn't powered by the variable power supply, but then the volt and amps would be fixed values.

How would a variable power supply know what a fixed current is, when the display rely on showing a value for the current? I imagine that voltage can be measured, but how would the power supply working with specific amp values as a tangible thing? Only thing I can imagine, is the power supply somehow has predetermined values for how many times it pushes out increments of electrical power to maintain a certain current. Admittedly, I feel I still don' understand the meaning of amps, other than the size of a current being similar to the amount of electrical power flowing. So, relatively speaking, the higher the amps at an output, the more different electronic gadgets or components could be powered with such an increase in current.

Edit: Wait, does perhaps the knob for adjusting current/amps, simulate a circuit with resistance in it? Such that you get a predictable current? As if, you could just lower the current, instead of adding a bunch of resistors to a circuit?

[bd139]

There are two modes: constant voltage (CV) and constant current (CC) which it switches between. By default it starts in CV mode i.e. when you say "I want 5V"

If you set the power supply to 1A at 5V for example, it starts in CV mode and it will deliver 5V until the current reaches 1A. If you try and pull more than 1A, it will switch to CC mode and the voltage will drop but the current will stay at 1A.

Power (watts) is a function of V*I (P=VI) so the above will deliver a maximum of 5W to the load. If the thing connected to the supply tries to draw 2A, the voltage will drop to 2.5V

These are not "resistors" as such. They are as close to possible an ideal current (CC) and ideal voltage (CV) source. The voltage knob sets the voltage in CV mode. The current knob sets the current in CC mode.

An application: if you connect an LED across a power supply without a current limit and set it on 20v, the LED will go pop because current is unlimited! If you set 20v at 10mA, the power supply will attempt to deliver 20v in CV mode, find that the device connected looks like a short, switch to CC mode and drop the current to 10mA by reducing the voltage. The voltage will happen to be the junction voltage across the LED.

If you stick 5 LEDs in series, it'll still deliver 10mA though all of them and just knock the voltage up.

Current is electrons per second through a point. Voltage is potential energy. An analogy of voltage is rolling a ball up a hill. 1m up is 1V, 2m up is 2V etc. Batteries pushes a ball up the hill, things doing work roll it down again. So an LED to give out a certain amount of light may need 10mA (which is a fuck load of electrons really) though it and you have to overcome a hill which is 2V high to do this. If you put lots of LEDs in series, you have a bigger hill to overcome (2v times the number of LEDs) but still 10mA of electrons need to go though.

Literally every textbook, apart from perhaps Art of Electronics, has a pretty rubbish explanation of all of this. Basically for every interesting electronic component you can draw a graph of voltage versus current which describes the behaviour of the device. A diode looks like the following:



If you look, as you increase voltage, the current rises infinitely and the diode explodes. If you limit current, the voltage stays constant and the diode does not explode

[kalel]

Edit: I see bd139 posted a far better explanation.

Disclaimer: I have not used such a supply yet (with current adjustment). I am also a beginner.

Here is how I think it works:

Say you set Voltage to 5V and Current to 1A (and turn on current limiting, there is probably on/off setting somewhere),  this will limit the current to 1A max (rather than e.g. try to increase current until it reaches 1A).

So, if you connect a 5V device that uses 500mA, you will see 5V 500mA on the screen, no problem since your limit is 1A.

What if your device tries to use 1.5A? Since current limiting is on, the power supply will decrease the output voltage until the current drops to 1A. So, it will be your 5V minus whatever is needed to ensure output current used by the device is not higher than that.

Of course, if the device then stops asking for so much current and starts drawing 500mA again, the output voltage will go up to the 5V you set, as long as the current is below 1A (which you also set, of course this could be anything within the specs of the supply).

[bd139]

You're spot on and there's less rambling than mine

If you want a quick start in fundamentals, this is pretty good: http://lcamtuf.coredump.cx/electronics/

[glarsson]

An application: if you connect an LED across a power supply without a current limit and set it on 20v, the LED will go pop because current is unlimited! If you set 20v at 10mA, the power supply will attempt to deliver 20v in CV mode, find that the device connected looks like a short, switch to CC mode and drop the current to 10mA by reducing the voltage. The voltage will happen to be the junction voltage across the LED.

Note that many power supplies are not quick enough to apply the current limiting (perhaps due to a large capacitor on the output), so it is very likely that the LED did go pop due to a brief 20V pulse even with current limit set to 10mA.

[rstofer]

I have a Rigol DP832 which has adjustable voltage and adjustable current limiting.  I was bringing up a new Z80 project and I was anticipating that there might be some bus contention going on.  So, I set the PS to provide 5V and set the current limit at 100 mA.  Sure enough, there was an oopsie and the current tried to exceed 100 mA.  The output voltage dropped down to something under 1V.  The important part is the magic smoke stayed inside the chips.  This is what the current knob is used for:  Protecting the load!

Had I connected the project to a 5V supply with a current rating of 10A, the entire project would have melted into an expensive blob of molten plastic.

I don't care for the term Constant Current because that simply doesn't happen.  For such a thing to work, the PS has to be able to put out an infinite voltage (consider trying to pump constant current of 1A into a 1 Mohm resistor - this takes a million volts!).  The voltage setting isn't used at all.  The power supply puts out whatever voltage is necessary to pass a specific current - in my view of Constant Current.

What really happens is the maximum voltage is limited to the set value and the current setting puts a current limit on the output and the voltage drops until the current limit is satisfied.  But it only drops!  It never increases beyond the voltage setpoint.  I consider that current limiting, not constant current.  Other opinions vary and I accept that.

I suppose a person could make the convoluted argument that "my power supply is a Constant Current supply within the constraints that it can only put out 3A at  30V".  OK, that's one way to define Constant Current.  I prefer the description as "my supply will put out up to 30V with an adjustable current limit of up to 3A".

[bd139]

That's actually quite fair. Decent power supplies, well my PL310 at least, is a good precision current source with 30v compliance

Note that many power supplies are not quick enough to apply the current limiting (perhaps due to a large capacitor on the output), so it is very likely that the LED did go pop due to a brief 20V pulse even with current limit set to 10mA.

The capacitor is there to support CV mode. The power supply's feedback loop from when it measures voltage to the time it can react and change the pass transistor current is actually quite slow. The capacitor keeps the voltage up long enough to squirt more current into the circuit. If it isn't there, then when there's a load transient i.e sudden current dip, the power supply will not react quick enough, then overcompensate, then not react quick enough etc, which is oscillation and then lots of bad juju happens with low impedance sources. Bang, zip, pop, woosh etc.

Unfortunately the capacitor can dump a shit load of current out pretty quick into a device expecting CC if it's pre-charged enough. If you're going to use it as a CC source on something fragile, start with the supply shorted with a banana lead, then pop that out rather than just turn it on. .... Or build yourself a simple current source with a transistor and a couple of resistors or something.

My daily driver power supply, an E3630A has a fixed CC mode of 0.5 / 2.5 amps depending on rails. Never a problem with CC explosions if your CC isn't fixed. A 100 ohm 1/8W resistor in series with your circuit is a cheap way to find mistakes

[KL27x]

What really happens is the maximum voltage is limited to the set value and the current setting puts a current limit on the output and the voltage drops until the current limit is satisfied.  But it only drops!  It never increases beyond the voltage setpoint.  I consider that current limiting, not constant current.  Other opinions vary and I accept that.

But as long as you don't run out of voltage, you can just turn the voltage up until the voltage limit is irrelevant for the circuit in question. Just because you are using it as "current limiting" doesn't mean it won't also function as a constant current source, within its maximum parameters.

On the flip side, if you put a 0.1R bit of nichrome wire across the terminals of your PSU, it doesn't matter how high you set the voltage. The output voltage is going to drop to a few V, max. If you say "constant current" is a misnomer, wouldn't that mean "constant voltage" is equally a misnomer? It depends on the circuit/load which limit is hit first.

[rstofer]

You can certainly make the case that the PS isn't Constant Voltage either because it can't deliver infinite current.  Fair enough.

In my limited experience, I want to control voltage.  I have never seen an application for constant current.  I'm sure they're out there but I haven't run into one.  OTOH, everything I do with a PS is somehow related to a fixed voltage output.  And, yes, I know I can do remote control of the voltage and thereby get a voltage output that varies over time.  And, no, I have no use for such a thing.  There is no possibility that I will network my lab tools.

Since I am only interested in constant voltage with current limiting (to protect the load), I'll just consider the supply to be Constant Voltage with Current Limiting and let others pick their own definitions.

[CharlieWorton]

I have the Korad KA3005P power supply.  They've increased their quality dramatically since Dave's review.  Here's a link to my review on Amazon.  Regards, Charlie
https://www.amazon.ca/gp/customer-reviews/ROUW2KIJCDHJG/ref=cm_cr_dp_d_rvw_ttl?ie=UTF8&ASIN=B0085QLNFM

[Decoman]

CharlieWorton's unit seem to have 4 memory slots on the display, with the "lock" feature where the M5 is seen on other units. Looking at photos on eBay is so confusing, never knowing if the sales item actually looks like the one you will get, or not.

I asked a seller if he could be so kind and test a unit for me, with an oscilloscope, re. volt overshoot on the output, and I am curious to see if the seller will do that for me.

[Decoman]

If you set the power supply to 1A at 5V for example, it starts in CV mode and it will deliver 5V until the current reaches 1A. If you try and pull more than 1A, it will switch to CC mode and the voltage will drop but the current will stay at 1A.

Thanks. Nice to know.

And thanks to everybody else chiming in.

If you stick 5 LEDs in series, it'll still deliver 10mA though all of them and just knock the voltage up.

Aah. I see. Sounds practical. It sounds like one were to build a circuit with several 20mA led's and then set the limit to 20mA, and then you could ideally just wait and see with the PS in CC mode, to learn how many volts the PS pushes out. Assuming there wouldn't be any real volt overshoots from the output of the powersupply. I am tempted to buy a cheaper one, so without any real experience with this, reading reviews is my only hope, unless some seller can test a unit for me before selling it to me.

Presumably, with V set to 1V and A set to 20mA, in CC mode, the power supply will start ramping up voltage starting at 1V right? Or would there still be a risk of volt overshoot in that case with bad power supplies in general? I guess, starting at 1V in CC mode sounds better with regard to risks of volt overshoot, than being in 'CV/constant volt' mode set to the maximum allowed voltage value, risking overshooting for sure if there is ANY volt overshoot.

[bd139]

You will have to set the voltage to above the required voltage if you are using it in CC mode. The power supply will deliver only the set voltage maximum. If you set it at 1V the LEDs will never light.

[Decoman]

Ah ok, but eh, would using 'constant current' mode be a way to simply avoid damages stemming from voltage overshoot on the output in any case? As if the power supply NEVER had any "current overshoot" and could always guarantee that the mA limit was real with no issues, unlike issues with voltage overshoot on the output. I mean, for working with LED's that only handles 2-3V or some such, it sounds nice to me, if a simple current limit in milliamps could be a sure way to avoid burning out your installed LEDs, or would I be wrong to think that?

And, could there be such a thing as a "current overshoot" the same way you would have 'voltage overshoot', specifically on the output?
Making me wonder if LED's would fry, or not, with both a professional, and with a cheaper power supply, if suddenly outputting power with constant current mode at 20mA, connected to some leds. I guess it would be crucial what voltage was set before enabling the output in constant current mode. E.g setting 4V and 20mA might be better for LEDs when powered by a constant current limit, as opposed to setting 30V and 20mA. I have no experience with this so I have no idea what to expect, neither from professional equipment and the cheaper ones, that most likely aren't as good for a variety of reasons.

[Decoman]

Another thing: If I were to put 200 led's in a series, each rated eh 2.5V and 20mA, then I would need to power this with 500V? If so, suddenly LED's aren't so innocent anymore, as I imagined them to be previously.

This question makes me wonder if having lots of LEDs in a series, would make the voltage be larger for LEDs on one end, but smaller at the other end. Maybe required to use capacitors inbetween to feed the LEDs? Or, maybe I am to understand that a constant current would light the LED's anyway, equally well, as long as there is enough voltage coming from the power source.

[Assafl]

An approach that works really well in buying power supplies is searching eBay in your country (or city) and finding scrapyards that are trying to sell power supplies (such as the TTi models suggested above or HP, etc.).

These things are heavy so the dealer will factor in a few 100's of dollars for shipment, which makes them sit on their shelves for a LONG time. So haggle.

I got the three channel TTi (PL-330) with GPIB and RS232 control for about $250 locally. I took the best looking one from the pile. I tested it with a DMM I brought from home a power resistor. The pile is still there listed for $400 && $200 shipping on eBay .

 

[Decoman]

I tested it with a DMM I brought from home a power resistor.

I am not sure what this means.

[ogden]

Ah ok, but eh, would using 'constant current' mode be a way to simply avoid damages stemming from voltage overshoot on the output in any case? As if the power supply NEVER had any "current overshoot" and could always guarantee that the mA limit was real with no issues, unlike issues with voltage overshoot on the output.

Unfortunately most of lab/bench supplies regulate current much slower than voltage. Manufacturers do not even specify current regulation transient time. For example let's check quite decent & new Keysight supply: E36311A. Voltage regulation transient recovery time specified: < 50uS. Nothing said about current regulation recovery time - when load suddenly requires another input voltage during CC mode. Only overcurrent protection reaction time specified: 5mS. 100 times slower!! Even very specific supply named "Dynamic Measurement DC Source", 66332A does not regulate current as fast as voltage. Dunno about other manufacturers thou.

[bd139]

Another thing: If I were to put 200 led's in a series, each rated eh 2.5V and 20mA, then I would need to power this with 500V? If so, suddenly LED's aren't so innocent anymore, as I imagined them to be previously.

This question makes me wonder if having lots of LEDs in a series, would make the voltage be larger for LEDs on one end, but smaller at the other end. Maybe required to use capacitors inbetween to feed the LEDs? Or, maybe I am to understand that a constant current would light the LED's anyway, equally well, as long as there is enough voltage coming from the power source.

You usually use multiple chains of LEDs across a voltage. So if you have 12 volts, you will stick 4 LEDs in series and a series current limiting resistor to limit the current to say 20mA. Then when you want more LEDs, you add more chains across the voltage. All 12v. Each additional chain takes more current.

[paulca]

If your intent is to just put some LEDs on models and be able to dim them slightly you don't need a whole lot of equipment.  All this talk of current limiting power supplies and PWM is (from a beginner to beginner perspective) NOT step one.  It's about step 25.

A mobile phone charger, cut a USB lead and pull out the 5V and 0V, usually, not always black and red.  This will supply at least 1A, which will power a HUGE number of LEDs perfectly fine.

You mention 20mA LEDs.  As I learnt on here, they may say "20mA", but you wouldn't run them at 20mA unless you want extremely bright LEDs.  Most modern LEDs will provide a glow for indication use at less than 1mA and be adequately bright to use as a torch and find your way round a dark room at 4mA.  At 20mA they will be more like a cheap 2xAA battery torch.  If you had two or three of htem they would be as bright as the torch on your mobile phone!  So a mobile charger will happily power you 50 LEDs at low brightness or 20 or 30 at high brightness.  Some chargers go up to 2Amps so will power twice as many LEDs.

Current limiting the LEDs with the power supply is not how you would go about it. If you connected 50 LEDs to your mobile phone charger it would probably work, the chargers lack of output would limit the LEDs, but the charger would get heavily loaded to 100% and heat up.  If it's a good charger it might cut out, if it's a cheap one it will get hotter and hotter and burn itself out.  Worst case is it could potentially go on fire, but that is extremely unlikely if it's not a totally cheap knock off.

Current limiting LEDs is usually done with a series resistor.

Power (V+) -> LED -> Resistor -> GND (0V or V-)

The resistor value is easy to calculate, there are hundreds of you tube videos explaining how, but basically, if you want 10mA and have 9V you want 9 - 2V (for the LED) / 0.01 = 700Ohms, I would just use a 1KOhm in place of this.

If you wired 50 LEDs into a model and wired them all in parallel (ie. each having a wire back towards the power supply) you would need a resistor on each LED.  50 LEDs and 50 1KOhm resistors would draw a total of around 500mA (half of what a small cheap mobile phone charger will put out) and everything will run cool and happy.

You 'can' group LEDs in parallel sets and share resistors between them by adding up the current you want and using a single lower value resistor to limit current through the set.  It's probably not "proper" as there is the potential for one LED to take more of the current than another, especially if they LEDs are not exactly the same.  You can also put LEDs in series (one after the other) with a single resistor but as the LEDs "consume" voltage you are limited to your supply voltage.  With a 9V supply, using blue LEDs which might drop 2.9V you could only get 3 in series before you ran out of volts.  With a 5V supply, you might get 2 in series before you run out of volts.  Different colour LEDs drop/consume different numbers of volts.

I would say you want to get a fairly decent mobile phone charger, modify a USB lead and a set of 1/4 watt carbon resistors from fleabay.  That an a multi-meter.

The Ben Eater videos above are worth a watch on this topic.  In other videos he runs a whole 8-bit computer on about 16 breadboards, containing about 100 LEDs from a mobile phone charger, though he does comment he needed to beef it up.

On a current limited power supply.  The current limit mode, for beginners is often used as a fail safe limiter.  If you short something out in your circuit and power it on the PSU current limiter can be set to the maximum current you expect to use and if you have a short it will click into current limit.  You will hear the relays clatter and see the voltage drop back.  You get to see you have messed up without damaging anything or letting any magic smoke out.

If you are using a small mobile phone charger it is unlikely to put out enough current to set anything on fire.

As to dangers, at sub 12V, sub 2A, unless you stick the voltage on your tongue you'll be fine.

Oh, to dim the LEDs you can just vary the resistance.  It would only be when that becomes impractical due to dimming large sets of LEDs you would consider PWM.  For PWM you can use the 555 timer chips which are high school electronics 101 type stuff.

[Decoman]

Power (V+) -> LED -> Resistor -> GND (0V or V-)

I think the resistor must be placed between the power source and the LED, not after the LED, or the LED will probably fry is my impression from all of this. But ofc, anyone, correct me if I am wrong.


Hm, I guess I find the use of a variable power supply appealing, because of how it at least would work as a fancy potentiometer, which regulates the voltage in a circuit as I understand it (I use one for my external speakers on my PC, a big knob made by 'TC electronic'), while also giving interesting feedback in form of measurements. I know I use a smaller device to power things, I did this for my custom made spray booth, which uses two 12V LED power supplies, one for the lights and one for the fans, so I had fun rigging that.

For working with LED on a complicated hobby project, I am thinking that I feel I could use a variable power supply, even though I might not know about all its features, or need to use all of the features. Adjusting the strength of LED's for testing is appealing. I guess a simple potentiometer would work in a circuit, but I like the idea of turning a knob on a power supply better.

Update: So the seller I talked to was not willing to actually test a power supply I wanted to buy, which would have been so helpful. I can understand that they might not want to open sealed products, or even worse, multiple packages, but it isn't unheard of, of a seller opening the package either for some other stuff. I did not like how the seller seemed to not know at all if the unit I inquired about was known to having volt overshoot on output. I guess I could compensate, by always remembering to lower the voltage from a memory setting, such that a voltage overshoot ideally never exceeds the initial voltage value set with the memory button. As for product quality I am willing to accept some risk, but I will want some assurances, and then at least I can try judge if I find that trustworthy or not myself.

[jpc]

As long as the resistor is in series with  the LED it doesn't matter though it is perhaps more usual to have it on the positive side of the supply.

I.E. +ve-resistor-LED-0V
or    +ve-LED-resistor-0V

will both work fine.


The only clue many online sellers have about the items they are selling is what is written on the box, if even that.

[paulca]

If you have some more of those 12V LED power bricks then the DPS3003 power supply that someone linked would give you a variable power supply with voltage and current limit.  They are sub £15 in the UK.  They just need a DC power supply input greater than your required output voltage.  As you have 12V DC supplies, these would give you at least 9V output from the little module.

A single potentiometer to limit a large number of LEDs would need careful consideration.  You would need to workout how many volts the potentiometer will drop and the total number of amps the circuit will consume.  The power lost as heat in the potentiometer will be equal to the voltage drop across it multiplied by the current.  For a 9V supply to parallel LEDs this could easily be 7V * 1A = 7 watts of heat.

Also note that using the PSU to limit the current for this purpose will not do as you expect.  Current is by demand based on the resistance and voltage of the circuit.  The PSU cannot change the resistance of the circuit.  All it can do is change the voltage to limit the current flow.  So your 9V power supply limited to 1A will drop to to considerably lower voltage if you try and draw 2A through it.  I'll take a stab at 4.5V.  So any LEDs in series will go out.

This is where you get into PWM as suggested previously.  Rather than trying to limit the current at all, you simple supply the full current in pulses.  if you supply the required 1A but only 50% of the time you effectively limit the LEDs to 50% and consume an average of 500mA.  You will still need the current limiter resistors on each LED though.  The heat dissipated is orders of magnitude less. 

If I were you I would start with smaller sets of LEDs, choose your power supply, multimeter and test/experiment.  When you gain some experience, then consider 555 timer circuits and transistors which will be needed to control larger arrays of LEDs or higher current.

555 timers (maybe with a few logic ICs) can do some really cool things with LEDs too.  Flashers, sequencers, bar graphs, etc. etc.  Lots of fun.

[rstofer]

Aah. I see. Sounds practical. It sounds like one were to build a circuit with several 20mA led's and then set the limit to 20mA, and then you could ideally just wait and see with the PS in CC mode, to learn how many volts the PS pushes out. Assuming there wouldn't be any real volt overshoots from the output of the powersupply. I am tempted to buy a cheaper one, so without any real experience with this, reading reviews is my only hope, unless some seller can test a unit for me before selling it to me.

Look at an LED datasheet!  20 mA isn't some magic number, it is simply a number within a range of currents over which the LED may operate.  For brief periods, like overshoot if it exists, the LED can stand a  much higher current.

https://www.vishay.com/docs/83171/tlur640.pdf

The surge current has an absolute maximum of 1 AMP but you will see graphs where the LED is characterized out to 90 mA.  Note that the luminous intensity is given for 10 mA.  There is no particularly good reason to operate an LED at 20 mA.

Presumably, with V set to 1V and A set to 20mA, in CC mode, the power supply will start ramping up voltage starting at 1V right? Or would there still be a risk of volt overshoot in that case with bad power supplies in general? I guess, starting at 1V in CC mode sounds better with regard to risks of volt overshoot, than being in 'CV/constant volt' mode set to the maximum allowed voltage value, risking overshooting for sure if there is ANY volt overshoot.

No...  The output voltage will jump to 1V or 20 mA, whichever comes first.  The voltage will not ramp up above the setpoint.

For LEDs, I wouldn't spend a lot of time thinking about overshoot.  Further, I doubt that the overcurrent control loop is fast enough to prevent voltage overshoot.  After all, the voltage control loop should be faster and it is the one allowing the overshoot.

I think you're worrying about things that don't happen.  LEDs are pretty robust as are most other components.  It would have to be a very poor power supply to overshoot enough to damage anything.

That PS you're looking at has an on-off switch.  I assume it is just ahead of the terminals so the output voltage from the supply is stable long before the load is connected.  But I admit, I'm just guessing.

[Decoman]

Although I don't know much about electronics, I should point out that for my particular project, it is imperative that I get to have the LED's live as long as I can have them, which imo would be one good argument to avoid any funny overvolt outputs onto the LEDs. Another issue for me is to try figure out what I can do in the selection of LED's to get rig the LED's for a long time without turning black, with little hope of fixing it, unless I take the time and effort and cracking open a glued shut scale model filled with LED's. I intend to have some LED's run for some time in an ideal setting outside the model, and then I would get to play with extra LED's on the side to try learn something about how quickly the LED's might die on me.

[bd139]

If you are worried about lifetime in a scale model, it's worth building an LED rig in the middle of whatever you somewhere that's easy to maintain and and use fiber optic cable/light pipes instead. Plan for replacements not lifetime.

TBH the only problem I've seen with LED longevity is from HP equipment from the late 1970s where the red LEDs start to go orange. That's a pretty good lifespan!

[kalel]

To make the LED last longer, it is a good idea to use it at less current (which of course gives less brightness).
Test with one LED first to find the optimal current before placing anything inside.

A resistor will help to protect the LED during testing. So, if you wanted to hook up and LED directly to the power supply for tests, you can still add a resistor in series somewhere. But as others have mentioned a very short burst of over-current should not destroy the LED (unless the voltage is high enough to cause that).

In order to have them work long, it's important for them to stay cool.
Reducing the current (and brightness) will keep them much cooler, which is good for cases where you can't place them on a heatsink.

If you only run them at say 5mA or even 10mA instead of say 20mA they should last much longer.

[rstofer]

LEDs will run forever as long as they aren't abused.  Try running them at lower current.  It is often argued that a person can't tell much difference in brightness between an LED at 10 mA versus an LED at 20 mA.

If you design for 10 mA and the LED is characterized out to 90 mA, that's kind of a lot of headroom for overshoot!  From the graph if If vs Vf, you can calculate how bad the PS could be.  What Vf corresponds to If=100 mA.  Multiply that value by the number of LEDs in series and add the resistor drop.  What voltage do you get?

[rstofer]

And design the thing for a constant voltage with a series resistor.  Have an appreciable voltage drop across the resistor.  Don't stack 10V worth of LEDs in series on a 12V supply.  There is no rule and, mathematically, it makes no difference but a common 2V LED at 20 mA on a 5V supply uses a 150 Ohm resistor and drops 3 of the 5V available.  In a long series stack, the resistor should probably do something similar - drop about 1/2 of the supply.  So, for a 12V supply, no more than 3 LEDs, maybe 4, but no more.  Yes, there is a lot of wasted power in the resistor.  That's a cost of doing business.

If you are running all the LEDs in parallel, you just need one dropping resistor per LED.  Take that 150 Ohm resistor above.  If the LED shorts out, the maximum current in the loop is 5V / 150 Ohms or 33 mA.  There is no condition under which the LED can get more than 90 mA.  Even if the voltage went to 10V, given a 2V LED, the max current would be (10-2)/150 or 53 mA.  Not ideal but still withing the rating of the LED.

It gets even better if you design for 10 mA.  In that case, for the 2V LED and the 5V source, you would use a 300 Ohm resistor.  If the PS spiked to 15V, the maximum current would be 43 mA.  Well under 90 mA for which the LED is characterized.  Heck, it could spike to 27V and only pass 90 mA.

This just isn't a problem!

[Decoman]

Thanks for the info guys!

[Decoman]

I see that there is a Rigol DP711 & DP712 variable power supply for some affordable price ($300 new on eBay). The differences seem to be mainly that one is 30V/5A and the other is 50V/3A.

Would it make sense to suspect that one version is more accurate, or more careful when working with LEDs? Though, I suppose there just isn't enough info to even suppose such.

At least these units are labeled EAC, which according to Wikipedia seem to indicate that there is some safety requirements/standards to adhere to with such a labeling on products.
https://en.wikipedia.org/wiki/Eurasian_Conformity_mark

[rstofer]

The other way to do things is to string a bunch of LEDs in series and drive them with a much higher voltage (perhaps as high as 24V) and control the current with a 2-transistor constant current driver like:

http://www.electroschematics.com/9383/led-constant-current/



The current source control will act very fast so overshoot is a non-issue and the version linked above can work on up to 24V.

One problem with long strings of LEDs is that they are all about the same brightness.  That may not be the best lighting scheme.

There are lots of current sink circuits and the above is just the first I looked at.

Do pay attention to the power dissipation in the upper transistor.  A heatsink might be a good idea depending on the dissipation.

It seems a shame to waste a perfectly good power supply on driving LEDs.  You can use a 12V wall wart to power 4 or 5 LEDs with this circuit.  The 20 mA isn't an issue for the wall wart so several strings can be connected in parallel.  Like 10 strings in parallel if the wall wart can provide an honest 2A.  Half that many if it is overrated.

[Decoman]

*reads about 'burden voltage' re. DMM's.*

Haha, so my digital multimeter will be inaccurate for reading off mA measurements with the DMM probes, because it also lower the voltage in the same process as measuring the current at low current values?

*reads about the uCurrent product*


Edit:

It seems a shame to waste a perfectly good power supply on driving LEDs.  You can use a 12V wall wart to power 4 or 5 LEDs with this circuit.

Yes, yes. I would want to use a variable power supply when working with the setup, and then finally connect the LED to some kind of generic 9V or 12V power supply.

[rstofer]

*reads about 'burden voltage' re. DMM's.*

Haha, so my digital multimeter will be inaccurate for reading off mA measurements with the DMM probes, because it also lower the voltage in the same process as measuring the current at low current values?

*reads about the uCurrent product*

Even experts get caught out from time to time.  First, watch the video on current sources I linked above and then watch the follow-up.  There is a lot to learn from this video:



So, if you are measuring voltage, you need to be sure your meter doesn't load the circuit.  If you are measuring current, burden voltage comes into play.  If you are measuring resistance, you shouldn't hand-hold the parts because skin resistance enters in.  In fact, there's a reason that bench meters offer 4-wire resistance testing (hint: it is to eliminate the effect of voltage drop in the test leads):

https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/kelvin-resistance-measurement/

Nothing about electronics is a simple as it seems.