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).