Constant current power supplies

300mA+/-5%. Now, why on earth would I want to power my Arduino board using something
like this? Can anyone explain why my board would want to have a device trying to drive a
fixed 300mA current into it? Makes no sense, unless maybe the p/s were jiggered after the
fact.

You would never want to power an arduino board with a true constant current power supply. An arduino board with nothing wired to it's output pins or 5V or 3.3 pins draws only around 80ma, and a true CC power source would raise it's output voltage as high as it could trying to force 300ma into the arduino board, which would most likely burn up many components on the board due to the high voltage.

There are some Asian DC constant current LED driver modules designed to power 1 and 3 watt power leds that also have a TTL level input that allows for PWM control for the module effectively allowing a dimming function if one wants to use such a feature, or as a simple on/off control using an arduino digital output signal. However you have to look carefully at the maximum PWM frequency that is allowed for the specific CC drivers that have this extra feature.

Lefty

draythomp: can you post a link to the actual modules that you have purchased? And/or photos or reverse-engineered schematics (without chip numbers, since they're unreadable.)
eBay will let you link to completed auctions for quite a while after they're over...

Also, what voltage to these end up putting out when you connect them to an Arduino?

Here's the closest thing to what I got that is on their website. I actually ordered it through sourcing map and they don't have it up any more. After I got it and played with it a little there was a series of mails back and forth with them in extremely broken English and Chinese (we were both using google translate) to determine the lowest voltage (6V) and the highest (12V) instead of what's on their site. Seems there was a mistake somewhere. I paid practically nothing for them; I'm unclear on the exact price though because there was a few cents adjustment after shipping for currency differences that happened during the day or so it took to get it actually ordered.

http://www.microtek-led.com/ArticleShow.asp?ArticleID=116

So, you see that the voltage, 6-12, was right in the sweet spot that the arduino likes and its regulator would take care of the voltage, and of course, experimental results were different from either the specs or the conversation. I was pretty bummed when I saw how much noise it puts out, but nothing ventured, nothing gained.

Letfy, you're right, no one (well maybe me) would want to put a perfect constant current supply on any device that has some limits on acceptable voltage; it would blow up in a heartbeat. However, there ain't no perfect CC supply, and this ones parameters showed possibilities for a really cheap and easy power supply.

oric, you don't have to do this. I never suggested you do this. Don't do this. For crying out loud, I brought up a subject for discussion and got attacked like a teenager that told her dad she got a tattoo on her face.

I asked for thoughts, not a diatribe.

For crying out loud, I brought up a subject for discussion and got attacked like a teenager that told her dad she got a tattoo on her face.

Good, so you learned a lesson. :smiley:

Lefty

Yep, I ain't never getting no darn tattoo on my face.

Straight out, it is unlikely something like this can be used for a device needing a constant voltage.

The topology, however, is likely a fly-back smps - assuming that it is isolated. That means the led string is floating above a current sampling resistor that controls the output voltage to maintain that constant current (thus constant voltage over the current sampling resistor - likely via a zener + optocoupler).

So all you need to do is to identify that resistor and put in place a divider. That resistor is very easy to identify: it must be between the cathode (led-) and the ground.

After that, you have turned this constant current regulator to a constant voltage regulator.

Sorry folks, I just have to do this:

<flame_on>
Oh, for goodness sake, I asked a question. Yes a question. I started off light-hearted and wanting to have fun with the possibility, then I was told that I was basically an idiot for even bringing up the subject, it wasn't constant current, had a step-down transformer, didn't have certifications, it didn't make sense to attempt such a thing, and that others would never ever do such an unsafe thing. Fine, don't do it.

As for the number of people that thought it was a bad idea, a couple of them actually said something reasonable about why it might be a bad idea. Things like isolation, noise, etc. That's the kind of thing the inexperienced folk should be learning, not that someone THINKS it's a bad idea because they hadn't looked into the possibility.

The tone of this board is sad. I tolerate it because there are a lot of people that have really good ideas and experience that have helped me a lot in the past. However, there are some that just want to belittle the poster as quickly as they can. I'm way too old to be put off by people that use sarcasm as their initial response because I know there are others that will contribute really good ideas.

So, thank you very much to the folks that came up with good suggestions and novel ideas; the rest of you,...

Have a nice day.
</flame_on>

I just have to do this:

It is a process thing: you will learn to talk to those whom you can talk to and don't talk to those whom you cannot talk to.

http://www.microtek-led.com/ArticleShow.asp?ArticleID=116

It looks very much like the "fake" Apple power supplies (one or two transistor flyback), though not misleadingly labeled and bare for anyone to look at the details (and nicely priced, and not needing "decasing.")

It doesn't look like they'd come much closer to meeting safety specs. I love the "fuse" implemented as a squiggly little trace on the PCB (clever and probably effective, but probably not "legal.")

Here is one out of a broke led light bulb I bet it's made right off the data sheet
the chip on this one is LNK562-564. It was rated at 300mA

The circuit shown in Figure 5 is a typical implementation of
a 6 V, 330 mA, constant voltage, constant current (CV/CC)
output power supply.

http://www.powerint.com/sites/default/files/product-docs/linklp_family_datasheet.pdf

Led Suppy.jpg

If you prowl around that site a bit there's example after example of flyback power supplies and how they can be made. I gather that, given their chips, it all boils down to the flyback transformer design and choice of resistors that are used. They even offer a transformer prototyping service that can build a transformer for you based on their application notes. I don't want to build one, I want to use one.

Now, all I have to do is find a tiny, cheap, constant voltage power supply that isn't already stuck inside a wall wart to experiment with.

See, I have this dream of a device that houses an XBee, a minimal arduino and a couple of screw terminals all built inside a wall wart like this: GS-2415 | Wall-plug Electronics Plastic Enclosure . That way I can deploy a sensor and read its data remotely. Things like the inside temperature of the chicken coup, voltage of the generator battery, open-closed state of the North gate, become easily done. Add a few more terminals and a couple of tiny relays and you have a sprinkler system, a remote gate opener for the UPS guy, an auto drain for the horse watering trough (mosquito prevention). With a simple sealed float switch I can remotely tell when the septic tank starts to rise so I can go out and clean the output filter and hose out the input. Yes, the chicken coup has power, this is Arizona, they even have a swamp cooler in the summer.

I have practical uses for this kind of thing.

find a tiny, cheap, constant voltage power supply that isn't already stuck inside a wall wart to experiment with.

The simplest would be your cell phone / usb chargers. Buy one and break it apart.

LNK562-564.

That is one of those gated oscillator smps types: it turns itself on/off between 1.69v - 0.8v (on the feedback pin), making it suitable for low-power applications but largely not usable for a device needing a constant voltage.

The grandfather of those designs are the Linear LT1107.

@ dhenry I think you have some details incorrect... Again. The first switchers I remember were made WELL before integrated designs for chips were out of the development stages., Remember PWM? it was in discrete form (3 or 4 IC's) and the real cheap ones used a flyback oscillator... and a linear regulator.
I remember the first one I saw, very clearly... It was plugged in and promptly went up in smoke, along with a $300.00 basic 4 function calculator.

Bob

Bob, I think he was talking about the designs to turn off the device. In 'the good ol days', they used and on-off switch and a human to turn them off. Later, inactivity timers took the job on and did a pretty well. These days, they actually sample the current and shut down when the battery or other device their hooked to stops pulling current; actually, they don't shut down completely, just drop really, really low. It's a green thing.

It's a green thing.

A gated oscillator is actually quite different from what you were talking about.

OK, I give up.

the LNK chip seems to be exactly the sort of thing you're hoping that the LED drivers would supply. A small and only marginally regulated replacement for "ye old" 60Hz transformer and DC rectification circuit, that converts AC line voltage to something appropriate for the input to Arduino-like circuits, in a hopefully cheap circuit. That's what it says in the datasheet:

LinkSwitch-LP switcher ICs cost effectively replace all unregulated isolated linear transformer based (50/60 Hz) power supplies up to 3 W output power.

dhenry is also correct that it's a gated oscillator circuit. It says THAT in the datasheet too:

Unlike conventional PWM (pulse width modulation) controllers, it uses a simple ON/OFF control to regulate the output voltage.

With the suggested circuit having 220uF of output capacitance, and probably running that gated oscillator "as needed"

A lot of the confusion seems to surround the use and mis-use of the term "constant current" WRT power supplies.
A proper LED driver should a TRUE constant-current supply, holding the current constant by varying the output voltage "quite a bit", in order to accommodate at least variations in Vf of the LED string, and perhaps a relatively wide range of number of LEDs in series. You can see this on chips specifically designed to be LED drivers; for example the Allegro Micro LC5220 series supports Vled of 6 to 90V in its "buck mode" configuration. (Datasheet) Meanwhile the "CCCV" supplies are more "Constant voltage first", with some current limiting capability, and "well defined behavior" as the current increases beyond the limit (unlike, say, a 7805-style regulator.) I don't know whether these chips are designed to be operated in their constant-current "operating region"; it looks more like the CC feature is a protection mechanism. It might take a power-supply engineer to figure it out :frowning:

So... Does a random "LED Driver Module" do anything useful, WRT powering hobby electronics? The fact that they're advertised as "3 x 1W driver" or similar implies a pretty low output voltage range, so they're probably not "true" constant-current drivers. And there's no sign that any two supplies operate similarly, or use the same circuits. You might get lucky, or you might not, and I don't think there is any way to tell without either getting much more detailed specifications than tend to be listed on eBay, or actually buying one and analyzing the circuit and/or behavior.

Given the average level of Power-supply-design expertise of Arduino forum readers ("nil"), I find the suggestion that they might be able to use some random cheap LED-driver module from eBay as a power supply to be ... dangerous. That's probably the reason that tempers in this thread are running a bit hot.

Given the average level of Power-supply-design expertise of Arduino forum readers ("nil"), I find the suggestion that they might be able to use some random cheap LED-driver module from eBay as a power supply to be ... dangerous. That's probably the reason that tempers in this thread are running a bit hot.

Don't misunderstand, I'm not trying to insult you but, BULL! Go back and look at the very first page and the first few posts. With the exception of Lefty, all the posts are insulting and sarcastic. But, I've gotten used to that over the years; it seems there is always a race to be the most deprecating first. If one is really worried about the newbie getting in over their head and blowing something up or getting shoved across the room by mains voltage, just say so. Something like, "This is messing with mains power and can really hurt you, and here's why it may be a bad idea" would probably do the trick.

And you folk that do this kind of thing know full well who you are.

Like I said, I keep coming back here because there are a large number of really helpful folk that actually respond.....eventually.

Go back and look at the very first page and the first few posts. With the exception of Lefty, all the posts are insulting and sarcastic.

I think you get what you deserve and in this case it is dhenery.

What do you expect from such a washy washy question coupled with your arrogance.