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Author Topic: Constant current power supplies  (Read 6426 times)
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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?
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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.
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Left Coast, CA (USA)
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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-grin

Lefty
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Yep, I ain't never getting no darn tattoo on my face.
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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.
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New River, Arizona
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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>
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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.

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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.")
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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

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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 (31.19 KB, 415x213 - viewed 26 times.)
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New River, Arizona
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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: http://www.polycase.com/gs-2415 .  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.
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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.
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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.
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@ *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
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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.
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It's a green thing.

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