Constant current power supplies

That yellow thing is the transformer that's normally used on a switching power supply, not a step down transformer. And, as for price, I already told you the deal I got on two of them by just searching around a little bit. The example I cited was simply the first one I came across on ebay; there's tons of them out there in many, many configurations and prices. Once again, these things are constant current supplies and would require a voltage regulator to keep the voltage correct for a processor circuit; maybe an extra 50 cents or so if the board doesn't already have one. The ones I have supply a constant 350ma until the load resistance drops enough to cause it to shut down. On the high side, they max out at a little over 11V and don't go any higher. Carefully hooking it to the VIN on an arduino (UNO), they seem to work fine. The problems may come when the combination of arduino and other devices need more than the 350ma and the thing shuts down. But I won't trust it until I have much more experience with it.

It's obvious that you only want to argue rather than look into possibilities for cheaper smaller devices, so get the wall wart and be happy with it; I have dozens of them and they work fine if you want that kind of thing. I also gut them and use the power supply inside directly for some projects.

However, if one wants a self contained device that just plugs into the wall and works in the smallest possible size, there may well be better solutions. That's what I'm exploring.

I don't see the point.

  1. Arduino is designed to be connected to a constant voltage power supply.
  2. Constant voltage power supplies are equally common and cheap.

Essentially, you're buying "wall wart guts", but not the type that Arduino was designed for...

You can add that a supply designed for LED lighting may not provide isolation from the house power wiring, making it suitable for self-contained lamps of various sorts, but NOT for an experimental device like an Arduino with exposed wiring that you poke your fingers at.

Also, cheap Chinese import power supplies (of all types) are getting significant publicity recently for being poorly designed, lacking safety certifications (and in fact having poor safety design aspects that would make it impossible to get safety certifications), and having very questionable performance (poor regulation, high ripple, etc) For example Crappy PSU analysis - YouTube

westfw, that's the kind of response I was looking for. Good point about the isolation, I'll check that in the next few days, but I suspect you're right in that there may well be a potential difference that sets the Arduino above ground and could cause problems. Regarding Chinese manufactured power supplies being crappy, I've see good ones and bad ones. However, I haven't seen wall warts or small power supplies that were made anywhere else.

Every cotton picking one I have in the house was made in China, even the ones that are 10-15 years old and are stored in a box in the garage. The one that powers my Japanese alarm clock was made in China??? So, it appears that we're stuck with Chinese devices until we start building our own in the garage.

The point I'm trying to get to is that I'm tired of the power supply for the device I'm building being twice the size of the device itself and having a big lump that plugs into the wall. Things got a little better when Apple came out with their small USB supplies (also made in China), but it's still annoying.

draythomp:
The ones I have supply a constant 350ma until the load resistance drops enough to cause it to shut down. On the high side, they max out at a little over 11V and don't go any higher. Carefully hooking it to the VIN on an arduino (UNO), they seem to work fine. The problems may come when the combination of arduino and other devices need more than the 350ma and the thing shuts down.

But isn't that an important issue? These LED drivers are sensitive to the load they're driving; too little load and they shut off, too much load and they shut off. Only under fairly specific circumstances would you have a circuit that could keep that constant load.

I haven't seen wall warts or small power supplies that were made anywhere else.

Good point. The danger sign is probably "cheaper than it seems possible", rather than "made in China."
Although "shipped direct to you from China, bypassing regulatory approval processes" is probably also a bad sign.
The Apple USB-supply cube is impressive engineering (much smaller than the LED supplies you linked), now widely (and sadly) "faked" with a much less worthy/safe circuit. Much of the recent publicy has surrounded fake Apple chargers. EEVblog #388 - Fake Apple USB Charger Teardown - YouTube
http://www.arcfn.com/2012/03/inside-cheap-phone-charger-and-why-you.html
(Real Apple or Amazon chargers aren't "inexpensive.")

draythomp:
Has anyone else noticed that there are a ton of constant current power supplies out there for extremely low prices that are very small and quite powerful? I've got a couple I'm experimenting with that supply 6-11 volts at 350ma. I take it that that means they go as low as 6V and as high as 11V trying to supply 350ma to a fancy led or string of leds.

Well, why can't we hook one of those up to an arduino and have it run just fine. There's already a regulator on the arduino that will take the incoming voltage and hold it at 5V for the board, so it shouldn't matter that the CCPS (constant current power supply, I just made that up) is varying its voltage as the current load changes.

Right??? Thoughts.

A constant current source (theoretically) would supply infinite voltage if it were open circuited. Of course, real-world current sources are limited by their design. The maximum open circuit voltage is called the "compliance" voltage.

If you tried to power a voltage regulator (like the input to an Arduino board) with a constant current source, one of two things would happen:

(1) if the Arduino needed more current that the CC source supplied, the voltage would be too low and the Arduino would not even boot up.

(2) If the current source was set to a higher current that the Arduino required, the current source would swing right up to it's maximum voltage (the compliance voltage).

Since both 1 and 2 are worthless :slight_smile: You see that you don't want to power an Arduino (or most anything else) with a constant current source.

What they ARE good for, however, it powering LED's and LASER DIODES. These devices have a nominal forward voltage drop, but it varies from part to part and varies with temperature. You NEVER want to power an LED or laser diode with a constant VOLTAGE source.

A constant current source is like an "electronic spring". It will flex - give and take a little bit to accommodate the LED or laser. The nominal voltage drop across the LED or laser, times the constant current equals the input power to the device. As the device warms up, it's forward drop will change slightly, and the current source will simply adjust itself and keep providing the correct current to the device.

Without the electronic "spring" to take out fluctuations in operating parameters, the device could work fine one minute and burn out the next.

That's why, in simpler setups, you always use a resistor in series with an LED... to limit the current. The resistor acts like an electronic "spring".

Did all this make sense?

The little transformers provide ("are supposed to provide") as much isolation as the traditional large "step down transformer." They can be smaller because they operate at a much higher frequency. You can think of a switch-mode power supply as modulating the high-voltage side of the supply so that the frequency and waveform are varied in a way that causes them to provide exactly the right amount of power needed by the load.

westfw:
The little transformers provide ("are supposed to provide") as much isolation as the traditional large "step down transformer." They can be smaller because they operate at a much higher frequency. You can think of a switch-mode power supply as modulating the high-voltage side of the supply so that the frequency and waveform are varied in a way that causes them to provide exactly the right amount of power needed by the load.

So George Westinghouse was right after all? :slight_smile:

I read this stuff and wounder is this for real. Constant current supply's Are mainly used for driving Led's or led displays, Now there no way anyone can tell the OP any thing about his supply that's not a guess. Now if he would run down the chip used on this we would have a place to start. I use a constant current led driver you set the dang thing to 25 mA which is changeable in code to a point meaning you can change the range a little up or down to hit the 25 on the head.

Let's see a show of hands how many of you have used cel phone charger even like the iphone charger or any of the newer smart phone chargers. Well how may hands are up in the air Guess what there constant current supply that do to things one
there output is fixed at 5 volts till you over load them then it drops off to a point that they shut down.

So the bottom line is if this doesn't put out more then the happy 7 to 12 volts it would work fine as long as it doesn't need to be loaded.

But you may have to add filtering and a regulator to keep the power clean and under the happy zone of 12 volts to 7.

Let's see a show of hands how many of you have used cel phone charger even like the iphone charger or any of the newer smart phone chargers. Well how may hands are up in the air Guess what there constant current supply that do to things one
there output is fixed at 5 volts till you over load them then it drops off to a point that they shut down.

Not sure what you are trying to say here exactly. However as far as cell phone charger modules most are indeed just fixed DC voltage regulators, typically rated at 5.1 or 5.2 vdc output and at some maximum current rating. The chip that handles how much constant current to allow to charge the phones Li battery and when to shutdown charging current when the cell is fully charged is inside the phone itself. I find these power modules at second hand stores for usually $1-2 and they work fine for powering small digital projects drawing an amp or less.

Lefty

I believe that the root of the question is essentially: "look! Cheap switchmode power supplies being produced in the millions for the LED lighting market! While they're supposed to be "constant current", don't they have a practical limit on output voltage that falls in the range needed at the input of an Arduino?"

Answer: maybe. But the cheap switchmode power supplies being produced by the millions for the cellphone/etc market are more appropriate and not that different in price. (although, the price goals of the supply for a $10 lightbulb are probably lower than the goals for the supply for a $100 cellphone/videogame/whatever.)

Cel phone charger are a constant current supply fixed at 5 volts. That's the newer one's like the iphone use's and most smart phones use

What part don't you understand I'm talking about the charger that plugs in the wall
not how it disconnects the battery from it.

be80be:
Cel phone charger are a constant current supply fixed at 5 volts. That's the newer one's like the iphone use's and most smart phones use

What part don't you understand I'm talking about the charger that plugs in the wall
not how it disconnects the battery from it.

No, you are giving these cell phone charger modules more functionality then they actually have. I've used many in my projects, they put out a constant regulated +5vdc voltage only, and not at a constant current. I have wired one up to power my arduino board via It's 5V and ground shield pins and the board draws about 80ma and the voltage is fixed at 5.1vdc. The constant current part is performed by a chip inside the cell phone that handles the chore of properly charging the battery at a proper constant current, using the regulated voltage supplied by the external cell phone charger module.

Again a typical 'cell phone charger module' is simply a switch mode DC voltage regulator outputting a regulated nominal +5vdc (most are rated at 5.1 or 5.2vdc) and can supply any current demand from 0 to typically 1 amp.

Lefty

I not giving them any thing there I just tested about 100 of them as to there rating
and did a lot of digging on the net

TND329
5 W Cellular Phone CCCV
(Constant Current Constant
Voltage) AC-DC Adapter

better charger like the smart phones and iphone use these in them and there
Not this same chip but one like it.

This reference document describes a built-and-tested,
GreenPoint solution for a cellular phone Constant Current
Constant Voltage (CCCV) AC-DC adapter. This design is
intended for isolated, low power, universal input off-line
applications where a constant current/constant voltage
output (CCCV) is required for charging NiCd, NiMH,
Lithium-ion or similar batteries. Typical applications would
include cell phone chargers or cordless phone chargers.

Back to the Op you may be able to use the supply you posted if like I said they fit the happy spot of 7 to 12 volts but may need to add a regulator and filtering

See I have a bunch of these old ones and new ones and got to wondering why some of the old one where no way near there rated voltage and the newer one where dead on 5 volts and I got my kids two new phones so we had 4 of the same
chargers that open the door to cut one open and have a look and as I figured
they have CCCV
So draythomp test it for output voltage and see what it is you may be able to use
the E-Bay supply but I have to say one thing if the output is 350mA and it's 2 watts it's only good for about 5.7 volts at that load

be80be:
I not giving them any thing there I just tested about 100 of them as to there rating
and did a lot of digging on the net

TND329
5 W Cellular Phone CCCV
(Constant Current Constant
Voltage) AC-DC Adapter

better charger like the smart phones and iphone use these in them and there
Not this same chip but one like it.

Well I guess we will just continue talking past each other. I will state that from an ohms law point of view that a 'constant current constant voltage output 'module' could only work with single fixed value resistance load, and thus would be of very limited usefulness. You can't have both a constant current and a constant voltage at the same time with a variable resistance load.

A DC power supply module either changes it's output voltage to maintain a constant current when presented with variable resistance loads, or it maintains a constant voltage by supplying more or less current to match a variable resistance load, it can't do both simultaneously except at one 'perfect' load resistance value. I've used these modules with different value loads, the current didn't stay constant but the voltage did, hence they are indeed simply fixed output voltage regulators.

If you study the datasheet for the TND329 device you talked about, you will find that it is in fact a regulated constant voltage regulator when operating at any current demand from 0ma to 1 amp. If however the load attempts to draw more then 1 amp (load resistance decreases below 5.1 ohms) the module enters a current limiting mode where it will lower it's output voltage to maintain the maximum current drawn by the load to a constant 1 amp value. So that one acts like a simple fixed 5.1vdc output voltage regulator up to it's maximum rated output current, above that value it enters a fixed current limiting mode by lowering it's output voltage to maintain 1 amp. So no ohms laws broken and does never at any time maintain a constant current and a constant voltage at the same time, the load resistance always has a say in the matter.

Lefty

lefty I'm not trying to get you mad or any thing but your missing a part here

Take a Iphone charger there made to do to things one is supply 5 volts .
Now if the battery is dead you want a fast charge these supply's can dump a amp on the battery and still keep the voltage at 5 volts and charge current at there rating but at no load they just keep the voltage at 5 volts they use CCCV

I'm pretty sure that a constant current switchmode power supply (LED Driver) and a constant voltage switchmode power supply (cell phones) are awfully similar. One would take feedback from the load voltage, and the other would take feedback from current sense resistor om series with the load. Any switchmode chip ought to be able to implement either type of supply with minor changes in wiring.

In "Lab" power supplies, "CCCV" generally means "constant voltage with a current LIMIT", and defines the way the supply behaves when the current limit is exceeded. This is supported by the TND329 datasheet:

(section 5: "Circuit operation")
For output currents less than 1 A the circuit performs as a constant voltage source. ... Although very simple, this current sense circuit will provide a constant current output of approximately 1 A all the way down to an output voltage of 1 V.

In other words, a "CCCV supply" provides a constant voltage up to the current limit, and then decreases the voltage as needed not to exceed the desired current. (as opposed to simpler regulators, that might just shut down in over-current situations.) Some Li-ion battery charging chips assume that the power supply has a current limit built-in to it, but a cell phone supply will normally operate in a constant voltage mode.

In general, that's not quite the behavior you'd want from an LED driver style of CC supply. In that case, you want the voltage to go to any value needed to force the target current through the load. This permits you to use varying numbers of LEDs in series in your light, for instance. I haven't studied the current flood of "LED driver" chips to see how they're different from other switchmode chips; there may be other simplifying factors that permit a cheaper design and justify having a separate chip (oh, like not needing to provide clean DC, for instance.)

westfw:
I'm pretty sure that a constant current switchmode power supply (LED Driver) and a constant voltage switchmode power supply (cell phones) are awfully similar. One would take feedback from the load voltage, and the other would take feedback from current sense resistor om series with the load. Any switchmode chip ought to be able to implement either type of supply with minor changes in wiring.

In "Lab" power supplies, "CCCV" generally means "constant voltage with a current LIMIT", and defines the way the supply behaves when the current limit is exceeded. This is supported by the TND329 datasheet:

(section 5: "Circuit operation")
For output currents less than 1 A the circuit performs as a constant voltage source. ... Although very simple, this current sense circuit will provide a constant current output of approximately 1 A all the way down to an output voltage of 1 V.

In other words, a "CCCV supply" provides a constant voltage up to the current limit, and then decreases the voltage as needed not to exceed the desired current. (as opposed to simpler regulators, that might just shut down in over-current situations.) Some Li-ion battery charging chips assume that the power supply has a current limit built-in to it, but a cell phone supply will normally operate in a constant voltage mode.

In general, that's not quite the behavior you'd want from an LED driver style of CC supply. In that case, you want the voltage to go to any value needed to force the target current through the load. This permits you to use varying numbers of LEDs in series in your light, for instance. I haven't studied the current flood of "LED driver" chips to see how they're different from other switchmode chips; there may be other simplifying factors that permit a cheaper design and justify having a separate chip (oh, like not needing to provide clean DC, for instance.)

Yes, you have explained it all very well and better then me I' sure. There is a difference between a DC power module that has automatic current limiting protection and a true constant current regulator. I'm just trying to make sure that people understand that ohms law is still applicable here no matter what name a DC power module uses. :wink:

Lefty

be80be:
Let's see a show of hands how many of you have used cel phone charger even like the iphone charger......

Let's see a show of hands how many of you built your first computer in the 1970's with an 8008, 256 bytes of static ram and toggle switches for address and data input.

Let's see a show of hands how many of you know the difference between a triode and a pentode (and have actually built circuits using both).

Lots of people here have forgotten more about electronics than you know. Don't be smug.

(rant off).

Me on all counts, although it was 2650 processor and it was 1976.

Cel phone charger are a constant current supply fixed at 5 volts

That sentence is so wrong unless the charger has the ability to change the impedance of its load. But as others have pointed out this is just sloppy talk.