Voltage blockers/buckers for LED power supplies?

[randgust]

Like most modelers, I grew up in the 16VAC accessory terminal era, accumulated a lot of accessory 'stuff', and wired the main layout for no fewer than four different power accessory busses.   All were eventually resourced to separate small DC packs for better control over power.

Over time, pretty much all tiny lights are evolved over to LED's.   Nearly all of them come 'out of the box' with a current limiting resistor so that they can still be connected to 12-16v. of something, in my case, DC.  I've still got about 60% incandescent and now 40% LED's, if I add up all the indicators, buildings, streetlights, signals, and miscellaneous other powered stuff like crossing gates.

I've bought a lot of TCS and Richmond Controls micro golden-white LED's, and on my first projects, set up a separate power bus of 3v from a Radio Shack wall wart rather than pay the resistor game on about 20-30 LED's here.    I still put in a small current-limiting resistor as the LED's were just too darn bright, but on the power supply itself rather than every LED.

Recently I discovered a power alternative - inexpensive voltage-blocking 'thingies' off of the auction site, that can adjust voltage (not current) from the 30v. range down to 1.5v.    I got one, meter-tested it, has an adjustable potentiometer so that you can trim the output voltage (and resulting brightness) as you see fit.  http://www.droking.com/dc-to-dc-buck-converter-step-down-voltage-regulator-sheet/ 

The 'thingie' is rated, however, at 3a  passthru ; i.e. it's a voltage, not current, blocker.  I'm putting these back on my original 12v DC 1a. power bus and knocking voltage down to 3v for the same project I was using a wall-wart for.   

So, the reading I've done on these tiny LED's usually indicates that the killer is excess current accidentally shot through the wrong direction (polarity) and you still need protection from a current-limiting resistor..... true??   Or am I basically OK with blocking the voltage only and limiting the input to 1a?    What I love about LED's is that they'll last about forever, or have I managed to invent an LED killer?

Because of the adjusting potentiometer, this rather nicely solves the glitches of LED's that are just too darn bright, and I also don't need to keep adding separate wall-wart circuits - just step-down the voltage by zone to power specific projects.    But I'm still thinking I need to put current limiting resistors in... or maybe not.... you guys are the experts.

[riogrande491]

Yes, an LED current limiting resistor is still needed. Current through an LED increases exponentially with voltage and can vary with temperature. With a 5 Volt supply, a 200 ohm 1/4 watt resistor would limit the current through a single LED to about 10 milliamps (mA.)

Also be aware that many, if not all, of the buck regulators sold on auction sites as "LM2596" contain counterfeit regulator chips. There has been much discussion of this on the internet. Two clues are 1) they oscillate at the wrong frequency (too low); 2) a genuine 2596 part costs more than the entire module, even when purchased in large quantities. Sometimes there is also evidence of a nonstandard part number and logo on the regulator chip.Today Mouser quotes a price of $2.30 each in 5000 quantity.

I personally measured a batch of LM2596 buck regulators I bought at an internet auction site and confirmed with an oscilloscope that it does not meet the manufacturer's data sheet specifications. It oscillated at about 50 KHz whereas the data sheet states 150 KHz. My chips are counterfeit.

These quickly overheat and fail if run at a 3A output current. I still plan to use the units I bought, but keep the output current well below 0.5 A. By running each LED at 10 mA (which is still plenty bright) it would light up to 50 LEDs.

An alternative would be an inexpensive 5V, 1 or 2A "wall wart" intended for powering small electronic circuits.

Sorry for the bad news.

Bob

[randgust]

That's really not bad news.... my power supply only puts out 1a  input anyway.   I'd already read some internet gossip about these overheating under full load, not necessarily a shock (pardon the pun).

That does confirm my suspicion that I should probably have current-limiting resistors in there as well, as I'm running 3.5 volts out (checked with a meter) and have a good stock of smaller-value resistors to experiment with.     I'd already put them in the previous circuit with the wall-wart just to dim the LED's down a bit.

I already have so many various power supplies and wall-warts on this layout the object literally is to thin the herd where I can, and this separate wart to power half a dozen LED's was a logical first shot, as well as put more LED's on it.   I've got two main track power, the capacitor discharge switch transformer, two lighting circuit transformers, test bench power, and wall-warts for the electroluminescent power, two sound systems, grade crossing circuits, LED light circuits.... and it's still growing.   It's all DC cab control, all power switches on the main, and every switch on the main has GH illuminated pushbuttons showing position.  Add a bidirectionally-signalled two track main line.... and the forest of wire that's taken me 30+ years to grow welcomes simplification!

I went back on their website (drok) and there's not much more for information but nobody seems to be griping about them either.

Thanks for the response!


   

[peteski]

As it has been said, LEDs are not light bulbs - they really need current  limiting resistors or a current-regulated power supply.  Powering them directly with the voltage they are rated for (Vf) is not a good way to go.  But I see that done often by modelers who are not familiar with electronics (which is a majority of modelers). I cringe everything I see an article in a modeling magazine showing LEDs hooked up directly to a battery.

In my view a standard 12V power supply is ideal for powering LEDs (with a current limiting resistor added in series of course).  With 12V at your disposal you can hook up 3 white LEDs in series (those will "need" about 8.5-9.5V, depending on the current you will run through them), then you have the remainign 3V or so to have across the current limiting resistor (which is pretty much the ideal way to go about it).

If you Google "how to power LEDs" or "LED resistor calculator" you'll find planyt of good info on how to correctly use LEDs (but you might find some bad examples too).    But as long you remember to always use a series current-limiting resistor, you will be all set, your LEDs will live a long life and you'll be able to fully control their brightness.

[riogrande491]

You're welcome.

My application is to provide daytime and nighttime building and street light power on a friend's huge O-scale layout. Main power comes from a 24V switching supply which runs through buses under the layout. The buck regulators will locally drop that down to about 9V, depending on desired brightness.

On the modules I bought there is a widespread suspicion that they actually use a clone of an older TI chip that runs at 50 KHz and is only good for 1A.

A web search for "2596 fake" leads to a number of independent reviews for these modules. The photo below is one of mine. Yours may be different and may not be a fake.

Bob

[railnerd]

the biggest drawback for many of the "fakes" is that if they get into an overcorrect situation, they will fail in a way that feeds the raw high voltage right into your low-voltage lighting circuit.

For small projects, I've had good luck with these boards from $4.95 boards from Pololu (they'll take up to 36VDC in and regulate to 5V @ 1A)

https://www.pololu.com/product/2843

You'll want to make sure you aren't feeding AC or DCC into the circuit.

-Dave

[peteski]

I really think that using a regulated switching power supply just to drive bunch of LEDs is a needles complication and an overkill (and an additional device which might break).  A simple wall-wart will be quite satisfactory.

As I mentioned a 12V supply is most flexible to me. 5V will work too but you won't be able to hook up the LEDs in series.

If you need to power up about 60 white LEDs at about 5mA for each LED then using 12V wall-wart and 3 LEDs in-series for each branch, they will consume 100mA total. A 12V 500mA wall-wart will power them up pwith planty of reserve current available.

Using a 5V supply (with each LED with its own current limiting resistor) they will consume 300mA.  A 5V 1A wall-wart will again be more than capable of powering those LEDs.

[peteski]

Randy,
I read about your additional struggles with this project in this post in the Yak shaving thread.  That is when I decided that an intervention is called for. 

You are doing all sorts of contortions to avoid using resistors with LEDs. Well, as we told you - resistors are really needed and it really isn't that hard to figure out. What kills me is that you are willing to have all those separate power supplies (voltage blockers?!) to get the job done. That seems really messy, convoluted  and overcomplicated. 

If you can do basic math, you can figure out the LED resistor values. Or use an online LED resistor calculator (there are many of them on the Web).  If I was to power up dozens or hundreds of LEDs on a layout I would use 12V DC power supply. Why? Because 12V is a very common voltage used on model train layouts and in many household or industrial applications.  You don't need a precisely regulated voltage. A plain old wall-wart which provides filtered unregulated 12 Volts works well in this application.

You probably have a 12V DC wall-wart hanging around the house from some old electronic device which stopped working. Or you can buy them very inexpensively. Here are few:
http://www.goldmine-elec-products.com/prodinfo.asp?number=G21027  (this is actually a regulated 12V DC 1.5A supply on sale for $1.50!)
http://www.allelectronics.com/category/914/ac-dc-wall-adapters/pwr-supplies/1.html (there are several 12V DC supplies to choose from).

Here is a sample hookup diagram.  I would run a 12V bus under the layout and power all the LED lights from that bus. Nice and simple!
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As you can see, the circuit is really simple and as an added benefit you should also be able to power all your other devices (like a RR crossing flasher or whatever else you have under your layout). As I mentioned, most of the electronic circuits for model trains can run off of 12V.

That diagram also shows you that you can hook the LEDs as a single, double or triple LED and a resistor circuit.  It really isn't complicated.  To me it is much simpler that what you want to do with all those separate circuits.

I have in the past posted some info here on calculating resistor values in this and this post. It really is simple and nothing to avoid or be afraid of. 

Resistors are literally cheaper than dirt and you can even buy entire resistor kits which contain all the standard resistor values from 1 ohm to million ohms.  But you really don't need all those. Having about 10 values of resistors should be more than enough.  For white LEDs running from 12V supply I would pick up some 220, 330, 470, 560, 680, 820, 1k, 2.2k, 3.3k, 4.7k ohm resistors (1/4 W power rating).  That should pretty much cover 1, 2, or 3 series-connected LED circuits and gave you a range of brightness (current) selection.  Resistors are available in quantities for pennies each from eBay or from electronic suppliers like Digikey or Mouser electronics.

So, with a 12V power supply, bunch of resistors and LEDs you'll be all prepared to light up your layout.  No more yak shaving.  If you need help, the TRW electronic expert crew is here to assist. 

Let's say that you figure out that for the desired brightness you need to pass 5mA of current through your streetlight LED and figure out the resistor value, you can just repeat that circuit for all the other street lights. Or if you want to conserve the current, you can hook up 2 or 3 street lights in series and then figure out the resistor value for that circuit. That way you can have 2 or 3 LEDs illuminated still only consuming 5mA from the 12V supply.

If you have a 1.5A supply you should not load it more than about 75% of its rated current, or about 1.1A. If you were to use all the same single LEDs with resistor consuming 5mA each, you could power 220 of them. If you hooked them up in sets of 2 LEDs per resistor, you could power 440 LEDS or 3 LEDs per resistor then you could power 660 LEDs.  No need for multiple "voltage blockers".

I'm also curious:  I have been involved with electronics for several decades (both as a hobby and professionally as an electronic technician) and I have never heard the term "voltage blocker". When I hear that, a diode comes to mind (since it only allows current to flow in one direction and blocks it in reverse.
What you are describing converts one DC voltage to another (usually lower one). That is a power supply or voltage regulator.  They don't block the voltage, they just reduce or regulate the voltage. If the input and output have to be isolated from each other then those are DC-DC converters or inverters.  But I never seen either type being called a "voltage blocker".

Did you coin that name yourself or heard it somewhere?

[John]

very good advice from Peteski .. I recommend this solution highly

[Greg Elmassian]

I prefer to use the CL2 regulators from Suntek (and others) and then use higher voltage supplies like Petski said.

This way, you get constant current (the CL2 regulator is a CURRENT regulator fixed at 20 ma)...

They operate at up to 90v DC.

So you could have a 24v supply, and run a whole bunch of leds in series with one CL2 in series with the "LED chain"... fewer wires.

And no matter what voltage you feed it, you get 20ma through your circuit. You can parallel LEDs if you want to drop back to 10ma, for example.

This way no regulated power supplies are needed, and also works at widely varying input voltages.

Greg

[peteski]

I prefer to use the CL2 regulators from Suntek (and others) and then use higher voltage supplies like Petski said.

This way, you get constant current (the CL2 regulator is a CURRENT regulator fixed at 20 ma)...

They operate at up to 90v DC.

So you could have a 24v supply, and run a whole bunch of leds in series with one CL2 in series with the "LED chain"... fewer wires.

And no matter what voltage you feed it, you get 20ma through your circuit. You can parallel LEDs if you want to drop back to 10ma, for example.

This way no regulated power supplies are needed, and also works at widely varying input voltages.

Greg

That is a very good (and most efficient) solution when you want 20mA through lots of LEDs. I, in most applications find 20mA of current results in way too much brightness in most of my N scale models. I also run various LEDs in different locations at different brightness.  For example, a single or couple if LEDs illuminating interior of a building will run at 20mA, a street lantern might only need 3mA and a modern street light might need 10mA to look good to me.  Using a 12V bus and current limiting resistors allows me to control the current of individual LEDs (or groups of LEDs).  With resistors costing pennies a piece, the cost is not a problem either.  I could also use a 24V bus but 12V seems to be a voltage most commonly used  in model RR circuits.