CC regulated drivers in series?

[MrNaz]

Can I arrange a string of drivers in series? E.g., Lets say I have 3 separate 1050mA CC drivers that take input in the 3.8V to 4.2V range. Can I string 3 of them in series across a 12V power supply?

Also, is there any reason that using these drivers, which are usually used in torches, is a bad idea to use with mains as a 12VDC supply?

I'm trying to build a flood light, and I have piles of cheap 1050mA flashlight drivers, and I also have a large number of mains to 12V adapters which I scavenged from from MR16 fittings. So I was thinking that I could use them both, provided that there is nothing wrong with what I'm planning to do.

 

[BillyNoMates]

The exact behaviour will depend on the type of driver - linear regulator or switching regulator (ie buck/boost), but I think there will be problems with this configuration.

This is my reasoning.....

The function of the CC regulator is to provide a known current. When circuit elements are connected in series the current MUST be the same ('cos there are no other paths). You need to consider what happens if the CC regulators have slightly different currents (as will be the case in practise). The element with the lower set point will begin regulating before the element with the slightly higher set point. The limiting element will start dropping more voltage as it attempts to limit the current, while the other one will still be trying to minimise its voltage drop because it hasn't reached its set point. There is a good chance that the first device to limit will exceed the its voltage rating and go pop.

 

[nleahcim]

It'd take a very strange driver to be OK with that.

99/100 will not work in that configuration.

 

[jtr1962]

Not going to work. Besides the problem of balancing the voltage each driver receives, those drivers sound like they might be linear as opposed to switching. A linear driver isn't a problem driving an LED if the input is within a volt or so of the LED's Vf. On the other hand, even if it worked on 12V, it would need to dissipate the difference between 12V and the LED's Vf times the LED's current. In this case that would be about 9 watts-quite a bit of heat. There are any number of switching regulators available which drive an LED off 12V. That would be a better solution. Perhaps you can trade some of those other regulators for a proper one in the Marketplace?

 

[MrNaz]

I'm not sure that I've explained myself clearly. I think some people thought that I was going to stack them on top of each other, which I know won't work. Here is the configuration that I have in mind:

If I understand correctly, each driver sees 4V, which is within the 3.6-4.5V range of operation, and outputs accordingly.

If this configuration can't work, can anyone explain to me why?

 

[Al Combs]

I believe someone already did explain why it won't work. Read BillyNoMates post again. But here's a really interesting post that only requires one of the type of regulator you are talking about. Again the notion is current is everywhere the same in a series circuit. So the reason three regulators can't all be trying to regulate the current, is the same reason only one has to try. Download really hit on something brilliant here.

 

[BillyNoMates]

I see what you are attempting, I still think it won't work.

A driver will try to drive a fixed current throught the LED, independent of the Vf of the LED and the applied voltage. The current through the + and - supply terminals will be the sum of the current driven throught the LED plus the current used by the driver. By connecting the drivers in series, you are forcing the currents through all three to be the same (kirchoffs current law), but what happens if the drivers are different? I think that the driver with the lowest set point will regulate first, and by its function will take the lions share of the voltage. The others will not have reached their limit and will consequently try to reduce their 'effective impedance' (in order to try to reach their set-point).

I don't think it will work, but that shouldn't stop you trying if you really want to....I'd be just as interested to find out for sure.

 

[MrNaz]

I believe someone already did explain why it won't work. Read BillyNoMates post again. But here's a really interesting post that only requires one of the type of regulator you are talking about. Again the notion is current is everywhere the same in a series circuit. So the reason three regulators can't all be trying to regulate the current, is the same reason only one has to try. Download really hit on something brilliant here.

OMG that's bloody brilliant! :twothumbs

 

[BillyNoMates]

I believe someone already did explain why it won't work. Read BillyNoMates post again. But here's a really interesting post that only requires one of the type of regulator you are talking about. Again the notion is current is everywhere the same in a series circuit. So the reason three regulators can't all be trying to regulate the current, is the same reason only one has to try. Download really hit on something brilliant here.

Just read the post in the link.

One driver, three LEDs - very clever. It seems plenty of people have used this to good effect.

 

[Al Combs]

I had to laugh to myself the first time I read his post. It was a very clever idea. The only problem I can foresee is the AMC7135 linear regulators Download used were of a specific type that can take up to 6 volts. Being linear they generate a lot of heat at 6 volts, but they do work.

Say the Vf of your LED's is 3.7 volts. No way to know for sure other than a power supply and a DMM with at least 1 amp current scale. Were you going to use a 12 volt SLA? An automatic charging circuit would raise that some.

Anyway the two unregulated LED's take their 7.4 volts (roughly) and leave 4.6 volts for the one regulated LED. Whatever buck regulator you are using needs to be able to handle at least 4.6 volts. Or more if you have a charging circuit attached to your SLA.

 

[LukeA]

I had to laugh to myself the first time I read his post. It was a very clever idea. The only problem I can foresee is the AMC7135 linear regulators Download used were of a specific type that can take up to 6 volts. Being linear they generate a lot of heat at 6 volts, but they do work.

There's only one type of AMC7135.

In download's configuration the AMC7135 chips only dissipate the difference between the sum of the LED forward voltages and the supply voltage multiplied by the current.

 

[Al Combs]

My bad... I just read your first post again where you said you were using a wall-wort. So the only other issue besides voltage limit of the buck regulator is can the wart deliver ≈ 10 watts output power.

BTW, if you're not sure which type of regulator you have, generally the switching regulators are the ones with inductors on them to smooth the waveform. Much more efficient than linear when you go outside a specific voltage range. That way heat won't be an issue.

If you have a bunch of regulators just go ahead and try it. If it blows, it will more likely just open and the LED's should be OK. You'll only have lost one regulator.

 

[MrNaz]

I just had to try this idea immediately. It was too clever to leave for tomorrow. Here's my first try, I got a single AMC7135 board regulating current at 350mA to three spare Crees I had lying here.

Here it is, and yes, it works. I tried taking a photo of it with the LEDs on, but even at 350mA, those Crees burned out the photo every time.

This has to be one of the best ideas I've ever seen, and one of the most useful. The basic concept can be applied to just about any multi-emitter array, and not limited to this three emitter configuration. It could be used for any number of LEDs from two on up.

It's such a "why didn't I think of that" idea. Every time I planned a resistor into the circuit to step V down I kept thinking about the wasted watts. I don't know why I didn't think to use LEDs to step the V down given that the current is already guaranteed.

Any idea who actually came up with it? He or she needs a medal!

 

[Al Combs]

Yes but MrNaz didn't say at first he was using an AMC7135. He said he was using a regulator with a 3.8 to 4.2 volt range. And that might be a problem. His regulator needs to be able to handle 4.6 volts.

There's only one type of AMC7135.

 

[Al Combs]

AFAIK Download came up with the idea. You should drop him a PM to say thanks. Or maybe do it in his thread to bump it up to the top of the list. I'm sure he'd appreciate it.

Any idea who actually came up with it? He or she needs a medal!

 

[MrNaz]

Yes but MrNaz didn't say at first he was using an AMC7135. He said he was using a regulator with a 3.8 to 4.2 volt range. And that might be a problem. His regulator needs to be able to handle 4.6 volts.

Please excuse me. I don't know the terms "regulator" and "buck driver" and so I use them incorrectly when referring to circuits. Would you mind elaborating on what the AMC7135 is, what a buck driver is, what a regulator is, and what the difference is between them all.
Thanks!

 

[Al Combs]

No problem. I'm not an expert on the subject but I'll be glad to help with what I know. A regulator controls either current or voltage. Many constant current regulators are simply voltage regulators that monitor voltage drop across a "sense" resistor. The idea is many devices (like LED's) don't behave in a linear fashion as far as voltage to current ratios. Resistors are for the most part linear. AMC7135 is a linear regulating chip that will allow 350ma of current to pass. Any extra voltage through a Linear regulator has to be dissipated as heat. That can be a problem when you have a lot more voltage than you need. They put three of them in parallel on the board to give 1050ma current limit. By contrast a switching regulator turns itself on and off at very high speed (500khz or so). It monitors the current rise and just shuts off when there is too much current. A combination of ceramic capacitors and inductors smooth the waveform to give filtered DC. Buck means it will stop the flow of extra voltage that would ordinarily cause current to rise. The other type of regulator is Boost. If you have a 3.7 volt cree running on a 3 volt battery, a constant current boost regulator will raise the voltage until a preset current level is reached. Also there is Buck/Boost which has some of the characteristics of both. You find there in many single cell CR123A lights like Fenix the P2D. That was the type I thought you might have. Buck/Boost typically can't handle too much over voltage. I've run 4.2 volt RCR123A's in my Fenix P2D even though it's not recommended. A P2D regulator in your 3 LED's on 12 volts would blow. You'll be fine with the 1050ma AMC7135. Maybe pot it onto something metal with something like Arctic Alumina epoxy so heat won't be a problem.

Please excuse me. I don't know the terms "regulator" and "buck driver" and so I use them incorrectly when referring to circuits. Would you mind elaborating on what the AMC7135 is, what a buck driver is, what a regulator is, and what the difference is between them all.
Thanks!

 

[Al Combs]

Here is the pdf on the AMC7135 if you want to look at it. I was looking before and couldn't find it. My bookmarks are a little out of control. The AMC7135 is a Linear Buck regulator. One of the important characteristics of this chip is it's of the Low Dropout Voltage type. You see the term LDO every now and then. If you look in the pdf, the AMC7135 only has a 120mv drop. Roughly speaking only 1/8th of a volt is lost in the chip. That is very low. If you have a voltage source close to what you need, this regulator is very efficient.

Say your Cree needs 3.7 volts. You only need a 3.82 volt source to give full power to the LED. But the chip can handle up to 6 volts, albeit while generating lots of heat. A Cree and a single Li-Ion is perfect for this regulator. There is very little extra voltage so not a lot of heat for the chip to dissipate. And it works down to 2.7 volts. I'm not sure if that means it just shuts off. Which of course would be perfect to protect the battery.

The downside of Buck drivers is below 3.82 volts the regulator goes into what you sometimes hear called direct drive. Not the kind like a 3D MagLite with a P7 and just wires. But since it makes no attempt to boost voltage, the output tapers off gradually. My Buck/Boost Fenix P2D by contrast draws 735ma from a freshly charged AW RCR123A Li-Ion. I have an L2D battery tube and the same head draws ≈ 1,400ma from 2 NiMH AA's. I have no idea what the voltage drop on these batteries was. But it's less efficient on the AA's, even though it's just as bright. A buck regulator doesn't behave like that.