Author Topic: Make my Own Variable Constant Current LED Driver?  (Read 3668 times)

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Offline The_Almighty_Bacon_Lord

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Make my Own Variable Constant Current LED Driver?
« on: May 08, 2016, 02:33:19 am »
Hey all! So I'm planning on making a bicycle light with a 10W LED (making my own shitty lenses because I refuse to pay $4 for one on Ebay), a small voltage boost converter, with 2 18650 cells in series with this protection board: http://www.ebay.ca/itm/Dual-MOS-Polymer-Lithium-Battery-Protection-Board-for-18650-/351712441610?var=&hash=item51e3b1f50a:m:mIpBJH68Nf6Dr2-zB6nYOnQ

My problem: I really don't want to have a fan + heatsink to cool my LED (because it'll most likely break on a bike after a while), so I want the LED to be passively cooled, however, whenever I try to run my LED at 10V 1A, that shit goes crazy and starts to pull over 1.5A and just keeps getting hotter and hotter (thermal runaway). If I add more voltage, shit gets more hectic, but I stop it before it kills the LED.

I want to do one of three things to solve this issue:
1) Make my own constant current device (plan to use a POT connected to a DC-DC Boost Converter to control the amount of volts going to the LED to change brightness, whether that affects the constant current device or not, I have no clue, just putting it in to possibly help out).
2) Buy something online which can do constant current. If it can do constant current + change the voltage via a POT, EVEN BETTER, since I haven't ordered a boost converter yet.
3) GIVE ME YOUR IDEA AS TO WHAT I SHOULD DO! :D

Here's a basic drawing as to how I plan to do have it on my bike:
(may need to zoom in to read)

P.S Yes, it'll make everything water RESISTANT with hot glue and other types of glues that I have laying around.
 

Offline The_Almighty_Bacon_Lord

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Re: Make my Own Variable Constant Current LED Driver?
« Reply #1 on: May 08, 2016, 02:44:32 am »
If I had something like this: http://www.ebay.ca/itm/5A-Constant-Current-Voltage-LED-Driver-Battery-Charging-Module-Voltmeter-Ammeter-/400830390526?hash=item5d535a60fe:g:2KMAAOSwPcVVuaIa
As to where I can control the voltage, and amps, that would be amazing. Having it display voltage is just an added bonus. However, this one is too expensive to me.

Just letting the helpers know to give them an idea as to what I want.
 

Offline The_Almighty_Bacon_Lord

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Re: Make my Own Variable Constant Current LED Driver?
« Reply #2 on: May 08, 2016, 03:21:26 am »
Is it possible to make a constant current boost converter with an XL6005 IC? The thing supports 4A which is good enough for my needs?
 

Offline Marco

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Re: Make my Own Variable Constant Current LED Driver?
« Reply #3 on: May 08, 2016, 03:40:30 am »
You almost always drive a LED with semi fixed current, either with a constant current driver or through a current limiting resistor. Trying to use the LED's own resistance is not recommended (COBs do it any way of course between strings, exception, not the rule). You have a 3x3 COB LED module I assume since you are trying to feed it 10V? Single chip LEDs (ie. 3V range) more common for this kind of thing, most of the ready made flash light modules seem designed for it.

I think this will work.

The XL6005 can work.
« Last Edit: May 08, 2016, 04:00:10 am by Marco »
 

Offline The_Almighty_Bacon_Lord

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Re: Make my Own Variable Constant Current LED Driver?
« Reply #4 on: May 08, 2016, 08:54:03 pm »
I used my LED on it's own resistance for a few tests, which is how I found out how many amps it pulls, as well as when I keep it cool. When I keep the LED sufficeintly cooled, it'll stay at a constant amperage without increasing.

I am trying to feed 10-11V straight into the LED.

I took at look at the module you linked, and it is quite expensive, a bit too expensive for what I want to do with it (and possibly accidently break). I'm wondering if there are others on the market which do the same thing, for cheaper, or how I could make my own, possibly using the XL6005? I would have no clue how to make my own.
 

Offline Marco

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Re: Make my Own Variable Constant Current LED Driver?
« Reply #5 on: May 08, 2016, 10:59:37 pm »
Here's a cheaper one. Interesting circuit by the way, the LM2577 doesn't have a CC mode and since the feedback voltage threshold is too high to directly use with a shunt there's an extra opamp on there to amplify the shunt voltage (and to combine it with the constant voltage mode). If you really wanted to make something yourself this technique can extend the number of boost converters you can use. Also it's a SEPIC converter, but that's less relevant.

Here's someone making a high power boost converter on perf board (constant voltage, but it only differs from constant current because it feedbacks the voltage from a divider on the output rather than a shunt).

« Last Edit: May 08, 2016, 11:09:06 pm by Marco »
 

Offline The_Almighty_Bacon_Lord

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Re: Make my Own Variable Constant Current LED Driver?
« Reply #6 on: May 09, 2016, 12:34:13 am »
If I replaced the LM2577 with, for example, an LM2587, would I be able to achieve a higher amperage, as well as a higher output voltage from this circuit, or would those 2 be limited by the design of this circuit?

IDK What a SEPIC converter is, and have no clue what any of this means: constant voltage, but it only differs from constant current because it feedbacks the voltage from a divider on the output rather than a shunt

I'm quite a noob regarding electronics and circuits. I only know the very basics, and learned it all from self teach, so I don't know anything about calculating circuits and stuff.

Regarding the video, i'm pretty sure that doesn't deliver constant current because the voltage changes, and if it does deliver constant current, how would I make it so it always delivers a max of 1a, no matter the voltage?
 

Offline Marco

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Re: Make my Own Variable Constant Current LED Driver?
« Reply #7 on: May 09, 2016, 02:47:08 am »
constant voltage, but it only differs from constant current because it feedbacks the voltage from a divider on the output rather than a shunt

Look at the datasheet of the XL6005 at the typical application circuit, notice the how there is a resistor below the LED and the voltage across it goes back to FB. That's a shunt, a resistor to measure a current by converting it to a voltage. The XL6005 compares that voltage to a threshold voltage (0.22V) and if it's too low it starts pulsing the internal MOSFET, which raises the output voltage, which raises the current through the diode and shunt, which causes the the XL6005 to use shorter or no pulses.

Now look at the datasheet of a constant voltage boost converter, lets say the XL6009. Instead of the shunt there is now a resistor divider across the output, which leads to FB.

The major difference between XL6005 and XL6009 is the threshold voltage, because the XL6005 is designed to develop that voltage across a shunt which sees a large current it requires a low threshold voltage, 0.22V. The XL6009 uses 1.25V because it gets feedback from the large resistor divider.

Quote
Regarding the video, i'm pretty sure that doesn't deliver constant current because the voltage changes, and if it does deliver constant current, how would I make it so it always delivers a max of 1a, no matter the voltage?

It was more to give you a general idea on how to construct a circuit  around XL6005 if you want. These would be the changes you would need to make relative to the circuit he made :

- no need for the compensation network, the XL6005 has it build in.
- no need for the feedback divider, instead you need a shunt as explained above of 0.22 Ohm (0.22V/1A) ... making it adjustable would be slightly awkward, potentiometers at such low resistance get expensive.
- inductor is a little too small, needs to be around 22 uH (and needs to able to stand a DC current of around 1.5A)

PS. the supplies I linked earlier might actually have a problem ... the ebay page warns the constant current limiting stops working below Vin = 7V, I assume it's because of the opamp/comparator they used.
« Last Edit: May 09, 2016, 02:58:29 am by Marco »
 
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Offline The_Almighty_Bacon_Lord

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Re: Make my Own Variable Constant Current LED Driver?
« Reply #8 on: May 09, 2016, 08:56:30 pm »
That was a pretty damn good explanation of a shunt and what it does. I understood very little of it because I am still a noob and I barely know how to use a resistor (don't even ask me if I know how a capacitor or inductor or anything else works...), but it was quite interesting.

" no need for the compensation network, the XL6005 has it build in." No clue what a compensation network is. When I said I was a noob previously, I meant to say huge noob. I know almost nothing regarding circuits and what the do.

TBH, I mainly came here for someone to link a circuit diagram of a constant current/current limiting device, so then I could either make it myself on bread board, or for someone to link me to a cheap current limiting module (which you have done, Marco).

Would you know how to make a device which delivers constant current/limit current, as well as change the value of the limited current/limited voltage (using a POT), or is that out of the scope of your knowledge?
 

Offline The Soulman

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Re: Make my Own Variable Constant Current LED Driver?
« Reply #9 on: May 09, 2016, 11:01:35 pm »
Not the most efficient solution but a cheap one, a lm317 and a power resistor.
It could need cooling but google around on lm317 and you will find plenty of examples and circuits.
 


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