I'm working on a project using a 100w COB LED and want to do something like what DIY Perks does in this video of a DIY 100w flashlight. He gets into the circuit around the 4 minute mark. However, he doesn't really go into details on the hardware he is using. He calls it an Adjustable Resistance Divider. (See attached screen capture)
Basically, I want to be able to use a knob (potentiometer) to adjust the brightness without having to use a screwdriver. The power will come from a 19v 7A laptop supply running into a DC-DC boost converter. The converter has both Constant Current and Constant Voltage trim pots. The LED needs 3.1A max but I will probably look to set the upper limit to 2.7A or 2.8A.
Will using something like this "Adjustable Resistance Divider" accomplish my goals?
Goals:
-Use a knob for easy adjustment
-Have current limited to 2.7-2.8 amps (knob can't raise current past that level)
-be able to dim to an "off like" state (or at least close to it)
I'm sure there is something I'm leaving out.
If you have any thoughts or suggestions, I'm all ears.
PS -- I just watched through that part of the video again and realized he is using a Constant Voltage source. So, can this be done adjusting the current? Or does it need to be a voltage adjustment?
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The proper term for an "adjustable resistance divider" is potentiometer, and they come in a few different shapes and sizes. The small blue ones are commonly called trimpots. They are designed to be small and precise, and require a large number of turns (around 15) to move the wiper from one side to the other and they require a small screwdriver to turn. Because of their precision and the inconvenience of adjusting them, they are commonly used to adjust parameters that do not change often, such as calibrations. The inconvenience can be a feature; you use them for things you don't want to be easily changed.
The larger potentiometers that are meant to be used as part of a human interface will have a shaft where you can mount a knob handle, and will require less than a full rotation to take the wiper from one end to the other. These are best for coarse adjustments that need to be made very frequently, like volume on a stereo (or brightness on a flashlight).
In the DIYPerks video, the trimpots are used to set maximum and minimum limits for the regulator's adjustment. This is a good use for them, because you usually don't want those things to change easily. If you pay attention to the video he also mounts a potentiometer with a knob on the handle that he uses as his brightness adjustment, which adjusts the actual output of the flashlight within the limits allowed by the trimpot adjustments.
@Jiggy-Ninja Thanks for the info.
The LED needs 3.1A max but I will probably look to set the upper limit to 2.7A or 2.8A.
For that sort of current you need a wire wound rheostat.
rheostat
Anyway it is a crap way of controlling a high power LED because the forward voltage changes with temperature and component age and so you will need to be constantly changing it to prevent you from over currenting it.
Grumpy_Mike:
For that sort of current you need a wire wound rheostat.
rheostat
Anyway it is a crap way of controlling a high power LED because the forward voltage changes with temperature and component age and so you will need to be constantly changing it to prevent you from over currenting it.
Thank you for your thoughts.
Don't take this as "arguing". I'm just, shall I say, a bit confused. I know I'm not using a Meanwell LED driver. And maybe it's comparing apples to airplanes. But I don't believe Meanwell drivers (which I've never had open) use rheostats. Nor do any of the thousands upon thousands of boost and/or buck converters available.
The output of the boost converter I have is rated at 400w. Well beyond the 90, or so, I'll be setting it at. And it uses a 3296 trim pot for the voltage and current alike (Similar to this Bourns. But Bochen brand).
I've hooked this up and ran it for an hour or so each time and have not seen issues with heat. But I have a good fan and heatsink taking care of that. As for age affecting it, I have no idea how these components will react to time.
Back to the rheostat, is the one you linked what you are suggesting to be used? Or just an example? Reason for asking is that the one you linked to is only rated to 50w.
OK my mistake I missed the fact that they were trim pots on a converter. I thought you were only using a resistor to limit the current. Sorry.
Grumpy_Mike:
OK my mistake I missed the fact that they were trim pots on a converter. I thought you were only using a resistor to limit the current. Sorry.
I've been there. It's all good!
I guess the part that I'm having the hardest time wrapping my head around is calculating the resistor values. The trim pot on my converter appears to be a 5K ohm (reading 4.88 on the multi-meter) and I have 2K, 10K and 50K potentiometers.
I don't want to know what resistors I need. I want to know how to calculate the resistors I need in this case. I'm assuming its not as simple as calculating for a single resistor.
From the video it appears the author is trying to create a resistor where:
- the single turn pot with a knob, when at its minimum and maximum meet certain operation goals.
i.e.
at the fully CW position of the knob, the output is limited to XX
at the fully CCW position of the knob, the output drops to YY.
The resistance from the green to the black line(wire) is:
Rtotal = (Rknob + 11k) * (Rblue + 11k)
(Rknob + 11k) + (Rblue + 11k)
This is simply the equation for two parallel resistors: R total = product / sum
See the attached for the schematic version of your diagram.
If you plot the Rtotal vs Rknob you will find it has somewhat of a curve. There are other configurations that would result in a more linear resistance vs knob rotation.
So now, the question is: what resistor values are needed.
To an earlier part of your question. I have ordered similar devices (although not as powerful). It was my understanding a COB LED (in the raw so to speak) is a current controlled device. I am planning on setting the voltage to something above what the LED will ever need, then controlling the brightness by controlling the current.
JohnRob:
To an earlier part of your question. I have ordered similar devices (although not as powerful). It was my understanding a COB LED (in the raw so to speak) is a current controlled device. I am planning on setting the voltage to something above what the LED will ever need, then controlling the brightness by controlling the current.
That is my understanding as well. These, much like any other LED, are touted as being CC controlled. So my intention was to use the knob pot on the CC part of my converter. I plan to set the converter to around 40 volts (my COB needs 32 I think, don't quote me).
JohnRob:
If you plot the Rtotal vs Rknob you will find it has somewhat of a curve. There are other configurations that would result in a more linear resistance vs knob rotation.
Would any of those "other configurations" be better than that of DIYPerks? I have the stuff to create the circuit he used. But I can wait if there is a better way.
Maybe this?
"RANGE" should be 0 - LIMIT%.
Could be a better option for your board.
I would start with a pot (aka potentiometer) and determine the resistance range you need.
If it is zero based (i.e. off or dimmest is attained when the pot is at zero ohm) then edgesmoron suggestion could work.
Depending on the range you need you might only need a pot and a resistor. The video used two pots to tweak in the desired condition so the author didn't have to read the values and change the blue pot to a fixed resistor.
To make it more confusing, we don't know how the pot is connected on the board (I'm assuming your board already has a pot, which you will be removing, if this is not the case please let us know).
So:
Are you replacing an on board pot? Or does you board have provision for external control?
If it is no the board, is it wired like edgemoron's R1 or R2? You should be able to see the clad connecting two leads of the pot if it is like R1.
JohnRob:
Could be a better option for your board.
I would start with a pot (aka potentiometer) and determine the resistance range you need.
If it is zero based (i.e. off or dimmest is attained when the pot is at zero ohm) then edgesmoron suggestion could work.
Depending on the range you need you might only need a pot and a resistor. The video used two pots to tweak in the desired condition so the author didn't have to read the values and change the blue pot to a fixed resistor.
To make it more confusing, we don't know how the pot is connected on the board (I'm assuming your board already has a pot, which you will be removing, if this is not the case please let us know).
So:
Are you replacing an on board pot? Or does you board have provision for external control?
If it is no the board, is it wired like edgemoron's R1 or R2? You should be able to see the clad connecting two leads of the pot if it is like R1.
Yup. It's an onboard 5k trim pot that I will be removing. It's soldered on still. And the way it's mounted I can't really make out where the traces are going. Or even if all 3 legs are used. I can say that, if by clad, you mean the solder, no, they don't appear to be touching or connected in any way. Attached are 2 images. Not sure they'll be any help. Can't really see much.
edgemoron:
Maybe this?
"RANGE" should be 0 - LIMIT%.
That schematic actually makes some sense to me. Maybe there is some of that 30 year old knowledge still dancing around in the dark recesses of my brain? Thank you!
So, if I have my thinking cap on straight, I need to hook everything up. Set the trim pot so that the light is at it's dimmest. Disconnect power and measure the resistance (which should be at it's highest). Then do it all again with it adjusted to it's brightest (lowest resistance).
I have to go out of town. So, I'll try to do that next weekend.
You have no idea how much I appreciate you folks taking the time to help out.
In the 512-1 photo it certainly looks like the two left hand pot connections are connected to ground. If you have an ohm meter you can verify.
It that is the case, you will need no more than a 5k pot + a resistor in series (to limit the maximum current. You could use edgemoron's circuit with the left hand pot = 5K and the right hand = maybe 1k. Also the right hand pot should be wired like the left hand. i.e. the movable wiper connected to the now unconnected end.
Reasoning: If when turning the pot it goes "open" for a moment (due to a dirty spot on the control etc) When it is connected to the otherwise unused end the resistance will not go to open circuit but the max resistance of the pot.
BTW Is your DC-DC boost converter meant to run as constant current? Or is the current pot for protection only?
JohnRob:
BTW Is your DC-DC boost converter meant to run as constant current? Or is the current pot for protection only?
Good question. Its a Chinese made converter listed on eBay here.
The title says "...constant current..." so I think you are good.
Good luck
JohnRob:
In the 512-1 photo it certainly looks like the two left hand pot connections are connected to ground. If you have an ohm meter you can verify.
Good eye! They are indeed connected to ground.
