| Electronics > Beginners |
| LED control circuit, would appreciate feedback and suggestions. |
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| doublec4:
Hi All, I previously created this post here: https://www.eevblog.com/forum/beginners/battery-powered-led-pwm-dimmer-circuit/ which I did not want to continue with as this is taking a slightly different turn. Background: Previous thread was with regards to dimming a 12V LED module I have. When I took apart the module it turns out that it has its own programmable controller (running at 5V) that uses mosfets inside to flash the LEDs in programmed patterns, but it has no method of dimming the LEDs (it has a bunch of other components for vreg for the controller, current limiting resistors for the LEDs etc). Before taking apart the module I thought perhaps I could PWM the 12V power and dim the LEDs but I no longer see this working as I will likely just be rapidly turning the programmable controller on and off very quickly and messing with the flashing patterns. So that brings me to this post. Although the module nominal input is 12V, when I drop the input voltage using my power supply, there is no appreciable difference in perceived brightness down to 9V (current also drops from ~190mA to ~100mA). Below 9V I would estimate a 50% brightness drop when lowering input voltage to ~8.2V (current drops to ~25mA) So I have some 1n4002 diodes here and I believe on the datasheet it shows a forward voltage drop of 0.6-0.8V with current between 0.03A to 1A. If it put one in series with the LED module and run it at 9V I achieve the ~50% dimming I am looking for (voltage across the LED module should be in the ballpark of the 8.2V I'm assuming). If I used a ~30ohm resister in series instead of the diode I get roughly the same result as I am limiting the current. Here in lies my first question, by using a diode in series to drop the voltage, or using a resistor to further limit the current (since it already has resistors built into the module) what is the advantage or disadvantage to either solution? Second question, in the previous post, it was suggested to use a constant current LED driver but this was before determining all of the other components inside the module. I'm not sure what effect it would have trying to "push" a constant amount of current through the module with everything else going on in there? Third question, I actually have three of these LED modules in red, amber, white... which I would like to control individually (on/off). The brightness of all LED modules can all be tied together as I have no reason to run them at different brightness simultaneously. I came up with the following circuit (see attached) using MOSFETS for the ON/OFF and controlling the dimming with the diode as mentioned in my first question above. The red/yellow/white inputs from my controller would be normally low to turn off the lights, and then driven high to turn each of the colors on. A common dimming signal would be driven high so that path to GND avoids the diodes (or resistors, see question 1). Low signal on this line would put the diode/resistor in series to dim the lights. Does this look like it should work, or is there a better way to do it with less components? Thank you all for your patience with all of my questions... there are definitely some gaps to fill in my fundamental understanding of electronics. |
| doublec4:
Anyone willing to take a stab at it? Just to try and be more clear, I am only looking to achieve two discrete states with the LEDs. Bright and Dim, not a continuously adjustable brightness level. In the circuit I posted the LED components represent the LED modules I have. I did not remove the actual diodes from the modules or anything like that. Thanks! |
| DDunfield:
I'm not sure that dimming the LEDs by providing lower than the specified voltage to the module is a good idea. You would have to do some reverse engineering to determine if this is reasonable or not. Assuming it is, some thoughts to consider: There will be some sort of regulation going on inside the device. The fact that you see no difference between 9V and 12V suggests that there might be a 9V regulator. The fact that the current drops as you lower the voltage suggests that it is a liner regulator (switching regulators should draw more current as input voltage drops). So the question becomes: What is happening below 9V. First thing I would do is check the controller to see what voltage it is actually running on. It is likely to be 3V or 5V so there might be another regulator. You want to make sure that the controller is getting a proper power source at the lowest voltage you intend to provide, otherwise the device may not be stable. Both a resistor or a diode would lower the voltage, the resistor due to voltage drop based on E-IR, and the diode due to it's characteristic voltage drop. In this application I think a diode would be better because it will have a relatively constant voltage drop. If you use a resistor, the voltage drop will change based on the current drawn, which will depend on how many LEDs are on at a given time. Assuming the patterns will have different numbers of LEDs on at various times, then the brightness will change as the pattern changes. You are correct in that you don't want a constant current driver in this case, again as pattern changes required current would change. Can you modify the module? It may be possible to bring out the controller supply and LED supply separately which would likely be the best way to accomplish this as you could fool with the LED supply without compromising the controller supply. Dave |
| madires:
--- Quote from: doublec4 on June 11, 2019, 03:58:33 pm ---Here in lies my first question, by using a diode in series to drop the voltage, or using a resistor to further limit the current (since it already has resistors built into the module) what is the advantage or disadvantage to either solution? --- End quote --- The resistor also creates a voltage drop. It's a quick and simple way to reduce the LED's brightness but it will waste power (heat). And color balancing for RGB LEDs would require some effort to find the best resistor values. --- Quote from: doublec4 on June 11, 2019, 03:58:33 pm ---Second question, in the previous post, it was suggested to use a constant current LED driver but this was before determining all of the other components inside the module. I'm not sure what effect it would have trying to "push" a constant amount of current through the module with everything else going on in there? --- End quote --- If you have resistors on the LED module, like on LED strips, go for constant voltage. Constant current is meant to drive an LED directly. Again, any resistor will waste power. Therefore driving power LEDs with constant current is more efficient. --- Quote from: doublec4 on June 11, 2019, 03:58:33 pm ---Third question, I actually have three of these LED modules in red, amber, white... which I would like to control individually (on/off). The brightness of all LED modules can all be tied together as I have no reason to run them at different brightness simultaneously. I came up with the following circuit (see attached) using MOSFETS for the ON/OFF and controlling the dimming with the diode as mentioned in my first question above. The red/yellow/white inputs from my controller would be normally low to turn off the lights, and then driven high to turn each of the colors on. A common dimming signal would be driven high so that path to GND avoids the diodes (or resistors, see question 1). Low signal on this line would put the diode/resistor in series to dim the lights. Does this look like it should work, or is there a better way to do it with less components? --- End quote --- Instead of the second MOSFET and the diode you could add an logic AND to each driver MOSFET and control the brightness via PWM. For example, a 74HCT08 has four AND gates. |
| doublec4:
--- Quote from: DDunfield on June 13, 2019, 01:57:11 pm ---I'm not sure that dimming the LEDs by providing lower than the specified voltage to the module is a good idea. You would have to do some reverse engineering to determine if this is reasonable or not. Assuming it is, some thoughts to consider: There will be some sort of regulation going on inside the device. The fact that you see no difference between 9V and 12V suggests that there might be a 9V regulator. The fact that the current drops as you lower the voltage suggests that it is a liner regulator (switching regulators should draw more current as input voltage drops). So the question becomes: What is happening below 9V. First thing I would do is check the controller to see what voltage it is actually running on. It is likely to be 3V or 5V so there might be another regulator. You want to make sure that the controller is getting a proper power source at the lowest voltage you intend to provide, otherwise the device may not be stable. Both a resistor or a diode would lower the voltage, the resistor due to voltage drop based on E-IR, and the diode due to it's characteristic voltage drop. In this application I think a diode would be better because it will have a relatively constant voltage drop. If you use a resistor, the voltage drop will change based on the current drawn, which will depend on how many LEDs are on at a given time. Assuming the patterns will have different numbers of LEDs on at various times, then the brightness will change as the pattern changes. You are correct in that you don't want a constant current driver in this case, again as pattern changes required current would change. Can you modify the module? It may be possible to bring out the controller supply and LED supply separately which would likely be the best way to accomplish this as you could fool with the LED supply without compromising the controller supply. Dave --- End quote --- Hi Dave, when I took apart the module I scraped off the epoxy and did some part number look ups to find what I was dealing with. I have them on my work computer so I can post them tomorrow, but the only regulator I could find was a 5V regulator (very small current output so it can only be for the programmable chip not the LEDs). From my (limited) experience with a few other "high power" LEDs I find that running them at their full rated current vs running them at lower currents there isn't an appreciable difference in perceived brightness. The brightness vs current does not at all seem linear. I feel like that is what is happening here... once you hit a certain threshold the brightness has a steep drop off. I'll check again for any components I may have missed but I didn't see any other regulator, just the MOSFETs for turning them on and off. As for the flashing patterns, each module has 6 LEDs and they all flash on and off together. The patterns are simply just different flash rates, no sequencing of individual LEDs or anything like that. So the resistor would see consistent current drawn as well if I went with that instead of the diode. The module is pretty tiny and the chips are all tiny SMT components... not saying that it would be impossible to do, but it would be tough to modify. |
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