I'm planning on building my own 5W high power LED Lamp, and this is my current setup:


My power source is going to be 3 Li Ion Batteries in series, giving 14.8 volts for most of the time.

I then have the positive wire hooked directly to a Single Pole Single Throw switch, so if I have the circuit open there will be no current flowing, i.e. 0 power output from the battery.

These wires then lead to an adjustable buck boost converter with CC/CV, which I will set to a Constant Voltage of 12V and a Constant Current of 350mA as required by the LED

The output then goes through an adjustable PWM Dimmer with a potentiometerwhich then finally leads to the LED

This is my question - Is there anything wrong with having the PWM dimmer AFTER the Buck Boost converter?

I also want to know whether the entire circuit is going to be using 5W (exluding inefficiencies) all the time, regardless of whether I set the PWM dimmer to a duty cycle of 10%, 50%, or 100%?

Should I move the PWM dimmer BEFORE the Buck/Boost converter, so that I control the duty cycle before the power is sent on?

  • 1
    \$\begingroup\$ If you got an adjustable buck boost converter, then why do you plan to dimm the LEDs with PWM? \$\endgroup\$ Commented Jul 28, 2017 at 4:17
  • \$\begingroup\$ @HarrySvensson, the problem is that it is too expensive to have a variable current-sensing circuitry in LED driver ICs, so all manufacturers offer devices with a simple one-value shunt resistor, which sets the max current to LEDs. Then the dimming is achieved by PWM in one form or another. \$\endgroup\$ Commented Jul 28, 2017 at 5:28
  • \$\begingroup\$ OK, If I were to actually use the adjustable feature, would I decrease the voltage or the current? As far as I know, increasing the voltage by a tiny bit raises the current by a lot. This would end up causing a thermal runaway. \$\endgroup\$
    – Che0063
    Commented Jul 28, 2017 at 6:49
  • 2
    \$\begingroup\$ What, but you said that it's a CC/CV => Constant Current and Constant Voltage. That means that the voltage will never go above some threshold and that the current will never go above some threshold. So if you set CV to like 20 V and CC to 20 mA, then it will have some brightness, if you change the CC to 10 mA, you can expect less brightness. Why do you have to fiddle with the CV? This is so weird. \$\endgroup\$ Commented Jul 28, 2017 at 16:13
  • \$\begingroup\$ @AliChen but he said that it had CC/CV. And CC stands for => Constant Current => There is some current sensing on board. \$\endgroup\$ Commented Jul 28, 2017 at 16:15

3 Answers 3

Is there anything wrong with having the PWM dimmer AFTER the Buck Boost converter? ... Should I move the PWM dimmer BEFORE the Buck/Boost converter?

Neither. A separate PWM modulator for dimming is pointless duplication, since you already have a switching power supply in there anyway.

You say can you can run the switcher in current controlled mode. Modulate the light level by changing the current control input to the switcher.

You can think of the switching regulator as a sortof filtered PWM modulator already. Instead of feeding this regulator pulses, or drawing current from it in pulses, have it make the average current you want in the first place. This will be more efficient since there won't be yet another pass element in series, and more control logic to power.

Also, drawing current pulses from a constant current supply is a great way to make it unstable. Don't do that.

  • \$\begingroup\$ Researching more, I just realised how stupid I am. Of course reducing the current work! Thank you for pulling the wool out of my eyes. Looking at graphs, the current to brightness ratio is ALMOST linear - That means if I give it 1/2 the nominal current it will be 1/2 as bright, ignoring the logarithmic perception of our eyes \$\endgroup\$
    – Che0063
    Commented Jul 29, 2017 at 5:17
  • \$\begingroup\$ @Che0063 I'm really really happy that you chose this as your answer. Good luck in the future. \$\endgroup\$ Commented Jul 29, 2017 at 23:59

There are quite a few wrongs with this plan.

  1. 3 Li-ION batteries will give you about 3.6 x 3 = 10.8 V, not 14.8. If you cut the battery off at 12V, you probably will lose 50% battery capacity, if not more; it looks like a misprint, and you mean 4 cells, not 3.

  2. LEDs do not operate in CV/CC regime - depending on chip temperature (and it will go up substantially at 5 W), the voltage will vary. Common industrial practice is to fed LEDs with constant current, and the voltage would assume whatever it will assume.

  3. PWM dimming is usually incorporated into the LED driver internally, see for example LM3405, or right into the high-current output path in series with LED/LEDs, see the LT3519 example. Have you any chance to examine what the industry is offering in the area of "LED drivers"?

In summary, you should use industrial wisdom and proven IC solutions to build a 5W LED lamp, and not eBay toys. Technically what you are assembling will produce some light, but this would not be something that you should be proudly presenting to potential employers (or educational institutions) as an example of your qualifications.

  • \$\begingroup\$ Good stuff, +1. Having worked on Optonico's OEM LED driver IC systems (at infinitefactors.org/misc/images/p1000046_640x480.jpg and yes, that's my initials signed and dated on them), I very much want to emphasize your comments about taking advantage of the developed wisdom found in industry LED driver ICs. \$\endgroup\$
    – jonk
    Commented Jul 28, 2017 at 6:17
  • \$\begingroup\$ Very sorry about the misprint, I did indeed mean 4 lithium ion cells for 14.8v nominal voltage. As for number 2, I have to regulate the voltage SOMEHOW, I can't just feed it 16v directly. I've read a lot about the temperature/voltage relations with LEDs. I have also shopped around for LED Drivers both in the real world and online - None of which match the requirements of my usage. I don't care if the build is shoddy, I would like to know whether it works or not; whether the PWM dimming will have an effect on power consumption. I'm a student just entering thae ctrical side of things \$\endgroup\$
    – Che0063
    Commented Jul 28, 2017 at 6:45
  • \$\begingroup\$ @Che0063, no, you don't need (or want) to regulate voltage, you need to regulate only the output current, that's how a special class of devices, LED drivers, work. This is the fundamental idea behind the "LED driver". The PWM chopper from eBay will most likely work with a CV-CC-limited PSU (caution, see the answer by Olin Lathrop), however the effect on overall power consumption is unclear. And what is the problem with consumption? Do you mean overall efficiency of the setup? \$\endgroup\$ Commented Jul 28, 2017 at 16:47
  • \$\begingroup\$ That's really interesting, so you are telling me that I can simply give the LED 20v and the LED will self-regulate to 12v? I had a couple of small single 5mm LEDs and I gave it 12v for a try, it simply burnt up. And yes, I mean the efficiency. If I set the potentiometer to 10% Duty Cycle, will the entire setup use 90% Less Power? \$\endgroup\$
    – Che0063
    Commented Jul 29, 2017 at 5:07
  • \$\begingroup\$ Nevermind, thank you for your help. I'm scrapping the dimmer thing, I'll just give the LED less current. \$\endgroup\$
    – Che0063
    Commented Jul 29, 2017 at 5:20

Easy part first - keep the PWM after the regulator. The DC/DC converter assumes a constant input, not a large amplitude square wave.

Next, power - The circuit power consumption will decrease as the PWM duty cycle decreases. Not 1:1, because the circuits draw some constant power to operate that is independent of the output power, but there will be noticeable changes in battery life with changes in the load.

As for the constant current feature in the regulator versus the LED, that is a bit more complicated. Does the LED have current limiting built-in, or does it rely on external current limiting. It might be that the LED can handle a maximum or 12 V and a maximun of 350 mA, and expects both to be controlled externally. If so, then how the voltage converter and PWM interact is hard to say without a lot more information about both. However, it the converter is adjusted so neither of the LED's requirements are exceeded, then it probably is safe to connect all of it and see what happens.

  • \$\begingroup\$ Ok, I'll look further. I'll look around for more LED sellers who actually post requirements instead of shoddy pixelated graphs \$\endgroup\$
    – Che0063
    Commented Jul 28, 2017 at 6:47

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