Using pwm to switch transistor to dim LEDs

[kgthomas]

Hi I am currently in the process of gathering the info on an LED light I am gonna build. It will have 4 strings of 12 LEDs. I initially was planning to use 4 dimmable drivers to power each string. The dimming on the drivers I was looking at was by a 0-10 volt input. I initially was gonna control this with a simple 10 volt supply and potentiometer. I had worked this out on this forum with help from BobK. Eventually I was gonna change the control from a simple pot system to an arduino control converting the pwm output to 0-10 volt by smoothing it and then amplifying the voltage.

I work in a university and we have an electronics department and a very good electronics engineer. I ran my ideas by our engineer including eventually changing to a pwm system. He recommended to immediately use a pwm system.

His recommendation was to go for a fixed current driver. Then to use pwm signal to switch a transistor on and off providing the dimming effect.

As I understand it if I bought a pwm dimmed driver then the pwm signal turns the driver on and off rapidly thus making the LEDs look dimmer and brighter. So from what I can tell the option of using a transistor to turn the LEDs on and off rapidly should have the same effect. I was planning to use the 0-10 volt dimmer driver as I thought it would look smoother. However if I could get away with using 1 driver and still have dimmer control I would be more than happy with that.

What I am wondering is would this work? Also if it works why do people use pwm dimmer drivers instead of using a constant driver and a switched transistor? The engineer was telling me that as the transistor is either on or off then there would be minimum heat through the transistor, is this correct?

Thanks for any help anyone can offer with this.

Cheers Keith

 

[(*steve*)]

All the constant current driver does is ensure that the LEDs receive the same current, regardless (within limits) of how the Vf changes. This is the correct way to drive LEDs and your "good electronics engineer" is giving you good advice.

LEDs are often driven from a voltage source with a series resistor. This (within limits) can be almost as acceptable as a constant current source, and for low power LEDs is almost universally used.

All PWM is doing is turning the LEDs on and off very quickly so that the integrating nature of your eye perceives them as varying in brightness as the duty cycle changes.

Regardless of how you do PWM, you still need *something* to limit the current to the LEDs.

With a 10V supply, you're not going to be able to run a string of 12 LEDs without a boost SMPS to lift the voltage to between 25 and 40 volts (depending on the colour of the LEDs).

If you don't want one current source per LED string, you'll need to add some series resistance to the strings to help them share the current equally.

A simple constant current source involves 2 transistors, a current sense resistor (which drops 0.6V at the desired current) and another resistor to give some base drive to one of the transistors. These constant current sources can be made to follow a PWM signal by taking the resistor providing the base current to the pass transistor and driving this from your PWM signal rather than tying it to the supply rail.

The practical upshot of this is that you convert a single transistor turning the string on and off into a switchable constant current driver with the addition of a resistor and a further transistor.

A transistor used as a switch will not dissipate much power (as your engineer suggests it is either on or off), however if you set it up as a constant current source it *will* dissipate power as you have essentially caused it to act as a linear regulator dropping the balance of the voltage between what is required to drive the LEDs at the set current and the supply voltage.

 

[kgthomas]

Hi Steve

Thanks for getting back to me about this. The 10 volt I mentioned was to control the dimming of a 60w 48v dimmable meanwell driver. There will be 36 XTE LEDs and 12 XPE LEDs in my build. The XTE can be driven at a max of 1500 mA and the XPE can be driven at a max of 1000 mA. I am planning on driving both types of LEDs at a constant 700 mA that way they are run well below their maximum.

The driver I am now looking at is a meanwell driver again a NES-350-48. This driver gives out 48 volt and a maximum of 350 watt and a constant current which can be set anywhere from 0-7300 mA. I plan on using this driver as I can then potentially add more LEDs at a later stage in different colours if I wish. What I'm wondering is there a problem using such a large power supply and under using it? As I will be outputting only 700 mA constant current and the 48 LEDs I think will be driven at around 2-2.5 watt each so it's a potential wattage of 125 watt? I will be using an arduino to control the lights could I power this also from this driver or should I use a separate power supply? I have just checked and I can adjust the output voltage of the driver down to 41 volt so I can adjust it to suit a resistor and string of 12 LEDs.

Yeah the engineer in work had mentioned about having to add resistors into it to ensure no string got all of the current. I was emailing him last week but I'm going to go over next week to chat with him and try to figure out a circuit for this.

Cheers again for your help
Keith

 

[(*steve*)]

Using different LEDs can pose a problem if you're using resistors to balance a constant current source. The two types of LED may have different V vs I curves (and Vf vs temp) and this may result in an unstable sharing of current.

If I had to run 6 of 1 led and 6 of another (at the same current) I would consider 2 strings of 3 of type A and 3 of type B so that each string is identical to the others.

I would probably use a separate power supply for the logic. It's low power and the cost will be small. It means you don't have to worry about the constant current power supply's voltage fluctuating with the PWM (because it will).

You need to be careful that using the PSU to drive a load switched by PWM isn't going to cause problems with the power supply. Even if just to reduce the chance of creating RFI, you may have to limit the switching speed of the mosfets.

 

[kgthomas]

Hi Steve

Thanks for getting back to me about this. Yip I was thinking of what affect the PWM switching might have on the psu. I will have to look into this before I do anything as the light unit is for my marine fish tank which is in my living room so even rfi could get annoying.

As for the strings of LEDs, the driver I was looking as has 3 48 volt outputs so if I ran the 3 strings of 12 XTE LEDs on one output then ran the one string of XPE LEDs on another output would this sort the problem of the 2 types of LEDs having different V vs I curves?

Cheers Keith

 

[(*steve*)]

Assuming the outputs are independent, that would solve it.

I might go for splitting the load of the XTE LEDs evenly across 2 outputs if I could, but that's not a must.

 

[kgthomas]

Hi Steve

Thanks for getting back to me about this. I have been doing a bit more research into this and meanwell do a dc/dc driver the LDD driver range and this should separate the four strings of LEDs and make each string independent and the LDD drivers only cost £7 each and they have inbuilt filters to prevent EMI. I'm wondering now though I can get a ac/dc driver in branded for £20 with no pfc or the same spec driver made by meanwell again with no pfc for £30 or a meanwell driver with pfc for £60. I'm wondering is it worth the extra cost to go for a driver with pfc? Or should I just go for the unbranded driver.

Cheers Keith

 

[(*steve*)]

PFC may be important if you're building a huge array of these that will draw thousands of watts.

Otherwise you could dispense with it. Personally, I think it's a good thing, but at what cost?

 

[kgthomas]

Hi Steve

I imagine my total wattage will be no more than 300 watts as I eventually plan to add up to 7 strings of LEDs so I imagine that I wouldn't need the PFC but I'll see how far funds will stretch as its only £30 difference which I would save over time in electrical use.

Thanks again for all your help with this
Keith