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Author Topic: Power supplies - dimmable or not  (Read 1103 times)
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Hi,

I was doing some research on "dimmable" power supplies aka Meanwell ELN6-48D / ELN6-48P when a few questions bugged me.  Go easy on me, I've not touched electronics for almost 10 years.

1) As I understand it, if I have PWM signal going to the base of a transistor (o MOSFET), I should be able to vary (or dim) the brightness of a string of LEDs right ?  In other words, if the PWM signal from a Arduino switches the transistor on and off fast enough, it would dim the LEDs ?

If this is the case, can we safely say that the main difference between a dimmable power supply and a "non-dimmable" version is simply a additional transistor (and or some other filtering circuits) ?
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The term "dimming" should not be overstressed. You do not say: "I dim the speakers", when you turn the volume down...

Dimming by PWM or PDM is a mostly technical internal matter to reduce the dissipation of heat. The technique used is full range switching (rather than analog amplification) of transistors or triacs.  
« Last Edit: August 25, 2010, 10:25:41 am by mpeuser » Logged

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For a refresher course on PWM see:-
http://www.thebox.myzen.co.uk/Tutorial/PWM.html
In your case you do not need any smoothing filters as that will restrict the dimming range of the LEDs.
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Thanks for all the replies.

@Richard - I'm quite surprised when you said "Do NOT attempt to put a PWM signal into that power supply." .. how would I control the brightness of the LEDs then ?

@deSilva - Thanks, sometimes I get all confused over the terms used. No thanks to 10years of unused electronics knowledge in between the ears.

@Mike - Thanks for the link - I'll be reading through site as much as I can tonight.
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Those are well written articles with a pleasing lay-out - thank you!

Just 3 points came to my mind...
- It is not deciBells but deciBels, though this slip is excusable - who would think of calling some things milliWat
- There is also an issue with the backward voltage of LEDs. This is arround 5 volts (one never knows exactly), and can give rise to funny behavior.
- I would not second your opinion WRT to pulsing LEDs upto that consequence. You are absolutely right to whatyou are saying, but many hobby (!) devices are used for hours or days only anyway. So life-expectance is relative and there is  wear and tear, isn't it... An output generally fails when its internals overheat, which is generally a matter of power, not so much of current as such. Inside PC CPUs there are 10s of amperes flowing for pico seconds.... But I am overcritical - Very good work!
« Last Edit: August 26, 2010, 10:04:21 am by mpeuser » Logged

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That document says that the unit with "D" at the end of the part number is designed to take 0~10VDC, and the one with "P" at the end is designed to take PWM "100Hz ~ 3KHz".  So the "P" version will take PWM. http://arduino.cc/en/Reference/AnalogWrite says "The frequency of the PWM signal is approximately 490 Hz." so it should be compatible with the "P" version of the power supply, but NOT the "D" version

I can tell you from experience that the P-type needs a 10v PWM signal. If fed a 100% duty cycle 5v signal straight from an Arduino, they respond as if they're getting a 50% duty cycle signal. It makes me wonder if the only difference between the analog and digital version is some sort of simple filter included in the digital version, since both seem to respond to an analog signal anyways.

I can also report that the analog version responds fine to a (boosted to 10v) PWM signal from an Arduino. So, from the perspective of controlling these supplies from an Arduino, there doesn't seem to be much difference. In both cases, you need to boost your control signal to 10v.
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It all depends of what happens to this controll signal inside the box. It could be used in the internal voltage regulation, but this is most unlikely, as much higher switching frequencies are typical, and they have to be more precisely defined.
So it comes back to a comparator between filtered output and (filtered?) controlinput.
In the extreme case (no filtering of control signal), the output voltage would follow your input signal exactly.. This could stay mostly unnoticedas there will be some large capacitors at the end. It could be observed under heavy load with an oscilloscope.

In the digital version, there should be no relation to the amplitude of the PWM signal, as this would be against its primary definition... But maybe they did nothing but tune the input filter... This would be - naughty
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Richard, in general I would agree with you whole-heartedly, but in this case I don't see where the danger is, but of course I did not design the thing so I can't claim to be an expert. Prudence is always the best route when someone is unsure and operating outside "specified" practices.

That said, many people in another hobby community I belong to regularly use these very two devices interchangeably (to drive HP LEDs under constant current), and no one's reported a burned down house or even a failure.
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if you are trying to dimm/control led lights with an arduino then i would recommend using a PWM dimmer circuit after the power supply to avoid having to buy a special power supply and it is easy to connect and you can get a 3 channel (RGB) one that connects to an arduino for about $25 or you can make your own with some mofsets and a few other parts  
there is a connection diagram on fritzing

http://fritzing.org/projects/arduino-controlled-rgb-led-light-strips/

you could also buy a power supply that has a pot or knob to adjust the output voltage and see what voltage and range it is and get a digital pot that could be controlled by the arduino (i can't think of the name of them but they arn't cheap and you wouldn't have full range dimming but you would have some
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if you are trying to dimm/control led lights with an arduino then i would recommend using a PWM dimmer circuit after the power supply to avoid having to buy a special power supply and it is easy to connect and you can get a 3 channel (RGB) one that connects to an arduino for about $25 or you can make your own with some mofsets and a few other parts  
there is a connection diagram on fritzing

http://fritzing.org/projects/arduino-controlled-rgb-led-light-strips/

you could also buy a power supply that has a pot or knob to adjust the output voltage and see what voltage and range it is and get a digital pot that could be controlled by the arduino (i can't think of the name of them but they arn't cheap and you wouldn't have full range dimming but you would have some

These methods may be fine for typical gumdrop LEDs, but they won't practically work for HP LEDs, which is what the ELN driver is basically designed to drive.

That said, there are many constant current HP LED drivers (the ELNs included, or Buckpucks, and so on) that are just as easy to rig to an Arduino as anything can be, so it's not really a big deal.
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@Richard - My bad - I was half asleep while typing that reply, I stand corrected, it should be an ELN60-48 (specifications as mentioned by ill_switch as well - thanks buddy!)

@ill_switch - Thanks for the info - I got exactly what you said when I hooked up a 12V power supply to a single 12V LED - 50% brightness only .. guess I got to find a way to boost the 5v to 10v now  ;D Looks like I'm getting quite close to what I want to achieve.

I'm still puzzled though. If I'm to use a Meanwell ELN60-48P, is the PWM pin supposed or not to connect to DIM+ and DIM- connects to Arduino GND ? If not, can someone show a picture of how it should be connected in order to 'control' the LED brightness ?
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