# PWM dimmer: wire before or after drivers?



## rico-la (Oct 21, 2012)

I am building a few 20w (well 18w really) backup lights based off 10w 10-11v smd's. I bought 5x dealextreme 9w 12v in, 9-12v out drivers to power them (cheap as hell btw ~$6 for a 5 pack). I also purchased another 8a pwm rf dimmer the same as one i already own and am using for two 5w smd's. 

The dimmer i have dims from zero to full power, when hooked up to a 12v 1.25a wall wart for the input, and the output wired directly to 2x 5w 10-11v smd's in parallel without a driver. I realize this is not going to work off a dc battery, as it prob will push too much current and fry the smd's, but i haven't tested that theory yet. Prob will need a driver to limit the current.

Basically what i am wondering, is should i wire all the drivers together in parallel to push ~45w current to the dimmer (5x 9w = 45w to drive 40w of 10w x 4 smd's wired in parallel (obviously not running the dimmer at full power as that would push 1.25w over rated to each smd, and i don't want to fry them).

or should i hook up the dimmer to the 12v dc input (either a 12v 4a ac/dc WW, or a 12v 10a max in from a sla charge controller when no ac is available), then wire each smd to a driver and connect those in parallel to the dimmer? The problem i see there, is that the PWM dimmer goes from 8.5-12.25v, so the drivers would be undervolted when dimmed low. Or is that not be an issue with pwm dimming?

Something like this-


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## evilc66 (Oct 22, 2012)

You can't just randomly combine CC drivers and "dimmers". If you were to put the "dimmer" after the drivers, the drivers may not deal with having the output turned on and off at a high rate of speed. Some have open circuit detection that may mess with the results. There is also the possibility of some odd, potentially damaging results from connecting the LEDs again after an open circuit (load dump from the inductor).

Putting the "dimmer" before the driver may also give some odd results, depending on how much capacitance is on the front end of the driver (voltage input side). The pwm rate from the dimmer may be too fast to allow the front end capacitance to drain, effectively giving you no dimming at all.

There is a reason that there are dimmable drivers on the market. Combining two different components together will not necessarily get the same results.


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## rico-la (Oct 23, 2012)

Ok, i think i understand what your saying. Switching the drivers on/off rapidly via pwm dimming might cause problems, makes sense. Would the fact that these particular drivers accept a 12v ac or dc input make any difference? 

Alternatively, what about a low-pass filter to smooth out the pwm output? Perhaps something to convert the digital pwm to ac, which the driver can accept. 

Or would that cause it not to work as a dimmer at all? 

Basically a filter to convert the pulsing on/off digital output, to an analog signal that wouldn't cause issues with the drivers. 

I realize that a dimmable driver may be a better option here, but i really want to be able to dim/turn it on/off with a rf remote, without having to make it more complicated by introducing a microcontroller to be able to dim using a rf control. 

I guess after reading into pwm a bit more its not as simple as i thought. Is there any way to filter/smooth out the output of a pwm dimmer while retaining current dimming? Or is that not practical/possible?


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## evilc66 (Oct 24, 2012)

Adding more hardware isn't going to help. You are trying to put two devices together that were never intended to work together.

If you want to dim the LEDs using an RF remote, then get a driver that can be dimmed via a wall dimmer, and get an RF controlled wall dimmer to control the whole thing. This is an actual dimmable driver. Trying to make a non-dimmable driver dimmable, isn't going to work.


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## AnAppleSnail (Oct 24, 2012)

evilc66 said:


> Adding more hardware isn't going to help.



+1.

The driver basically wants to do EVERYTHING IT CAN to deliver xx current through the things plugged into it. If you put a dimmer 'upstream' of it, any smoothing capacitors the driver has will be run well over spec (Emptying fully and filling partly at high current) and fail. If the dimmer is 'downstream' of the driver, the driver may over-volt itself trying to jam 1 amp through a half-amp dimmer. This over-voltage will raise the current going through each cycle of the PWM driver and 'cancel out' the dimmer - again, you'll run parts over their rating and something will fail. Even if it somehow works, it won't work well.

Buy a part that's meant to do what you need.


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## rico-la (Oct 24, 2012)

Any suggestions for a driver at that wattage that can be dimmed via a non wall mounted (battery powered remote) rf dimmer, that takes a 12v dc input? 

Or what about a CC CV dc/dc converter acting as a current/voltage limiter/regulator, wired to the pwm dimmer, to the led's?

The point of the project is to run off 12v dc without an inverter, I'll compromise with buck/boost dc/dc conversion, but i don't want to have my 400w inverter running just to power the lights. Too much waste in efficiency for the application, and its simpler to just use the 12v regulated dc coming from the 12v 120w voltage regulator in my backup system attached directly to the battery.

Thanks for the advice BTW, I'd rather do it right than burn up a bunch of hardware i could use for other projects


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## rico-la (Oct 25, 2012)

Alright, so before i go hunting for a dimmable driver, i'm going to propose one last hardheaded idea for getting this done a bit cheaper than going that route.

ok, so lets say i take a 12v ~60ah sla, and connect it to this regulator i have-

http://www.ebay.com/itm/180941753878?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

Which gives me a max 10a 12v output (~2.5x higher than what i need for this particular application, or 4x 10w led's).

Now i need to limit the current coming out of that regulator to 4a max. If i have a 12v 4a dc source, i can wire this directly to the pwm dimmer, then to the leds. No drivers. I know this works since i have the same model pwm dimmer running 2x 5w leds off a 12v 1a ww (don't run it at full power of course). It works great, and dim's very well in this application. 

So to get my 4a 12v from 12v 10a, i imagine i could run two of these sepic lm2577 2a converters in parallel to give me a max of 4a 12v.

http://www.ebay.com/itm/2pcs-DC-DC-...846?pt=LH_DefaultDomain_0&hash=item3f1cb8ec6e


Heat sink them of course, since they're prob going to get hot running at close to their current spec. Then run the output of those into the dimmer, and from the dimmer's output to the led's. Am i correct in assuming that this will limit the current into the pwm dimmer to 4a 12v? Or would it just burn them up in a jiffy connected to a high amperage dc source (10a 12v)? 

I don't imagine so, based on my rudimentary understanding of buck/boost and sepics, but i may be wrong, correct me if i'm wrong.

That way i'm spending ~$13 for 2 sepics, rather than most likely 2-3x more just for the dimmable driver, and some kind of rf control to dim it on top of that.

Any thoughts?


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## PH1 (Nov 9, 2012)

rico-la said:


> Something like this-


Image link is dead.


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