PWM help for Pool lamp...

[KerimF]

Hi Mishu

By the way, are you looking to build just a dimmer for the LEDs or your board should do something else too?

Kerim

 

[aq_mishu]

actually I need soft starter for LED and also some halogen flood lights... but auto... like i'll just switch ON them (may be direct switch, may be RS232 (later project)) and they have to ON with soft. I'll just push... then circuit will do the rest.

I have to turn ON pool led, helipad light... etc.

Mishu~

 

[KerimF]

So you certainly need a uC.
Do you have an idea about your actual current (RMS) supplied by the AC source to your LEDs (All of them)?

 

[aq_mishu]

dear kerim sir,
sorry as i am late... was busy last few days.

Well, i've checked... it is taking total 120mA of current in AC for each LED. (transformer output for a single LED).

Now, I have total 30 LEDs. And also for Dimming ckt, i am planning to dim @ 220V part as because then the triac will need less current to handle. in 12VAC part, there is a lot of current that the triac have to handle. Thus the voltage dimming i'm planning to do is at 220VAC part... thus the dimming/regulation will be fed to the transformer and then will be stepped down to 12V. (thus only the transformer will be a BIG one...). So even the zero crossing, and all other things will be at 220VAC part.

Also making a little change in my rectifier as the resistor (100ohm 3W) is also getting hot. (but not burning). I'm planning to put the resistor before the bridge thus it will run cool.

Mishu~

 

[KerimF]

Hi Mishu,

If you don’t intend to use more than 30 LEDs, their current would be:
120mA * 30 = 3600mA = 3.6 A

Most low cost triacs have a rated current of 8A and even 10A (I use them a lot at work though I usually not exceed 5A; I have a habit to leave always a margin).

Though using a triac at the secondary side will let your hardware be simpler, this has one drawback in case you cannot change the LED resistor (current limiter).
As you know when a triac is on, there is a voltage drop across it about 2V. This will decrease a bit the LED current so the LED resistor value may need to be decreased too.

Of course I don't mean that your idea is not good. In fact for the full automatic battery charger I designed and produce 25 years ago (still working for my PCs), I had no choice but to use a triac at the primary side since the charging current is rather high.

Before going on, I wonder if you had already installed a simulator on your PC. If not, you may consider installing the free simulator I run, ‘LTspice’. For instance, if I have to design something similar to your project, I just draw a possible circuit on it then I read (on its trace window for example, like using an oscilloscope) what I need to know about voltages, currents and powers for every part/element on it. There is a Yahoo group for help, just in case.

What do you think? Is this possible for you?
If this is your first and last project then installing a simulator would not be real necessary.

Kerim

 

[aq_mishu]

Dear sir,
thanks for the reply...

Well, this is not the only project in my life... thus a simulator is dafinitely important to me... actually these few days, i'm very bz with study and some other works... thus can't give enough cool mind...

anyway... i'll always like and accept advise.

Well, I was thinking about the primary side of the transformer because of firstly low current, then also about this less voltage drop. otherwise i'll have to make a custom transformer for this job... (i can do that). Thus what you suggest?? sensing zero from primary/secondary?? i mean @ 220V or @ 12V side?? also what voltage i should supply through triac?? 220V or 12V??? I got a triac in my local market with current rating of they said 20A. And LED will not exceed 30 for this part. if exceed, i'll add another ctk because there is another branch for that where i can put a whole new things... (but those will never be used, i'm sure...)

Mishu~

 

[KerimF]

I heard you saying that you may change the place of the limiting resistor. I wonder why we don't revise the whole thing and make it as simple as possible.

What we have is just 30 parallel white LEDs. And we look to control their brightness (likely using a uC).

Let's throw away the 220V/12V transformer.
Let's throw away the 4-diode bridge of each LED.
Let's throw away the zero-cross detector.
Let's throw away the triac.

Let's decrease the heat dissipation of the current limiting resistor at each LED.
Let's make the software easy to write.
Let's make the controller easy to design and build.

Is it possible?!!! Let's see how:

We will use a 5V DC supply.
The LED driver will be a low cost n-channel MOSFET (having Vgs =< 5V for saturation).

First, we will find out which 5V supply is available for you. I usually import 50W, 100W and 200W.
That is, they are rated for 10A, 20A and 40A.
I usually don’t exceed half of the rated current.
Case 50W: total current 5A, LED current 5000mA/30 = 166 mA
Case 100W: total current 10A, LED current 10000mA/30 = 333 mA

I prefer to keep the LED current no higher than 250 mA. That is the total current would be 30 * 0.25 = 7.5A
In this case, the 100W will run with a good margin. (I don’t think there will be a big difference in the prices for the 100W and 50W).

The type of MOSFET I use lately for LEDs, is equivalent to FQP50N06L, FG50N06L and STP50N06L.
At Vgs=5V, its ON resistance is about 0.025 Ohm, so for 7.5A Vds < 0.2V and its dissipation is less than 1.5 W at full load (a small heatsink on it is enough).

So each LED in this case needs just a limiting resistor (no bridge), its value (approximately):

R = ( 5 – 3.5 ) / 0.25 = 6 Ohm
P_r = ( 5 – 3.5 ) * 0.25 = 375 mW
So R can be made with two 500mW resistors in parallel, each of value 12 Ohm.

Since the LEDs are now supplied by DC, you need to connect them with the right polarity to their common 2-wire line. And if you will notice later that the LEDs near the end of the line have a lower brightness you can add a third resistor in parallel to the former two.

About the software you just need to generate a PWM signal with a period of not longer than 5 msec (>200 Hz).

For instance, I didn’t suggest this simple and efficient solution earlier because I thought you are not free to choose the topology. I guess you could :wink:

Good luck.

Kerim

 

[aq_mishu]

Dear Sir,
Thanks for the reply... well, i was thinking of the same approach for a long time... basically i have several problems in my location/local market...

• The market has no LED driver
• There is not all type of resistor along with values.

And the site problems are:

• The cable is around 100ft AC wiring cable, and I can't change the wiring
• The LEDs have to be parallel (30 of them), for maintainance issue

I need to lit them all with equal intensity and a dimming (soft start) on.

Now, Last night, I found that feeding a 12V AC or DC makes the same voltage drop at the last part of the line is about 1.5V at no load condition.

I tried to change the resistor placement as told earlier, but that was the foolish thing... so I am simply forgotting that part for ever...

After analyzing the parameters, here is I'm thinking to go in this approach similar to the one you said...

A 5VDC PWM of around 100HZ with 50-50 duty (thus it will be a 200HZ type AC) [I may be wrong with the calculation] means on-off...

This PWM voltage will be fed into the Line.

The LED's will get only a current limit resistor in series. The value will be 3V LED driving voltage and 90mA driving current. Thus R=(5-3)/0.09 = 22 Ohm

In the case of last few LEDs, i will add another resistor in parallel witht he 22 ohm.

==============================================================
Method 2

*And just found another method... supply 8V PWM (as drop is 1.5V) like before.
*Add a series regulator with a pass transistor and 3.3v zener (instead of a 7803 or lm317 complexity) ( **broken link removed** )
*Just add a small resistor for current limit.

both method will run the resistor cool and also will increase the life of LED due to pulsating DC. And I found method 2 will add a regulator at each point (as some of the resistors are not available here... my luck :-(( ) Opinion????

Well, the PWM driver will be based on uC and mosfet and ofcourse with sufficient amount of current... planning of IRF Z44N.
Mishu~