Help on designing a LED circuit
Help on designing a LED circuit
(OP)
Hi all,
I need your opinion on a circuit:
I have 12 panels of 55 Leds each. Each panel is powered by 24 VDC (as internal limiting resistors. the total current is about 300mA). The 12 panels must be able to shwitch in sequence or all at the same time.
This is not a problem of course. The problem is that I must also control the brigtness of ALL panels at the same time using PWM.
So if I have the negative side of the panels connected to the ground via a MOSFET transistor that does the PWM, wath is the best way to switch on and off each individual panel while mantaining the PWM ?
Use anothe transistor at the 24V side?
(I cannot use relays because the switching frequency can be about 200ms)
Thanks in advance
M
I need your opinion on a circuit:
I have 12 panels of 55 Leds each. Each panel is powered by 24 VDC (as internal limiting resistors. the total current is about 300mA). The 12 panels must be able to shwitch in sequence or all at the same time.
This is not a problem of course. The problem is that I must also control the brigtness of ALL panels at the same time using PWM.
So if I have the negative side of the panels connected to the ground via a MOSFET transistor that does the PWM, wath is the best way to switch on and off each individual panel while mantaining the PWM ?
Use anothe transistor at the 24V side?
(I cannot use relays because the switching frequency can be about 200ms)
Thanks in advance
M





RE: Help on designing a LED circuit
I hope you're running them with a micro?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Help on designing a LED circuit
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Sometimes I only open my mouth to swap feet...
RE: Help on designing a LED circuit
Yes I will use a micro. I know that I can turn them ALL off by putting the PWM to zero.
I need to have INDIVIDUAL on/off control of each panel. That's my problem.
This is for led (panel) arrows in a highway curve, The arrows will turn in sequence one after the other to alert the driver that the curve is dangerous. This is circuit is already working. The problem is that the high brightness Led's used (must be to be visible in the day light) are to bright at night. That's why I need to redraw the circuit to control the brightness while keeping the individual control of the arrows. One other approach is using a programmable step down regulator and bring down the voltage, but I cannot find such an IC...
thanks
M.
RE: Help on designing a LED circuit
RE: Help on designing a LED circuit
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Sometimes I only open my mouth to swap feet...
RE: Help on designing a LED circuit
Make one a master that also has a light sensor that maintains the required brightness based on the ambient light.
Meanwhile each segment is commanded with it's required turn on time, brightness, and duration. That will allow you smoothly or harshly switch segments, whatever you find most appealing or effective.
The boards will be so small and simple that you just make them all identical, even though one is to be the master.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Help on designing a LED circuit
RE: Help on designing a LED circuit
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Help on designing a LED circuit
Do you mean connecting all the sources of the panels MOSFET's to the drain of the PWM MOSFET and the drain of this one to the ground (keeping the positve side of the panels allways connected to the 24V)?
This was my original ideia, but I'm not sure if it works correctly.
TIA
M.
RE: Help on designing a LED circuit
RE: Help on designing a LED circuit
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Help on designing a LED circuit
Make sure that the circuit that will PWM the 24 volts only modulates the upplies to the LEDs, and not to the rest of the circuitry. Can this be done just with say a new harness that adds a little box somewhere, or should you modify each control boards to accomplish this?
There isn't just one good way of doing this. The best approach to take will depend on your system configuration, and how you plan to modify them (in the field or bringing them back at the factory). Can you tell more about that?
RE: Help on designing a LED circuit
I think we are talking the same language. Are we alone?
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Sometimes I only open my mouth to swap feet...
RE: Help on designing a LED circuit
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Help on designing a LED circuit
One of them will work!.
Thanks again
M.
RE: Help on designing a LED circuit
You know, I rarely get satisfaction from the crummy PWMs included in micros to provide PWM "since the competition has PWM".
I generally just use a timer.
Set one up for 1kHz and just mess with making one PWM based on that. Once you have that, think of how to run all the panels at once. It won't be hard.
You may even find it useful to sequence the panels as that will balance the power distribution too. Like Panel B is turned ON During panel A OFF times. Etc.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Help on designing a LED circuit
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Help on designing a LED circuit
I have been working on and off on a project here for about the last 9 months. The project started out with an 8 bit µC, one discrete input, one discrete output, one analog input, and a single pwm to drive a motor. All was well and good. I had it working fairly well. Then everything changed as it always does and now it needs to have 1 pwm input, 1 analog input and 2 pwm outputs I was using the internal timers for the single pwm but now I need 3 and it only has two timer channels. I don’t know were go, I was thinking of changing the µC. A software pwm work fine in theory, but how do you keep the signals exact?
Here’s what I don’t understand:
If you have your 1 KHz timer running, and say 8 different outputs being toggled from it, each on a different duty cycle, how do you keep the duty cycles exact? How do you keep the timer running at exactly 1 KHz? When the µC is servicing the pins it cannot be running the timer and visa versa.
RE: Help on designing a LED circuit
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Help on designing a LED circuit
You want to vary the brightness of all panels the same amount.
So, set-up one PWM output in the software. Then just software AND the new PWM output and the on-off output you already have. Make the AND output signals the new outputs to the panels.
You already have the required hardware....
RE: Help on designing a LED circuit
Not trying to hijack the thread.
The pwm is ONE (there are two) of my two newest and biggest problems. I am going to try to set up it in an interrupt. My specs call for a much slower clock than the 1kHz, that should not be a problem, but I have a pwm signal accuracy of +/- 50ns that has to be maintained, this is my worry.
RE: Help on designing a LED circuit
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Help on designing a LED circuit
As Keith suggested, start a new thread and we'll help where we can.
Dan - Owner

http://www.Hi-TecDesigns.com