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Automotive LED Driver Help

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911alertme

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I would like to make some axillary LED lighting for my vehicle. There are several ready to go solutions but all the ones that I have been able to find are lowish power buck solutions. I don't really want to buy a bunch of buck drivers to power three LEDs each. That could get expensive as I'm looking to drive probably a couple hundred watts worth of high power LEDs (30 - 60 maybe. I don't know yet). The best solution I can find so far is an IC from Linear and use 2+ of them.

It is the LT3755 IC. In the datasheet, it has a typical applications section. One of the examples is a 50 watt automotive LED driver. It gives a schematic but that is about it. The example can be found on page 18. It is a typical schematic that labels parts as D1, M1, etc. It gives part numbers for these parts but a search for some of them doesn't give me anything.

The listed parts other than reistors and capacitors are:
L1 - "COILTRONICS DR127-220" here
M1 - "VISHAY SILICONIX SI7850DP" here
D1 - "DIODES INC. PDS5100" here
M2 - "VISHAY SILICONIX SI2308DS" here maybe. No stock. And here.
RT1 - "MURATA NCP18WM1045" I have no idea what it is. Google search brings up 4 results.
D2 - "IN4448HWT" again no idea. Few results.

Also, I have no experience with electronics design. I could probably design the PCB and there is a local shop that will make it. As for soldering, I think I have that down too. All parts that can't be hand soldered can be done on a hot plate.

Also, I would like to be able to dim the LEDs. The IC supports PWM but I don't know much about that. The PWM section metions duty cycle but I need it to be on all the time. The datasheet mentions analog dimming but there are no details on it. Whatever decision I decide on for dimming, I want it to be via an external pot. Its late and I'm sure I missed some stuff in this post but will answer questions tomorrow.

Also, if anybody else has a better solution, speak up please.


Thanks in advance,
Spencer
 

Diver300

Well-Known Member
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Also, I would like to be able to dim the LEDs. The IC supports PWM but I don't know much about that. The PWM section metions duty cycle but I need it to be on all the time. The datasheet mentions analog dimming but there are no details on it. Whatever decision I decide on for dimming, I want it to be via an external pot.
The idea about PWM is that the turning on and off should be too fast to see.

100 Hz is often used but that is far too slow, and is intrusive if there is any movement. You should go to 5 kHz or more.

The reason that PWM is favoured over current reduction is to keep the colours and brightness constant at all brightness levels. Matched LEDs will only be matched for colour and brightness at one current level.

On that IC you can change the current by adjusting Vctrl.
 

bountyhunter

Well-Known Member
The idea about PWM is that the turning on and off should be too fast to see.

100 Hz is often used but that is far too slow, and is intrusive if there is any movement. You should go to 5 kHz or more.
That surprise me, I learned that the threshold of visible "flicker" is about 25 Hz, so 100 Hz should be enough to be invisible. Another thing about LEDs is they are sort of "capacitive" when driven and tend to average out the illumination effect that the eye is going to see anyway.
 
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Ghosty_Ghoul

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I can't use a CRT screen set at 60Hz because of the flicker.

I need it set to at least 85Hz not to see any flicker but 75Hz is bearable.

I can't see incandescent lights flickering because the thermal time constant of the filament.

I only see the flicker on fluorescent lamps driven from magnetic ballasts at the ends of the tube because the phosphor has some persistence time. Obviously I can see the flicker, just as the tube is about to fail.

I notice LED tail lights on some cars flickering as they're driving.

I agree with Driver it needs to be a few KHz at least to avoid flicker under most circumstances.
 

bountyhunter

Well-Known Member
I notice LED tail lights on some cars flickering as they're driving.
Car lights do indeed show visible flicker at some RPM ranges as the engine is speeding up because of the ripple from the alternator. You can easily see it if you let it drop to idle at night then slowly raise the RPMs.
 

bountyhunter

Well-Known Member
I can't use a CRT screen set at 60Hz because of the flicker.
I don't get that. 60Hz is the national standard for all CRT TV's made in the last 70 years. Are you saying you see TV's flicker?

My point was (it has been my experience) that LEDs have an "averaging effect" on light output so it would surprise me if they flickered as badly as a CRT or an incandescent will.

I do know that the cel phone makers used our boost converters to drive a string of white LEDs for the backlight and I know some were run at 100Hz without any flicker. Some ran the PWM faster but that was for circuit convenience.
 
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Ghosty_Ghoul

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I don't think it's caused by alternator ripple speed because I can actually see dots which is characteristic of a square waveform.

EDIT:
I live in the UK where we have PAL.

TV flicker isn't a problem unless I'm sitting very close to the set.
 
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Diver300

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That surprise me, I learned that the threshold of visible "flicker" is about 25 Hz, so 100 Hz should be enough to be invisible.
The human flicker threshold is between about 25 and 50 Hz at the centre of vision. It is higher at the edge. This makes faulty florescent tubes, where the ends only light on one half cycle, particularly annoying because they look OK when you look at them, but flicker when you look away.

100 Hz is invisible if you, your eyes, and the light source are not moving. However, that isn't the case for car lights. As you look towards or away from a light that is off for a few milliseconds at a time, the trail it leaves on your eye is not a line like everything else leaves. It is a row of dots. Again, really annoying as it is an effect that disappears when you look directly at the flickering light.

You can see this effect on the photo taken with deliberate movement.

Another thing about LEDs is they are sort of "capacitive" when driven and tend to average out the illumination effect that the eye is going to see anyway.
No they don't. The light from an LED is approximately proportional to the current through it, and the response time is in the nanosecond range.
 

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Diver300

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Car lights do indeed show visible flicker at some RPM ranges as the engine is speeding up because of the ripple from the alternator. You can easily see it if you let it drop to idle at night then slowly raise the RPMs.
I think that is just the dimming as the voltage drops at idle.

Alternator ripple is tiny, only a few mV at most, unless the battery is missing or really in a bad way.

Also flicker is very difficult to see if the light just dims a bit. It is obvious on PWM LED lights because they turn off for 9 ms at a time.
 

Ghosty_Ghoul

New Member
The physical vibration of the car probably has a lot to do with the apparent flicker.

The LED flicker looks like the red lights in your picture.

Some people are more sensitive to flicker than others. I'm just one of those people who is really sensitive to it.
 

Diver300

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I can't use a CRT screen set at 60Hz because of the flicker.

I need it set to at least 85Hz not to see any flicker but 75Hz is bearable.
When I had a CRT, I set it to 100 Hz and reduced the number of dots if needed to get 100 Hz. I could live with 85 Hz.

The problem isn't when looking at one point. It is when you move your eyes to look at something else.

It's not so bad with TVs as you are further away, and you tend to look at the whole thing not one point. Also, there is movement on a TV picture most of the time, so when there is some apparent movement it is less troublesome. Monitors have fixed pictures a lot of the time and apparent movement is distracting, like noise in a library.

I agree with Driver it needs to be a few KHz at least to avoid flicker under most circumstances.
Some cars seem to have faster PWM on the LED lights. I guess it is about 400 - 1000 Hz. I can tell that they are flickering, but I have to try. They are far less distracting than the 100 Hz ones. I think that a few kHz would be undetectable, but I would be quite happy if they were simply not distracting.
 
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Ghosty_Ghoul

New Member
The UK already has laws on vehicle lights, I think they need updating to ban low frequency flickering LEDS.:mad:
 

Diver300

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The UK already has laws on vehicle lights, I think they need updating to ban low frequency flickering LEDS.:mad:
The Road Vehicles Lighting Regulations 1989

There is no political will to enforce the existing laws. Now if those of us who can see the flicker could be classed as disabled, or an ethnic minority.........

Then again, about 10% of cars round here have a blown bulb somewhere, and there's no money in enforcing that.
 
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