Hello Gentlemen

Hello Ladies and Gentlemen

I have an application to replace a 4W 12V side marker lamp and reflector with an array of LED’s. The array is needed because I will no longer have the depth available for a single bulb and reflector, so I need to use several flat panel squares against an acrylic diffuser to give me the appearance of light without hot spots. Of course when I do this both the light and power are too high. I have already tried a set of 5 2” x 2” panels with 6x8 arrays of series 5050 LEDs. These panels run at 5W each for a total of 25W. I need to get my power down to below 4W as well as cut the light down by a factor of about 10. T do this PWM will be required.

I found this circuit that appears to do exactly what I need with the exception that the circuit show uses a 60V input and a 60W output. I need to modify this circuit for a 12 to 20V DC input and LED power under 4W at 12V. I’m expecting to operate this in the range of 5 to 10% duty cycle. Then I will bring the power back up to 4W with a resistor between the input positive and ground.

The attache PDF has a discussion on the circuit. The first picture shows the circuit. The second picture shows the original incandescent bulb and reflector. Third picture shows the side marker lens placed over 5 5W LED 8x6 arrays totaling 25W and acrylic diffuser. It is way too bright and can easily be lowered by a factor of 10 while consequently getting the overall power below 4W.

I would greatly appreciate guidance on how to either modify this circuit to accomplish that or any other circuit.

Thanks
John

 

The Schematic clearly states that the Circuit will operate on as little as ~8-Volts,
which is perfect for your application.

The connection scheme for your LEDs may have to be adjusted for best operation at ~8-Volts.
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The Schematic clearly states that the Circuit will operate on as little as ~8-Volts,
which is perfect for your application.

The connection scheme for your LEDs may have to be adjusted for best operation at ~8-Volts.
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My eyes have welders flash from too bright LEDs. I only saw the 60V. Well that's 1/2 the battle. Now I have to get the 60W down to under 4W. Can it be as simple as increasing the resistance of Rled.

 

That's not the preferred method,
simply reducing the PWM on-time is the best way to set the brightness.

Since You are asking this question,
using this chip may involve more complexity than You are currently equipped to deal with.
There are simpler, though probably less sophisticated, methods.

I would suggest that You simply enlarge the hole
( that You will have to have in any case for power ),
and recess a standard Side-Marker into the Fender.
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That's not the preferred method,
simply reducing the PWM on-time is the best way to set the brightness.

Since You are asking this question,
using this chip may involve more complexity than You are currently equipped to deal with.
There are simpler, though probably less sophisticated, methods.

I would suggest that You simply enlarge the hole
( that You will have to have in any case for power ),
and recess a standard Side-Marker into the Fender.
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OK that is the plan. But the power I need to reduce is at the input to the circute. If I PWM the voltage to the LEDs at say 400Hz and 10% duty cycle, wont the overall power to the circuit be ~ 10%. of all the LEDs peak power.

Setting the bulb deeper into the fender is not an option. I have to move them out to make room for projector Xenons. That's the whole reason for doing this.

 

If I PWM the voltage to the LEDs at say 400Hz and 10% duty cycle, wont the overall power to the circuit be ~ 10%. of all the LEDs peak power.

Correct. Personally that chip seems overkill.

 

Correct. Personally that chip seems overkill.

I'm certainly open to other circuits. That's just the first one I found and it was for an automotive application. I would like to stay with hardware that is automotive grade.

 

I would use a ready made option something like this:
https://www.amazon.com/Dimmer-Briidea-Halogen-Incandescent-Trailers/dp/B0936TRTHN?th=1

 

That suggestion will probably work just fine if mounted INSIDE the Car.
It's certainly not weather-proof.
It can easily Power both left and right side-markers at the same time.
The Pot can be removed and replaced by 2 Resistors when the ideal setting is found.

BUT,
there is still the problem of working out the wiring arrangement between the individual LED Chips
so that there can be a relatively efficient Current-Limiting scheme implemented.

Why have You chosen to use such High-Powered-LEDs that they must be run PWMed at~10% ????
Do these Side-Markers also act as Turn-Signals ?, If so, this changes the picture substantially.
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I would use a ready made option something like this:
https://www.amazon.com/Dimmer-Briidea-Halogen-Incandescent-Trailers/dp/B0936TRTHN?th=1

I guess that since it says 0 to 100% dimming that it must be a PWM dimmer. The only thing I don't like about it is its big and adjustable. I would like something that once I figure out the proper duty cycle it would be set permanently or at most with a trim pot. Also small enough to fit inside the area behind the LED arrays. I could easily do this with an Arduino nano 33BLE and I could get it to fit. That's also overkill but not expensive. The task says analog circuit all over it but my experience with analog circuits is about equal to my experience with walking on the moon.

That suggestion will probably work just fine if mounted INSIDE the Car.
It's certainly not weather-proof.
It can easily Power both left and right side-markers at the same time.
The Pot can be removed and replaced by 2 Resistors when the ideal setting is found.

BUT,
there is still the problem of working out the wiring arrangement between the individual LED Chips
so that there can be a relatively efficient Current-Limiting scheme implemented.

Why have You chosen to use such High-Powered-LEDs that they must be run PWMed at~10% ????
Do these Side-Markers also act as Turn-Signals ?, If so, this changes the picture substantially.
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The goal was to mount one in each side marker assembly. They are sealed but inside the car is an option is all else fails.

The arrays you see behind the lenses are these:

https://www.amazon.com/dp/B01LW2Q5AV?psc=1&ref=ppx_yo2ov_dt_b_product_details

Not saying these are the ultimate solution, but I can mount them and with the right amount of PWM duty cycle the power and light output will be OK. I believe these could be wired into that circuit as 5 parallel led strings.

The high power is merely because I need the LED's packed close together to get a uniform light out of them, so a large quantity is needed and it drives up both the light output and current. Normally a less denser set of lights would be placed about 2" behind the diffuser and light would spread out evenly, and then you could get away with 10 or 20. But in my case I have put them right up against the diffuser. On the US models of these cars the side markers do not come on with the turn signals. I believe on some of the European or Japanese models they do. It is something I may want to consider in the future, but I believe that would require two circuits in each light.

 

Those LED Panels, luckily, already have Current-Limiting built-in.

They are designed for direct connection to ~14.5-Volts.

They should probably be epoxied to an Aluminum-Backing-Plate to dissipate any generated Heat.

The above PWM-Controller should work just fine,
but You may find that no dimming is necessary.

NO ELECTRONICS will survive the exterior environment for very long,
unless maybe, You live in the Desert where it's always extremely dry.

You don't need the Plastic-Cover, Knob, and Pot, on the PWM-Controller,
the Pot can be replaced with 2 Resistors after the ideal brightness is found.

Only one PWM-Controller, inside the Car, is required for both Side-Markers.

You may need to put the PWM-Controller inside a Plastic-Box to
protect it from physical-damage, or Short-Circuits.
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The circuit inside that dimmer I suggested as an example could be duplicated with a NE555 timer and a N channel mosfet if you wanted to construct it yourself.

EDIT: Better frequency stability with duty cycle.

 

Those LED Panels, luckily, already have Current-Limiting built-in.

They are designed for direct connection to ~14.5-Volts.

They should probably be epoxied to an Aluminum-Backing-Plate to dissipate any generated Heat.

The above PWM-Controller should work just fine,
but You may find that no dimming is necessary.

NO ELECTRONICS will survive the exterior environment for very long,
unless maybe, You live in the Desert where it's always extremely dry.

You don't need the Plastic-Cover, Knob, and Pot, on the PWM-Controller,
the Pot can be replaced with 2 Resistors after the ideal brightness is found.

Only one PWM-Controller, inside the Car, is required for both Side-Markers.

You may need to put the PWM-Controller inside a Plastic-Box to
protect it from physical-damage, or Short-Circuits.
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I think I'm going to try it for the moment. I don't really consider it to be an exterior environment, if I can get it sealed correctly, but I do recognize that is a challenge especially with the components used in that particular unit. I would only use two to simplify the installation. But if I have to go in the car one will work. Yes there will be a plastic box.

Thanks
John

 

The circuit inside that dimmer I suggested as an example could be duplicated with a NE555 timer and a N channel mosfet if you wanted to construct it yourself.
View attachment 308720

 

Thanks. I actually feel more confident constructing it myself. That way I can pick out quality parts more suitable to the environment. In fact I believe I may already have these part on hand. But I will pickup a couple of those on amazon try out.

Thanks
John

 

If you connect the 5 x 12 volt LED panels in series and change the R LED resistor value to 3.75 ohms (Nearest standard value is 3.9 ohms.) then the circuit that you posted could be used. This is because the LT3761 is a boost regulator. As you require a total power of 4 watts with the 60 volts of the 5 panels in series would require a current of 67 mA. If you had an even number of panels you could use one of the cheap boost regulator modules to drive the required number of series pairs in parallel. You could either set the output voltage to a bit less than 24 volts to reduce the current to the required value or modify the boost module for constsant current output. (I can give you the information how to modify these cheap boost modules for contant current output.) I am assuming your LRD panels consist of groups of 3 LEDs in series with a current limiting resitor for each group of 3.

Les.

 

If you connect the 5 x 12 volt LED panels in series and change the R LED resistor value to 3.75 ohms (Nearest standard value is 3.9 ohms.) then the circuit that you posted could be used. This is because the LT3761 is a boost regulator. As you require a total power of 4 watts with the 60 volts of the 5 panels in series would require a current of 67 mA. If you had an even number of panels you could use one of the cheap boost regulator modules to drive the required number of series pairs in parallel. You could either set the output voltage to a bit less than 24 volts to reduce the current to the required value or modify the boost module for constsant current output. (I can give you the information how to modify these cheap boost modules for contant current output.) I am assuming your LRD panels consist of groups of 3 LEDs in series with a current limiting resitor for each group of 3.

Les.

Thanks Les. I have already ordered the one sghioto recommended and will also try building his circuit. But I certainly want to give this a try at least for educational purposes and if I am successful who knows. I do see a lot of challenges figuring out how to make this circuit run with standard automotive voltage of about 12V to 15V. Particularly how do I scale all the RLC components for 12-15V. The buck/boost operation of this chip completely eludes me. The paper explains that the chip itself is capable of inputs of 8 to 60V and outputs of 0 to 80V. It also states that this example is for a 60V input and it appears they are putting 60V across the LED string and Rled. I don't know any vehicles that use a 60V system, but maybe they meant it would boost any automotive voltage between 8V and 60V to 60V. I don't know how that magic occurs.

The LED's are series 5050, which I could only find one of and they appear to be either 3.1 or 3.3 Vf. Since the matrix is 8x6 and it is really designed for automotive use ~ 14.5V, I'm guessing they are in groups of 4 or else that would be over driving them by about 50%

Is there an advantage to boosting to a higher voltage and putting more LEDs in series. I don't understand why I need additional hardware to drive them in parallel.

 

The LED Boards that You linked to are already configured for the built-in Current-Limiting-Scheme,
which probably only consists of appropriately sized Resistors,
to allow direct connection to an Automotive-Electrical-System, (~14-Volts ),
so no further "Voltage-Adjustments" are required,
only a PWM reduction of output "IF" they turn out to be "too-bright" for your tastes.'

A PWM Dimmer doesn't require any special Voltages either,
it's just a fast switching "on-off" oscillator, with an adjustable Duty-Cycle.

And, You may not need any Dimming at all, as in, zero Electronic-Components.
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The LED Boards that You linked to are already configured for the built-in Current-Limiting-Scheme,
which probably only consists of appropriately sized Resistors,
to allow direct connection to an Automotive-Electrical-System, (~14-Volts ),
so no further "Voltage-Adjustments" are required,
only a PWM reduction of output "IF" they turn out to be "too-bright" for your tastes.'

A PWM Dimmer doesn't require any special Voltages either,
it's just a fast switching "on-off" oscillator, with an adjustable Duty-Cycle.

And, You may not need any Dimming at all, as in, zero Electronic-Components.
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Dimming is necessary for 2 reasons. 1, they are way too bright, 580lm each compared to the original 35lm bulb. 2 with 5 of them the power requirements will be 5 to 6 times too high and will cause a bulb out indication in the car or possibly damage the bulb out warning module. I do agree the PWM thing from Amazon will do the job and I have it on order. I would assume this particular array of LEDs is wired as groups of 4 in 12 parallel circuits. That would be slightly over driven and make no resistors required for a reasonable lifetime. But I took the double sided tape off the back and there are 8 resistors,

 

Any further discussion will require supplying the Wiring-Schematic for your Car.

Leave the original side-marker-bulbs attached in their original configuration, but hidden elsewhere.
Where to power the LEDs will depend on your Car's Wiring configuration.

Having Side-Markers that are brighter that the stock bulbs, isn't necessarily a bad thing.
Use just 1 LED-Board per side,
there is no need to use multiple boards when You are placing a Prismatic-Lens over them.
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