# LED PWM Help

I am trying to control a large bank of high power LED's using PWM. My setup is as follows.

When this bank is at 100% brightness it consumes 18V @ 3A. I am powering the bank of LED's with a MOSFET connected to a 150w 24V power supply. Since the power supply is a higher voltage than is required by the LED's I never let the duty cycle of the PWM wave exceed a certain percentage. At point 'A' I have a current sensing board so the Arduino never lets the current exceed 3A. I figured this would be a good way to control the current flow.

That said I have assembled the circuit. It works however, due to the large on and off current draw the power supply humms pretty loudly I placed a large capacitor over the power supply and it supressed it's loudness by about half. However I am wondering is there a better way to do this? I can only assume that the square wave high current draw is NOT good for the power supply.

Perhaps a current dropping resistor is in order.

24V - 18V = 6V (voltage drop across the resistor)

6V / 3A = 2 Ohms (Dropping resistor)

6V * 3A = 18 Watts (Power dissipated in the resistor)

If you can find a 2 Ohm 20 Watt resistor that should help. Then you can get rid of the current sensor and use the full PWM range.

Thanks John. I may have to try that to further optimize the setup.

In the meantime, does anyone know if it is bad to pull a large amount of current from a power supply using PWM? The PWM frequency is audible in the power supply. The fact that the PWM frequency is audible (to me) means that pieces inside the power supply must be virbrating (like the coils on transformers, chokes etc...) I wonder if prolonged use like this would cause damage to the power supply.

Keep in mind I know that tranformers hum because of the AC current flowing through. This is actually quite loud because it instantaeously draws nearly 150 w then a fraction of a second later it draws 0 w.

It is not likely that you have any '3.6v LEDs' since LEDs are rated by how much current should flow through them not by how much voltage should be applied to them. With the correct current the resulting forward voltage is a characteristic, not a rating. You may have some pre-assembled LED arrays that internally contain the required series resistance so that they operate correctly when supplied with 18 volts.

If you have 3A at point 'A' and if all the LEDs in your drawing are absolutely identical (not likely) then there will be 1A flowing through each LED. That's a lot of current. If these are pre-assembled arrays then each one probably consists if several parallel strings each with it's own current limiting resistor.

You should try to find a lower voltage supply, preferably an 18 volt supply.

Have you considered controlling each branch separately, with it's own MOSFET? That way you could have each branch on at a different time and equalize the current drawn from your supply.

Don

does anyone know if it is bad to pull a large amount of current from a power supply using PWM?

You are probably pulling a lot more peak current than you think. PWM only controls the average current NOT the peak current. This is what is overloading your power supply. If you don't fix it you will kill your supply and probably your LEDs as well. This is a very bad design.

Floresta,

These LED's are actually 3.7 volts max and 600mA. They are very bright and require heat sinks to dissipate heat. Thus I beleive the current across the whole bank should be 1.8A NOT 3A. My mistake.

These are the LED's I'm using Cree XLamp XM-L LED Star
This is the power supply that I'm using: http://www.allelectronics.com/make-a-store/item/PS-24150/24-VDC-6.5A-150W-POWER-SUPPLY/1.html

I would have liked to find an 18V supply, it's just that they are very hard to find in the wattage that I need without spending a few hundred dollars.

Grumpy_Mike,

PWM only controls the average current NOT the peak current. This is what is overloading your power supply.

That is a very good point. Thank you. Any reccomendations as to how to improve the design?

I would have liked to find an 18V supply,

That would not have helped you. You can't drive LEDs by just applying the forward volts drop to them because the forward volts drop is not a stable value. It changes with LED age, LED temperature and ambient temperature. So while you might have a design that draws the correct amount of current at 25oC, if it becomes a hot day say 30oC you might exceed the current ratings and severely shorten the life of the LED. I am not saying that the LED fails, just that the half life (time for the full brightens to drop to half the value of new lights) can be seriously shortened.

You need something in the circuit to limit the current, with low power LEDs this is done simply with a series resistor. However, as the current rises and so the resistor value drops and any affect on the forward voltage is not mitigated as much by the resistor.

With the PWM there is nothing to limit the current during the on part of the wave. This causes excessive ripple current from the power supplies capacitors that could cause them to explode. It also stresses the other PSU components as well as the LEDs themselves.
You need to feed the LED chain with a constant current supply, one for each series chain of LEDs (that's three for you).
There are lots of designs an options about and most of them can be PWMed to provide dimming.

Grumpy Mike,

Thanks so much. You have brought up many good valid points. I think I have to hit the drawing board again.

These LED's are actually 3.7 volts max and 600mA.

Where are you coming up with these figures. I followed the link you gave and downloaded the documentation. They look like big honking LEDs but they seem to have the same characteristics as any other LED (with larger numbers).

Don

Where are you coming up with these figures. I followed the link you gave and downloaded the documentation. They look like big honking LEDs but they seem to have the same characteristics as any other LED (with larger numbers).

Don

Don,

My apologies. ledsupply.com seems to have changed their datasheet for the Cree LED's that they now carry. The correct datasheet is below.

As you will notice on page 4 in the 'Characteristics' section

Characteristics Unit Minimum Typical Maximum
Forward Voltage (@350mA) V 3.3 3.9
Forward Voltage (@700mA) V 3.5
Forward Voltage (@1000mA) V 3.7

I figured I was going to run the LED's around the 700mA range in an effort to make them never reach their maximum temperature/current. I figured this would further extend the life of the LED's.

You have to understand how a device operates in order to correctly interpret the information on the 'Characteristics' chart. The maximum DC Forward Current values are ratings that you must never exceed and you must design your circuit so that they are not exceeded. On the other hand the maximum Forward Voltage values are characteristics that you can expect to observe. You will typically get values near those shown in the 'Typical' column but you will never get a value more than that shown in the 'Maximum' column. You use the typical forward voltage characteristic to determine the resistance that will keep you below the maximum forward current rating.

Whoever drew the graph of Forward Current vs Forward Voltage would have gotten a very low grade in any of my classes. The Forward Current is the independent variable (the one you control) and should be on the horizontal axis. The Forward Voltage is the dependent variable (the one that results from what you are controlling) and should be on the vertical axis. This graph gives you information for many more situations than the three in the 'Characteristics' chart, just remember that you start with the current you want and use that to determine the resulting voltage.

I figured I was going to run the LED's around the 700mA range ...

I don't know much about 'big honking LEDs' but the little ones usually work very well at half their maximum rated current and frequently work well at a lot less than half. No matter what current you choose you start your design with the current, not the voltage.

Don

Thank you Don!
Your clarification helped alot! Excuse me for being such a newbie I DO appreciate your patience!