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IC LED Driver, Low Current

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Chips & Chips

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I am designing a control circuit that uses 24VAC for the controls, ultimately switching 115VAC On and Off. In adding the bells and whistles, I want a couple of LEDs that indicate the ON condition and a safety condition. Both would be illuminated by the presence of that 24VAC control Voltage. I know I need to rectify the AC to DC and perhaps filter it, but that part is easy with diodes and capacitors. What I would like is to use a switching IC driver to give me the constant current needed for the LEDs instead of using resistors. The LEDs only draw 25mA, but even at that current level, the resistors calculate to 1.2K Ohms and 0.77W. So I need a 1W or two 1/2W resistors per LED and I will be wasting 1.5 or 2.25 Watts of energy when the LEDs are lit.

What I would like is a simple switching style IC driver where the current can be programmed with a single resistor or two. That would waste a lot less power so it appeals to my conservative side. Oh, and it should be cheap, of course. If it could handle 50mA that would be better as I may want to use two LEDs for the power on light. I am imagining a three or four pin device, preferably with accessible pins for soldering. I do not need any fancy things like dimming, just ON and OFF. I estimate the input Voltage, after rectification and filtering, to be around 33 VDC.

I searched before posting this so I hope I am not being redundant. I tried to search on Digi-Key but got tens of thousands of returns. It would take me forever to go through them. Any suggested ICs would be appreciated.
 
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There is no way around the problem, if you drop the current in silicon rather than a resistor, the same overall power will be dissipated. Vdropped x I = power no matter how you do it.

Your options are:
1. to drop the supply voltage to the LEDs so lower value resistors are needed and less power has to be lost.
2. drop the AVERAGE LED current using a pulsed driver but note that the peak current may be even higher and possibly exceed the LEDs maximum rating.
3. a combination of a resistor to limit the maximum to a safe level then a pulsed driver to reduce the average.

Option 3 is most power efficient but still produces some heat and it is most complicated electronically.

Brian.
 

Thanks for the reply. I guess you are saying that there isn't any cheap and simple LED driver out there. Or, at least, you don't know of one. But I seem to have heard the words "LED driver" somewhere. What does that mean? Are they just for high power LEDs?

I know that this is a rather trivial request to replace a couple of half Watt resistors with an IC, but I would have thought that something would be out there. What about digital versions of the 78xnns? Can they be used as a constant current mode like the original 78xnns can, using a single resistor?
 

"LED driver" generally refers to a device that can switch or control the LED current but they have no magical property that defies Ohms Law. I'm afraid they dissipate just as much heat under the same circumstances. Most work on switched mode principles where power is transferred through an inductive medium and their complexity, size and cost is much more than a single resistor, especially when you need one circuit per LED.

Using a 780x in constant current mode will work but if you calculate the power dissipated "(Vsupply - Vled) * Iled" you get exactly the same value as using a resistor but at the expense of an IC and capacitor as well. You also have to consider the maximum input voltage the regulator can withstand.

It would help if you could post a schematic of the circuit so we can see if there are other solutions. For example, knowing how the 24V AC is derived might give clues about alternative configurations.

As the circuit is AC, it might be possible to use a capacitive dropper but there are potential drawbacks, especially cost and capacitor size. You would need a diode and a NON POLARIZED 3uF capacitor instead of the resistor.

Brian.
 

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