# linear potentiometer with 30w LED (15v~18v)

#### StealthRT

Joined Mar 20, 2009
253
Hey everybody, I am looking into a project that involves placing a 30w LED set inside my old projector (to take the place of the burned out bulb). Problem is, i would like to very the brightness of it as it may end up being too bright for the projector (the light that was in it was rated at 1400 Luminous). Here are the states of the LED:

Lens Color: Water Clear
Emitted Color : White​
DC Forward Voltage (VF): 15 ~ 18Vdc​
DC Forward Current (IF): 1800mA​
Viewing Angle: 140 Degree
Color Temperature: 5500~7500K
Intensity Luminous (Iv): 1500~1800LM​

I have been trying to find a linear potentiometer capable of handling the 15 to 18v's with a 30watt but am unable to find such a thing. What all would i need in order to very the current between the 15v and 18v? I guess if i can not find any potentiometer to do this then i could always run the LED at 15v so it would have 1500 Luminous.

Anyways, i'd love to hear some suggestions on going about doing this since i can not seem to find anything myself.

David

#### SgtWookie

Joined Jul 17, 2007
22,210
You don't control LEDs by voltage, they're controlled by current.

Using a potentiometer (actually, a rheostat) would be very wasteful.

#### Bernard

Joined Aug 7, 2008
5,284
One way is to use a short string of diodes, with a shorting switch for each. V drops available from .5 to 2V @ 3A. with a gen purpose NTE156A, 1000V @ 3.5A,drop is max, 1.1V. So 18- 1.1=16.9.,15.8, 14.7. With 3 switches, any of the 4 voltages can be selected by shorting diodes.

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#### StealthRT

Joined Mar 20, 2009
253
Ah, ok. I was under the impression that it would not work if it had 14v or lower to it. I figured it was:

15v = 1500 Lum
16v = 1600 Lum
17v = 1700 Lum
18v = 1800 Lum

And 1v-14v would not even turn it on... What type of resisters are you using in your diagram? And i take it the "L" in your diagram is the NTE156A?

David

#### Bernard

Joined Aug 7, 2008
5,284
The package does not have its internal current limiting?? Best void my last post. What is your supply voltage? Might be able to use a IC voltage regulator connected as constant current regulator. Sgt. Wookie is the expert on VR's.

#### StealthRT

Joined Mar 20, 2009
253
Sgt. Wookie is an expert of all things electronics )

My supply voltage can either be 15v to 18v but i wanted to make something so i could interrupt it and supply 15.5, 16, 16.5, etc etc.. until i got just the correct brightness for the projector. Since this LED is more powerful than the bulb it will be replacing, I'm pretty sure I'll need to dim it a little in order for the image on the wall to be clear and not just bright white.

David

#### DC_Kid

Joined Feb 25, 2008
794
voltage controlled PWM IC and a fet.

#### StealthRT

Joined Mar 20, 2009
253
voltage controlled PWM IC and a fet.
Care to point me to those things in Mouser.com or Digikey.com ?

David

#### Bernard

Joined Aug 7, 2008
5,284
Your origional bulb @ 1400 lumens = 50W [BRL,FHR, GBE ,or sim.] @ 12V, AC or DC. LED array 32W, but would not like incadescent bulb temperatures. Is there provisyon for heat sinking?, with fan? Is your 18V supply DC? & regulated?,adjustable?

#### StealthRT

Joined Mar 20, 2009
253
I will have a heatsink on the back of the LED setup and either use the projectors fan or just buy a mini fan to attach to the heatsink itself.

The power supply is yet to be determined depending on if i can change the voltage. If not then i would prob. go with a 15v DC power supply or one that can be switched from 15v to 20v. (although, only using 15v-18v for the LED.)

David

#### StealthRT

Joined Mar 20, 2009
253
Something like so:

Input voltage: 1.2 - 37V AC or DC
Output Current: 1.5A max.
Voltage loss: 2.5V

• Output reverse polarity and back-voltage protection
• Output for LED power on indication
• On board heatsink for IC regulator
• Variable output voltage
• AC or DC input voltage
• Low noise (uses linear regulator)

David

#### DC_Kid

Joined Feb 25, 2008
794
voltage controlled PWM IC and a fet.
you can use a 555, a pot, two diodes, and cap to create variable PWM via the pot. the schematic is here on AAC (use the search feature). the FET can be N or P channel as the 555 will source or sink on its output pin.

found the schematic:

Last edited:

#### bertus

Joined Apr 5, 2008
20,099
Hello,

@ DC_Kid
The IC you are pointing at is for 500 mA.

Perhaps take a look at the LM3421.
This is a freely configurable IC with dimming possibility.

Greetings,
Bertus

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#### DC_Kid

Joined Feb 25, 2008
794
The IC you are pointing at is for 500 mA.
Bertus
the datasheet is odd. it's opening statement says ".... with integrated high side FET optimized to drive high power LEDs at up to 2A of continuous current."

#### bertus

Joined Apr 5, 2008
20,099
Hello,

When you look at the features statement is says the following:

The XRP7604 is 1 A, the SP7600 is 2A.

Greetings,
Bertus

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#### DC_Kid

Joined Feb 25, 2008
794
yep, thats why i say the datasheet is weird. perhaps it was to read "1/2A continuous" ..?

i am finding datasheets all over these days that have conflicting info. i am looking at a maxim fan controller ic (6643), on one page the datasheet says "6643 has a 'fainfail' input and will drive 100% when pulled high". but then on one of the app schematics it shows a 6643 with a "not-failfail" input tied to Vdd. either they mis-printed the ic # on the schematic or the text is wrong. waiting to hear back from maxim.

so to all: be weary of datasheets, they're not always correct.