What power supply for CAT4101?

reickler · 04/23/2012, 01:19 PM

Hello,

I'm looking to build some CAT4101 drivers to power 72, 3.7v LED's running at .7A. I'm an electronics novice, so I need a little hand-holding, so here goes...

If this stuff is sinking in, then I'd need 12 "CAT's" (72 LED's / 6 Led's per string/CAT4101 @24v), right?

So 3.7v at .7A comes out to be 2.59 watts per LED (I think). Does that mean I need about a 24v, 250 watt power supply, or how exactly would I power such a beast?

Also, can I put that many CAT's on a circuit, or should it be broken up into a few separate boards? I've not seen any schematics for that many. I'm not sure if it's because the design just repeats, or maybe because people don't need that many, or maybe because my fish will boil if I use that many?

Thanks in advance,

Ron

der_wille_zur_macht · 04/23/2012, 01:32 PM

Basically, you need to determine the Vf of your entire string, then aim for half a volt above that - the CAT4101 needs half a volt minimum to operate. You CAN go higher than that, but you're just wasting power, and after you get more than a few volts above the Vf, the devices will overheat and shut down.

So with 6 LEDs at 3.7v each you're looking at 22.2v Vf, which means ideally you'd have a supply with 22.7v. You won't find something with that exact value off the shelf, and doing math on Vf is always subject to a good 10 - 20% inaccuracy thanks to the nature of the LEDs, so the BEST policy is to get a power supply with a trimpot. Then, you can experimentally determine the correct voltage and adjust accordingly.

Typically-used supplies look like this:

http://www.mpja.com/24VDC-18A-POWER-...info/17434+PS/

See how it has a tiny trimpot labeled "Vadj" on the terminal strip? That's what you use to adjust the voltage. Basically, you'd build the array, start with the power supply turned all the way down, and then slowly turn it up while you measure voltage across the LEDs. At just the right point, the voltage across the LEDs will stop increasing (it'll be right around the 22.2v number). That's where you leave it.

If you are running multiple LED drivers on a single power supply, you need to check that the sum of the current used is within the supply's limit. If the LEDs are running at .7A then you might plan on .9A for each string (to allow for some headroom). So, if you needed 12 such circuits, you would need 10.8A. You could use a single supply that had more than that current, or you could break up the strings onto multiple supplies - for instance, there's a common model of 24v supply that provides 6.4A, so you might use two of them.

reickler · 04/23/2012, 01:40 PM

Well stated. Thanks for the reply.

04/23/2012, 01:19 PM	#1
reickler Registered Member Join Date: Apr 2012 Posts: 4	What power supply for CAT4101? Hello, I'm looking to build some CAT4101 drivers to power 72, 3.7v LED's running at .7A. I'm an electronics novice, so I need a little hand-holding, so here goes... If this stuff is sinking in, then I'd need 12 "CAT's" (72 LED's / 6 Led's per string/CAT4101 @24v), right? So 3.7v at .7A comes out to be 2.59 watts per LED (I think). Does that mean I need about a 24v, 250 watt power supply, or how exactly would I power such a beast? Also, can I put that many CAT's on a circuit, or should it be broken up into a few separate boards? I've not seen any schematics for that many. I'm not sure if it's because the design just repeats, or maybe because people don't need that many, or maybe because my fish will boil if I use that many? Thanks in advance, Ron

04/23/2012, 01:32 PM	#2
der_wille_zur_macht Team RC Member Join Date: Sep 2003 Location: NY Posts: 17,749	Basically, you need to determine the Vf of your entire string, then aim for half a volt above that - the CAT4101 needs half a volt minimum to operate. You CAN go higher than that, but you're just wasting power, and after you get more than a few volts above the Vf, the devices will overheat and shut down. So with 6 LEDs at 3.7v each you're looking at 22.2v Vf, which means ideally you'd have a supply with 22.7v. You won't find something with that exact value off the shelf, and doing math on Vf is always subject to a good 10 - 20% inaccuracy thanks to the nature of the LEDs, so the BEST policy is to get a power supply with a trimpot. Then, you can experimentally determine the correct voltage and adjust accordingly. Typically-used supplies look like this: http://www.mpja.com/24VDC-18A-POWER-...info/17434+PS/ See how it has a tiny trimpot labeled "Vadj" on the terminal strip? That's what you use to adjust the voltage. Basically, you'd build the array, start with the power supply turned all the way down, and then slowly turn it up while you measure voltage across the LEDs. At just the right point, the voltage across the LEDs will stop increasing (it'll be right around the 22.2v number). That's where you leave it. If you are running multiple LED drivers on a single power supply, you need to check that the sum of the current used is within the supply's limit. If the LEDs are running at .7A then you might plan on .9A for each string (to allow for some headroom). So, if you needed 12 such circuits, you would need 10.8A. You could use a single supply that had more than that current, or you could break up the strings onto multiple supplies - for instance, there's a common model of 24v supply that provides 6.4A, so you might use two of them. __________________ Inconveniencing marine life since 1992 "It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)

04/23/2012, 01:40 PM	#3
reickler Registered Member Join Date: Apr 2012 Posts: 4	Well stated. Thanks for the reply.