For the REAL EE's out there

SkiFletch

SkiFletch

New member
So with summer approaching, I've kicked my LED build into high-gear cause I just can't handle the halide heat anymore. Driving me flipping crazy. I've built a rig, mounted the LED's, wired them, tested them, etc etc. Built drivers based on dzwm's CAT4101 discovery and everything works great. One problem... the chips draw a decent amount of 5V power. I had originally sourced a TL760L series chip to regulate from 24V (my supply) down to 5V to power the CATs. However the current draw is a little higher than I had exepcted and my regulators get HOT, like 80+C hot even with a heat sink on top of them.

So my question is, do any of you have any ideas for how to drop 24V to 5V in a relatively compact package without creating more heat than I can deal with? I'd really rather not use another power supply and create another penatration for my box for this. Any advice would be great. :)
 
24VDC to 5V in one step is a lot especially at 500mA. you may want to take 2 steps:

LM7815CT (gets you down to 15VDC at 1A)
LM340T-5.0/NOPB (gets you down to 5V at 1A)

HTH
Nico
 
Based on the current spec of each component your running, how much current will you be drawing?

There are a bunch of options here, Mike.
 
SkiFletch said:
So with summer approaching, I've kicked my LED build into high-gear cause I just can't handle the halide heat anymore. Driving me flipping crazy. I've built a rig, mounted the LED's, wired them, tested them, etc etc. Built drivers based on dzwm's CAT4101 discovery and everything works great. One problem... the chips draw a decent amount of 5V power. I had originally sourced a TL760L series chip to regulate from 24V (my supply) down to 5V to power the CATs. However the current draw is a little higher than I had exepcted and my regulators get HOT, like 80+C hot even with a heat sink on top of them.

So my question is, do any of you have any ideas for how to drop 24V to 5V in a relatively compact package without creating more heat than I can deal with? I'd really rather not use another power supply and create another penatration for my box for this. Any advice would be great. :)
Click to expand...

Is that English??? :hmm3:
 
Lol, I think there's English in there somewhere Cully ;)

So I'm driving 12 CAT4101 drivers here, half of each with their own 5V regulator. If I'm reading the datasheet correctly, the quiescent current at 700mA drive current on the CAT4101 is 6mA. That means I'm using 36mA to drive 6 of these little guys. So P=IV, or P=0.036*19. P = 0.684 watts... OK, that's significant, but it doesn't seem like the kind of energy that would pop up to 80C with a heatsink on top of it?

Maybe I'll just try sticking a 7815 "in between" and take it in two half-steps. That might at least get me down to something where a heatsink can take care of the thermal issues...
 
Mike,

In your power equation, where are you getting the 19V from?

After looking at the data sheet for your driver, I assume you want the 5V to power the IC (Vin, pin 2)? Anyway, to answer the original question I think your best option would be a dc to dc converter. You could get one pretty cheap that you could power the remaining six IC's with.

I tried looking up info on the TL760L IC you referred to and came up with nothing.
 
19V dropping from 24 to 5. Converters like that will surely work, I'd just love to be able to do it with things I have on-hand rather than waiting forever for something to get shipped from ebay.

must have typed the wrong part number, will re-check when I get back into work tomorrow... Long story short it takes input voltages up to 35V and outputs 5V 150mA. We use them in a lot of our other applications powering 555 timers, opamps, etc but they're typically only dropping 12V to 5V and we're talking only a couple milliamps, so it's enough to handle that. I guess the CAT's just draw a little too much...
 
I got my number from your TL760 part number. It actually puts out 500mA but at 3.3 volts. So the lower voltage will increase the current even more.

It still is going to come back to the LED wattage because each CAT can drive upto 1A

Nico
 
How about 7 silicon diodes in series with an appropriate series resistor? The drop across the diodes will be 4.9v. You probably have those laying around.
 
OK so I figured, why guess as to my current draw when I can measure it... I have this technology :D. Measured the current at 38mA, I guess my math wasn't too bad. So in that case, I'm just gonna use a 7815 in the middle, do the drop in two half steps and I should be able to manage the heat that way. Thanks guys :)
 
Nico, it's 38mA on the 5V supply to the LED driver. The driver is a CAT4101 chip which is dirt simple in it's operation. Feed it 5V control voltage, add a capacitor for stability and a sense resistor to set the current, and it will regulate the LED current through the negative side of the circuit. The CAT4101 modulates the 24V supply down to 23.5 at 700mA, so it is indeed doing a fair amount of electrical work, but that's what it's supposed to do. I've heat sunk the pee out of those, they're not the problem. They do however draw more than what I'm used to for small IC's in terms of control voltage. I'm used to dealing with 555 timers, simple opamps, comparators, etc which draw control voltage in the microamp range, thus small simple linear voltage regulators are a cheap/easy/quick solution for that.
 
Now I'm a "real ee."

2011-05-18_20-13-49_588.jpg
 
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