parallel constant current drivers

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aj08

New Member
Hi there, some of you helped me last year with a problem, I have a new one. I need to control a very large LED array and want to do it with commercial constant current drivers, using a pic to vary the brightness with PWM.
The drivers I have identified for the job are these:

http://docs-europe.electrocomponents.com/webdocs/0aba/0900766b80aba022.pdf

There are not many complete drivers around with such a high Vout, unfortunately I need a higher current than these modules can supply individually, would it be possible to parallel their outputs?
I think this would work with, say two 500mA modules to give 1A but could I get away wiring a 500mA and a 350 mA together for 850mA? I've read that this will work but it sounds a bid dodgy to me can anybody exlpain how it would work?
Thanks in advance
 
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gengis

New Member
You can get away with parallel constant current linear supplies as a general statement, and then they don't share the load equally.

I wouldn't hazard a guess about a switching "black box" unless the manufacturer recommends it. Your best bet would be to inquire of the manufacturer.
 
A

aj08

New Member
Thanks for the link, the problem with that, and other similar regulators I have found, is I'm not set up for SMT work, and I need 6 of these:eek:

If I want to control the PMW pin on any of these regulators, I can do it straight fron an output pin of the 08M, Is there a limit to the amount of drivers I can control straight from the pic before I have to buffer it somehow?
 
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tiscando

Senior Member
aj08 said:
Thanks for the link, the problem with that, and other similar regulators I have found, is I'm not set up for SMT work, and I need 6 of these:eek:
Click to expand...
According to a thread in the forum:
  • Buy some soldier paste
  • create the PCB
  • apply the soldier paste over the pads where the SMD ICs are to be located on.
  • position the SMDs correctly on the soldier pads and paste.
  • Put the pcb in a frying pan (chips facing up), possibly on cooking oil to improve heat distribution, and if you have one, put a glass lid covering the PCB.
  • Have tongs etc. ready to lift out the hot PCB
  • Turn the hob on (max). When you see all the soldier paste liquify and be attracted to the IC legs and pads (seperating from each other hopefully), turn the hob off and lift the PCB out.
  • The ICs should be fully soldiered perfectly in place without any bridges. (the ICs seem to align themselves when the soldier melts.)
  • Soldier in any through hole components like normal.
I found this information very useful. :D

TC
 
T

tiscando

Senior Member
aj08 said:
If I want to control the PMW pin on any of these regulators, I can do it straight fron an output pin of the 08M, Is there a limit to the amount of drivers I can control straight from the pic before I have to buffer it somehow?
Click to expand...
According to the datasheet for the RCD-24-xxx:

@Vr=5V, the PWM/ON/OFF pin consumes 1mA

a picaxe pin can source/sink 20mA, so up to 20 units.

max. PWM frequency: 200Hz
 
B

BeanieBots

Moderator
In theory, you can simply add constant current sources to make a single source which is the sum of the currents. The problem is with 'real' constant current sources and how they measure and control the current.

From a quick look at the device you are proposing, I do NOT think it would be possible to use more than one to produce a higher current.

Do you really have LEDs which require 1A?
If you have the LEDs wired in parallel, then maybe use a seperate regulator for each string of LEDs.
Power LEDs tend to be either 350mA or 700mA max and there are plenty of drivers for those currents.
 
T

tater1337

Member
tiscando said:
According to a thread in the forum:
  • Buy some soldier paste
  • create the PCB
  • apply the soldier paste over the pads where the SMD ICs are to be located on.
  • position the SMDs correctly on the soldier pads and paste.
  • Put the pcb in a frying pan (chips facing up), possibly on cooking oil to improve heat distribution, and if you have one, put a glass lid covering the PCB.
  • Have tongs etc. ready to lift out the hot PCB
  • Turn the hob on (max). When you see all the soldier paste liquify and be attracted to the IC legs and pads (seperating from each other hopefully), turn the hob off and lift the PCB out.
  • The ICs should be fully soldiered perfectly in place without any bridges. (the ICs seem to align themselves when the soldier melts.)
  • Soldier in any through hole components like normal.
I found this information very useful. :D

TC
Click to expand...
this is insane.

but neat!

i'd caution about using a frying pan, someone, somewhere, at some time is goung to do this and end up making breakfast.
I wonder if a toaster oven would work, maybe with a heavy plate to act as your contact point. toaster ovens in my area get tossed out frequently due to being a fad.

maybe seal one, add some pipe fixtures to make a hot air rework station. hmmmmmm
 
A

aj08

New Member
According to the datasheet for the RCD-24-xxx:

@Vr=5V, the PWM/ON/OFF pin consumes 1mA

a picaxe pin can source/sink 20mA, so up to 20 units.

max. PWM frequency: 200Hz
Click to expand...
Thanks Tiscando, I should have seen that:eek:

Do you really have LEDs which require 1A?
Click to expand...
I've attached a pic, these have an absolute max of 750mA but I think with the thermal management I have in place there is a bit of extra headroom if I need it. In this case the problem isn't so much the current (there are a few 1A regs around) but the fact the LED's are wired in rows of 8, requiring a forward V of >30V, not many regs can supply this much power.
I will probably settle for the 700mA driver for starters, then try and get a 1A driver or parallel some smaller ones if I need more output.
 

Attachments

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BeanieBots

Moderator
Be careful!
Absolute max means what it says, no matter how good your thermal management.
Even if you use liquid nitrogen to keep your heatsinks at -40C, there is still a thermal gradient between the junction and the heatsink. Also, that destructive extra little bit of current won't give much extra light.

If you are going for very high voltage drive, it might be worth considering using a variable voltage source and series resistance. With careful design I doubt it would be any less efficient than using a constant current.

A simple method would be to use a 3V switching regulator. Fit your LED string between the supply and the regulator switch with a load resistor on the 3v output. A 3ohm load would give 1A and only dissipate 3W. Might be worth comparing with the disspation spec for a comparable constant current regulator.
 
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