Cooling several LM350s

[MagicMatt]

I have a conundrum

The heat tab (or whatever you call it) on the top of a TO220 package would be perfect to strap to a large heat sink. I have some from various computers, and I could fit quite a few TO220's to the copper heat sink and cool them gently with the fan already attached.

However...

The tab is not electrically isolated. It appears to be connected to Vout... so I can't strap several onto the heat sink as this is effectively strapping all the Vouts together - not a good thing when you're using them as current regulators!

Is there a way I could attach them and use the heat sink, or am I forced to use seperate heat sinks?

 

[duke37]

You can get insulating kits which consist of a mica or plastic sheet and a plastic bush to go in the TO220 hole. Heat sink compound should be used with the minimum possible thickness, just enough to eliminate any air gaps.

Could you modify your circuit so that the collectors are all at the same voltage? If you do not use insulators, you get better cooling.

 

[MagicMatt]

I'm driving LEDs... I don't know how I'd make the modification, since the different colours have different Vf, and I am intending to put a transistor on the output of the LM350 to give me some PWM control.

I considered using resistors instead of the LM350, but apparently regulated is better in case of an LED faulting.

 

[duke37]

Sorry, I did not read the requirement correctly, I interpreted the TO220 as a transistor so my second sentance was rubbish

 

[MagicMatt]

No problem.

Thanks for the info about the insulators - Is this what you mean?
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=7128225
I'm reading it right - a kit has 10 of each, so would do 10 devices...?

 

[duke37]

Quite correct, but your current source can do up to 3A. What current does the led take? Surely resistors would do.

 

[MagicMatt]

The current draw is set to 638mA, and Vf ranges from 3.9V (white) to 2.1V (red). In the current configuration there's 4 sets of 2 LEDs in series, so it could be drawing just over 2.5A from the PSU.

I'm using resistors on the LEDs for the power light, LCD backlight, etc. but they're only 20mA each.