MUFF WIGGLER Forum Index
 FAQ & Terms of UseFAQ & Terms Of Use   Wiggler RadioMW Radio   Muff Wiggler TwitterTwitter   Support the site @ PatreonPatreon 
 SearchSearch   RegisterSign up   Log inLog in 
WIGGLING 'LITE' IN GUEST MODE

Negative voltage regulators and heatsink surface...wtf..?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Negative voltage regulators and heatsink surface...wtf..?
devinw1
OK, so I've been beating my head against a wall trying to solve an issue with my power supply and figured it out:

My power supply happened to end up with the +15V and the -5V regulators next to each other, and the +5V and -15V next to eachother. THinking I was saving some work, i found some dual TO can heat sinks and bolted each of the above pairs to the heat sink. After reading the datasheets on the MC7900 (neg) series, they connect the INPUT pin to the heat sink surface. On the positivie regs, the ground is connected to the heat sink surface... makes sense! So basically I'm shorting the negative voltage input from the transformer to ground when heat sinking them together.

My question is, WHY on earth would they do that? Input pin to a heat sink just seems dangerous and stupid. Unless I'm missing something.
khakifridge
I understand that it's the terminal with the lowest potential (voltage) that's connected to the tab, so as you say: input for negative regulators and GND for positive ones. But I've not heard the reason for this. hmmm.....
fonik
Just use mica isolators for your TO regulators.
fonik
fonik wrote:
Just use mica isolators for your TO regulators.


something like this:
http://www.reichelt.de/mica-washers-for-housing-to-220-glimmer-to-220- p8194.html

and this:
http://www.reichelt.de/insulating-bushing-for-to-220-and-top-3-housing s-ib-16-p211665.html
Synthiq
khakifridge wrote:
I understand that it's the terminal with the lowest potential (voltage) that's connected to the tab, so as you say: input for negative regulators and GND for positive ones. But I've not heard the reason for this. hmmm.....

To minimize the thermal resistance between the silicon chip and the package, the bottom of the chip is connected to the tab. I assume for the process they use for these regulators, the bottom is electrically the most negative voltage.
devinw1
Synthiq wrote:
khakifridge wrote:
I understand that it's the terminal with the lowest potential (voltage) that's connected to the tab, so as you say: input for negative regulators and GND for positive ones. But I've not heard the reason for this. hmmm.....

To minimize the thermal resistance between the silicon chip and the package, the bottom of the chip is connected to the tab. I assume for the process they use for these regulators, the bottom is electrically the most negative voltage.


This makes sense, thermally.

Definitely a head scratcher, electrically haha. Wish there was a little warning in the data sheet. Having someone unknowingly end up with as much as 25VDC or so hot to the heat sink is sketchy IMO.
fonik
no head scratcher, though. there are specific technical solutions (see my post above).
devinw1
fonik wrote:
no head scratcher, though. there are specific technical solutions (see my post above).


Didn't see the links first, sorry! Thanks for that. But, still that is more a workaround than anything. I mean, Mica has a very low thermal conductivity, plus that doesn't address the screw/head/thread interface, which will probably also conduct, especial as the anodization inside the heat sink threads wears away a bit when the screw is tightened in. If one would make those insulating washers out of specialty ceramic like Aluminum Nitride, it would be better. AlNi can be as high as 180 W-mK or more. But again, I don't know what you do about the screw besides putting a sleeve around it and it's own insulating washer. At that point you've added 3 parts just to deal with an annoyance of how the regulator makers decided to do it. Bah.

But, I wonder what the difference in heat transfer coefficient would be if the regulator makers just connected the ground to the case on the negative models, vs. as-is and then using even a highly thermally conductive electrical insulator.

Anyhow, my workaround for the first prototype was to saw the gang heat sinks apart. I will re-pop the power supply board before I assemble the whole thing and think I will just use separate heat sinks for all 4 regulators and just deal with the fact that 2 of them will be hot to the (-) DC supply. seriously, i just don't get it
Revok
Use a mica or sil pad and an insulating washer. This is pretty standard.

Jarno
Not sure about the rationale, but I think they will have had a good reason to design regulators this way.
Apart from mica, you can also use a silicone pad (silpad) or ceramics (expensive).
The metal screw is an option, you could use a nylon screw. At work we have a module in which the power transistors are clamped to a heatsink using a sort of springy fingers.

Also, be aware that there often are package variants which are fully insulated, I couldn't find them in my search just now, but I am sure I have a few in my parts bin, 78xx (not the low power version). The drawback is worse thermal transfer from die to heatsink, but this might not be a problem for you.
cretaceousear
For my first dual rail PSU I connected up without the mica washers. Magic smoke with free bonus of magic pops. Dead Banana
devinw1
cretaceousear wrote:
For my first dual rail PSU I connected up without the mica washers. Magic smoke with free bonus of magic pops. Dead Banana


LOL. I was almost there, but I believe the black anodization of the heat sink was providing quite a bit of resistance (hard anodize is actually entirely electrically insulating, but not regular type 2), so it was just moving a LOT of current to ground but not an outright short. The first couple of times I tried to fire it up the negative regulators got CRAZY hot, but amazingly everything survived! Dead Banana
fonik
agaon what revok said, i posted links for both the mica layer and the srew isolator. these are no workarounds, these are the standard.
devinw1
Revok wrote:
Use a mica or sil pad and an insulating washer. This is pretty standard.



Thanks man!
MUFF WIGGLER Forum Index -> Music Tech DIY  
Page 1 of 1
Powered by phpBB © phpBB Group