[sdiy] voltage regulator insulation

[Oscar Salas]

> Hello Guys,

Hello :)

> I'm currently building a +/-15V, 1A PSU for my  dual Mega Percussive
> Synth and some other drum voices.
> I use an LM317 and LM337. I want to use the metal case as heatsink.
> Now I read somewhere that you need to put an insulation plate between
> the metal case and the LM's.

Accordingly to my measurements on LM317/LM337 regulators' cases and
junction temperature estimations, using a 6U 19" plate as a heat-sink
(2,5mm thickness, natural anodized aluminum)and assuming worst case
situations like 40 Celsius of ambient temperature and 10% of high mains
voltage fluctuations, the maximum current output should be 0.8A. Actually,
I did other measurements using other heat-sinks and my conclusion is that
using these chips I wouldn't output more than 0.8A for passive dissipation
assuming the above worst case situations. (36VCT transformer for +/-15V)

This is my power supply using LM317/LM337 and the case as heat-sink
http://electronic-sea.net/ps1.html
This is the one that I developed using MIC2941
http://electronic-sea.net/ps2.html which due its less regulator's voltage
dropout (lower voltage transformer is needed) and its lower -junction to
case- thermal resistance, the power dissipation becomes more simple.

> Is this really needed? What about the bolt that I'll use to hold the LM's?
> And where do I get this insulation plate? What kind of material is it?

Yes it is needed. For heat transfer and insulation. I use the pad "farnell
code 681118", an insulating bush "farnell code 936649", and then the bolt
the nut and the washers, for example "farnell Code 520160". It is VERY
convenient the "spring washer"

> And a related question, which ground do I connect the case with? Do I
> use the DC ground or the mains ground.

If the case is conductive it is mandatory to be connected to the earth of
the mains connector.

Also all the parts with mains voltage must be correctly insulated.
As ASSI said have a professional electrician to check it before connect to
the mains.

Oscar.
electronic-sea.net