you know when it all goes wrong? very simple question

[Si992]

i apologise if this isnt a straight forward simple question to ask you guys, uve more than once helped me on this project (to build a MOSFET amp for uni) and im more than grateful, but with 20 hrs to go my project supervisor has not ordered let alone recieved the MOSFETS i asked him to get. So a very quick drive to get to the electronics shop before it shuts (via one police road block due to an accident, a flat tyre and getting lost i make it just in time).

Anyway a little bit stressed i go in and ask for any MOSFETs they have, fantastically they have one exicon 10N20 and one 10N20 left which i hope will do the job nicely. i promptly buy them and a suitable heatsink, however theyre in a TO3 packet. not a problem you may think? or maybe it is i dont no.

http://www.maplin.co.uk/images/Full/19075i0.jpg

Only problem is, after 3 years of electronics degree and it being 100% theory no practical work (yeah stupid i know - big regret chooing this course) im completely thrown by them not being the standard transistor 3 legged packet. After looking at the data sheet from what it looks like i just attatch the source one of the large holes on the side? is this correct? does this then not conduct charge to the heatsink?

i apologise if im being very dumb here but my body is currently being sponsered by nescafe and redbull and i would really apreciate some one just telling me if thats correct or not.

i apologise again for the rant, as u can tell im a bit pissed off considering the rest of my amp has been complete for 2 weeks and have just been awaiting my MOSFETS but assured that they would be here by my irritaing supervisor :

[pinkmouse]

Yes, the case is part of the electrical circuit, and you either need to insulate the devices from the heatsink, or have separate heatsinks for each rail, insulated from the chassis, and not touchable by fingers.

[Si992]

Thanks, any suggestions on what to insulate it with? have fairly minimal resources at home.
Si

[pinkmouse]

Easiest would be to use two heatsinks, one per rail and mount them using plastic fittings inside the case. To isolate the devices, you really need special high temperature mounting kits, like these.

[Magura]

Originally posted by Si992
Thanks, any suggestions on what to insulate it with? have fairly minimal resources at home.
Si

Quick and dirty...a couple of sheets of paper soaked in whatever oil you got at hand.

Don't forget to insulate the screws!!!!

Magura

[Si992]

ive got 2 seperate heatsinks so thats fine, guess its gonna have to be a "do not touch" heatsink, thanks for the advice, ill get cracking on the best i can another cuppa coffee required first though!

[megajocke]

Originally posted by pinkmouse
Yes, the case is part of the electrical circuit, and you either need to insulate the devices from the heatsink, or have separate heatsinks for each rail, insulated from the chassis, and not touchable by fingers.

Umm... doesn't these have the source connected to the case. If there are no source resistors it should be possible to have them on the same.

[pinkmouse]

How do you work out that? The sources will be connected to the voltage rails, perhaps via source resistors, perhaps not, meaning you're feeding both V+ and V- to the same lump of metal. BANG!

[megajocke]

Sure? Which amp design is it?

The only amps with lateral mosfets in common-source configuration i have seen are some of these:
http://perso.wanadoo.fr/jm.plantefeve/sche.html

Most amps I have seen using lateral mosfets have the sources connected to output, for example Elektor Crescendo.

[Jean-Marc P]

Hello,

Yes, Exicon's 10N20 and 10P20 are "lateral mosfet" with case at source.

Common source or commain drain on diagram, you do insulate cases to heatsink, heatsink certainly grounding by the chassis amplifier.

Sorry for my poor english, however I try here my first message, thanks for your indulgence.

Jean-Marc Plantefève.