Another MoFo

[pre65]

That's the thought for today (it's Sunday now)...

My, how time flies when you're having fun.

Yes indeed. Go to bed about 2:30 and get up about 10:30, still get 8 hours kip.

[Cressy Snr]

I’m Burlington Bertie,
I rise at 10:30
And saunter along Temple Bar.
As round there I skip
I keep shouting 'Pip Pip!'
And the darn'd fools think I'm in my car.

[pre65]

I’m Burlington Bertie,
I rise at 10:30
And saunter along Temple Bar.
As round there I skip
I keep shouting 'Pip Pip!'
And the darn'd fools think I'm in my car.

I've never been a fan of getting up early, unless I have to.

Mind you, except for Thursdays, Sundays, and when Jean is away at her Mums I get up at 06:00 to do her breakfast, then go back to bed.

[steve s]

I was just expressing a preference, not criticising, Steve...

Sorry ray.. didn't mean to sound like that..

[Ray P]

Sorry ray.. didn't mean to sound like that..

No prob Steve

[ed]

Just out of interest Ray, what were your thoughts on the heatsinks ?

take the risk Phil, plough your own furrow....snigger
just a couple of thoughts from one of the "proud ignorant people lost in the woods" regarding heat sinks.
if you check out the Conrad product catalogue

http://www.conradheatsinks.com/products ... gue-05.pdf

it outlines the necessary processes required to determine what kind of heatsink is required. Most importantly it gives the
coefficient of all their sinks so that even if you don't use their heat sinks you can easily judge what kind/size of sink gives
what kind of coefficient.

Current through a fet at idle will give you a good idea of the heat generated in watts. It's just a matter of determining how
far above ambient you want to run and then getting the required sink with the necessary coefficient. In this respect spice can
be your very best friend.

My thoughts on fans are given in an earlier thread, best not repeated. But it's worth remembering that unlike laterals,
vertical mosfets are very susceptible to heat and do require to be run at certain temperatures to give of their best. Running
too cold(easily possible with a fan) may not produce optimum results.

from the example circuit, with P1 set at 44% the gate voltage is 5.98v giving 1.93 amps through the fet(with rail at 24v) and
1.35 volts on top of the choke.

therefore:
24-1.35v across the fet
1.93 amps through the fet
heat in the region of 22.65 x 1.93 = 43 watts

so to run this circuit at say 20 degrees above ambient you will need a sink with a coefficient around 0.5 degrees per watt.

you can do this by hand using the graph from the datasheet in exactly the same way you would with a valve...but imo spice is
more fun

I apologise if this is sucking eggs.

[pre65]

So, I stripped back one of the 50W Mosfet amp power supplies, and I've laid out the MoFo components as I think they will go for the prototype build.

So far, it's 22v toroid, bridge, small cap (1uf), 5mH @ 10A reactor, 47,000uf @63v cap, 10mH @ 5A reactor and 47,000uf @ 63v cap.

I've used the BIG heatsink and made space for the fan, just in case.

[pre65]

Thanks Ed, good advice. I was so engrossed with tinkering on MoFo I missed your post, and forgot to have any lunch.

[Nick]

It's just a matter of determining how
far above ambient you want to run and then getting the required sink with the necessary coefficient

I would say more important than that is to work out what die temp you will get with the combination of heat sink and power dissipated to ensure the device is working within its limits. And then if you are happy with the temperature of the fins. 50C is a good temperature for a class A heat sink, but CE is happy up to 80.

[ed]

pedant!

and there I was, just trying to be helpful...

snigger

[ed]

I would say more important than that is to work out what die temp you will get with the combination of heat sink and power dissipated to ensure the device is working within its limits. And then if you are happy with the temperature of the fins. 50C is a good temperature for a class A heat sink, but CE is happy up to 80.

Joking aside....I thought a bit more about what you are saying...I have limited experience of the differences between dye an sink but on the few(3 or 4) occasions I have measured it, the difference has only been 4 or 5 degrees or so.

Good point, but not sure if it’s relevant to Phil’s job-in-hand.

I’ve predominantly used sp400 but on the rare occasion I’ve used mica and goop I haven’t noticed any difference. The sp400 datasheet gives a figure of 0.5 degrees per watt per square inch. If I’m reading it correctly, it surprises me, but bears out what you are saying. It doesn’t tally precisely with my findings though.

In my defence I will say that a sink temp of 20 degrees above ambient will put any of the fets that we use for audio well within advised limits, irrespective of transfer losses.

[Nick]

In my defence I will say that a sink temp of 20 degrees above ambient will put any of the fets that we use for audio well within advised limits, irrespective of transfer losses.

Agreed, but I was just pointing out what the actual job of the heat sink was, and secondly if you start to run at or above 50 then its possible depending on how the device is coupled the its worth checking.

Also, if you are using a single device, it may become the limiting factor as the size of the sink increases.

[Paul Barker]

When I’m bored I idly lurk the odd conversation about mofos.

As a bystander I’m thinking ? There seems to be a lot of doing nothing for years except talking about heat sinks.

My bottles bring their own heat sink, saves years on a build.

[ed]

When I’m bored I idly lurk the odd conversation about mofos.
As a bystander I’m thinking ? There seems to be a lot of doing nothing for years except talking about heat sinks.
My bottles bring their own heat sink, saves years on a build.

Not sure of the point, seems to me you may be frustrated at the slow progress of these mofo builders and you're just itching to see the finished product because you secretly are in love with the SS way of life.

snigger

My bottles bring their own heat sink, saves years on a build

my experience has been the opposite(having played in both camps)....

bottles:

v.expensive (in comparison)
wear out
require expensive iron
cussing, cutting, drilling, bleeding
heat sinks are not child friendly
slow to build
difficult to breadboard(relatively)
can sound good

fets:

cheap
don't wear out
no expensive iron
can require expensive heat sinks(not comparable to transformers though)
heat sinks are child friendly
quick to build
easy to breadboard
can sound good(every bit as good as bottles)

I may have missed a few points.. I like both methods, they both have their merits....
but heat and speed of build?...I disagree Paul!

[Paul Barker]

When I’m bored I idly lurk the odd conversation about mofos.
As a bystander I’m thinking ? There seems to be a lot of doing nothing for years except talking about heat sinks.
My bottles bring their own heat sink, saves years on a build.

Not sure of the point, seems to me you may be frustrated at the slow progress of these mofo builders and you're just itching to see the finished product because you secretly are in love with the SS way of life.

You don’t know how much you lifted my situation with that humour. I genuinely laughed out loud when moments before I was almost crying no kidding.

As for the rest of you’re message it’s not a conversation that matters to me so I’ll not join the discussion. Which isn’t to say I don’t agree with you, I’m just not bothered, I’m glad to carry the torch for thermionic valves and I’m not bothered if nobody else takes that path.