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Old 1st January 2002, 02:31 PM   #11
ih is offline ih
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I've started figuring out exactly what I'm going to buy so I can get an idea of costs, and of course, more questions have come up.

Should the 5A fuses for the amp boards be fast, or slow blow? I'm leaning towards fast, since even fast is (comparatively) slow, but Rod's article doesn't say either way (or at least, I haven't spotted where it says, yet).

I can imagine some pretty huge peak currents through there, though, so maybe slow is better - or is that what C+ and C- are there for? I just can't imagine 100nF making a significant regulation difference against a few amps at 35 volts, although I guess there is a small amount of resistance in the fuse, plus the wiring for the power supply which would likely be mounted away from the amp board... am I sort of on the right track here? As I mentioned, my electronics experience is mainly digital interfacing and switching, and these sort of questions generally don't come up much...

Is there any reason I should choose between M205 or 3AG mains fuses?

On that note, I spotted in my Jaycar catalogue a combined IEC socket, switch and fuseholder (M205), which I will most likely get. Can I reasonably expect this to have the internal wiring done, so it'll have switched active (fused), neutral and earth pins on the back? I'm talking about cat. no. PP-4003, for those who have the catalogue.

Transistors, again - now that I check the catalogues thoroughly, I've found that only SOT-96 (and TO-220 2955/3055 pairs are available (I thought I was going to use TO-3 ones, until I realised that there's no 2N2955 available). I've used TO-220's before, and given their size, I wouldn't like to try conducting a few dozen watts of heat away from them - are SOT-96's much better?

Would it be possible to get away without using mica washers, so long as my heatsink is anodised? It sounds like a bad idea to me, but I can't see any reason it wouldn't work, so long as I don't scratch the heatsink while installing the transistor (and the bolt is insulated). Is this practical to do, or just too unsafe to bother with?

Thanks for all your comments.

Ian
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Old 1st January 2002, 02:43 PM   #12
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Just a bit of advice.... if it turns out that you need fast acting fuses use the ceramic ones as they offer greater protection under fault conditions.

As for the heatsink ... i wouldnt try it...
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Old 1st January 2002, 03:06 PM   #13
Geoff is offline Geoff  United Kingdom
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Ian

Use slow-blow fuses for the supply rails and ac due to the high capacitor charging currents and toroidal transformer inrush current.

The TO3 complement to the 2N3055 is the MJ2955 from ONSemi. Do not use the TO220 altenatives (MJE3055/MJE2955 etc) as the thermal perfomance of these devices is significantly inferior. Stick to TO3 (or TO264 if you must have a flat-pack device). There are many suitable complementary pairs in the ONSemi product range.

Do not rely on the anodizing for electrical isolation. Whilst in theory this is possible, in practice it will only lead to disaster.

Geoff
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Old 1st January 2002, 03:09 PM   #14
paulb is offline paulb  Canada
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Yes, the fun is beginning..
You should use fast-blow fuses on the DC supply rails, and a slow-blow fuse on the AC power line. Those 3-in-1 power entry modules are "prewired".
The complement of the 2N3055 in a TO-3 is a MJ2955. I thought SOT-96 was a surface-mount package.
You still need mica insulators, even if the heat sink is anodized. As soon as you tighten the screws it will make contact.
You can probably parallel another set of output transistors if you want to cut power dissipation down. Rod's original project 3 shows how to do this (and it might be worth asking him about it if you decide to do it).
[edit: just noticed Geoff posted while I was writing this. Geoff, the DC fuses are on the amp, after the filter caps. I think they should be fast blow.]
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Old 1st January 2002, 03:30 PM   #15
Geoff is offline Geoff  United Kingdom
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PaulB

I suggested slow-blow for the supply rail fusing to ensure reliabilty and freedom from false failure due to ageing. A fast-blow fuse will not protect the output devices, a semiconductor will always fail first. The rail fuses are provided to prevent further damage from occurring once the output transistors have failed. This being the case, they can be slow-blow and rated with a generous safety margin to increase their life-span.

Geoff
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Old 1st January 2002, 03:42 PM   #16
paulb is offline paulb  Canada
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Hi, Geoff. I was thinking that fast-blow fuses may just save your speakers. I think you're right, the transistors would be long gone by this time.
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Old 1st January 2002, 06:34 PM   #17
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Both Jaycar and Dick Smith have good cases. I think the 2u size is about $A80 or so.
Phil
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Old 2nd January 2002, 12:56 AM   #18
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I considered the rack cases, but they're way too big for what I want.

Can I use 2N3055/MJ2955 together? I mainly ask because the data I've got shows that they're quite different, whereas the other pairs have identical figures.
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Old 2nd January 2002, 02:25 AM   #19
bawang is offline bawang  Malaysia
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Default Fuses ?

I used a slow-blow fuse for the mains input and fast blow fuses for the DC supply on the amp PCB. These fast-blow fuses nay not save the transistors but they may save the speakers.

I think this is a better arrangement given the fact that this amp (ESP Project 3A) has no output protection whatsoever, which may be it's greatest weakness (or strength), depending on which way you look at it.

Still, I've been running this amp 24 hrs a day (it's NEVER OFF) since it was constructed over a month ago. Given a big and properly ventilated heatsink (better than 1 degrees C/W) and a good transformer, it should never overheat under most conditions.
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Old 2nd January 2002, 02:54 AM   #20
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"Can I use 2N3055/MJ2955 together? I mainly ask because the data I've got shows that they're quite different, whereas the other pairs have identical figures." There are many different versions of a '3055', the best way to deal with this is not to use any of them.Instead use the MJ15015/15016 pair.For USD$1.42/$1.52 you get 180W of safe area up to 60V, not the 50W at 50V that a TIP3055/2955 offer.You can also use them on +/- 50V rails.Carver used them in the M400 and Crown used them in the Macrotech MA600.
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