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Old 12th May 2011, 01:45 AM   #1001
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Can I just check, I have a new Transformer I want to use for convenience (has other useful voltages for my pre-amp remote volume and led volume resistors. It's 2x15VAC but on UK system runs to 18.3VAC.

With 18.3VAC hotrodded DCB1 running at 9.14 DC, how big should my MOSFET sinks be? Will normal small cast sinks do the job? Like these I have on at the moment:
AAVID THERMALLOY|ML524-30-B|HEAT SINK BIG CHIEF TO247/218 | Farnell United Kingdom

Many thanks
Lucas
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Old 12th May 2011, 10:00 AM   #1002
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Nobody?
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Old 12th May 2011, 01:10 PM   #1003
Salas is online now Salas  Greece
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You should give the running current also for an estimation.
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Old 12th May 2011, 01:19 PM   #1004
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Hi Salas,

Well, it's those standard Hotrod 5W 10R Dale resistors. Is that what you mean, or do you want me to take a measurement from somewhere?

I'm guessing it will be fairly hot with 18vAC transformer, right.

My last one worked great - it was 14vAC, but this new 18vAC one has some other useful secondaries for this amp, and is R-core so low EMI (It's a small box).

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Old 12th May 2011, 01:24 PM   #1005
Salas is online now Salas  Greece
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At ~ 200mA it will create 3W per CCS MOSFET if it will reach 24VDC on the filter caps. Those sinks will go +19C over in box ambient.
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Old 12th May 2011, 01:31 PM   #1006
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Hmmm - they're 35v caps, so just about within spec.

Bottom line is "no, those sinks are in no way big enough." Right?

19C over ambient could mean 49C in mid-summer highs. That's not good for those mosfets is it?
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Old 12th May 2011, 01:48 PM   #1007
Salas is online now Salas  Greece
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They could hold in England not in Greece. But enclosed, I doubt you would have 30C ambient. It accumulates from radiated. If it hits 55C+ on sink in the end is unhealthy even if those MOSFETS are TO-247 proven dogs, which they are.
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Old 12th May 2011, 02:02 PM   #1008
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OK, so I'll put some vents on the lid above the fets and try not to worry about it too much. Worst case scenario is a burnt out chip, right. OK, maybe the house burning down is worse than that, but let's not go there...

Thanks once again Salas.
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Old 12th May 2011, 02:07 PM   #1009
Salas is online now Salas  Greece
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Get a cheap IR laser spot thermometer gun from Ebay. Very useful for all stuff hot or cold. So you will know in your build.
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Old 12th May 2011, 02:46 PM   #1010
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I never knew about them. Maybe I'll get one. Thanks
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