Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: smackoj on April 26, 2014, 06:42:37 am
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I am building a 100 watt SS power amp that uses two output chips. I am wondering if there is a rule about when to use mica wafer and when to use the white heat transfer grease. I have some grease but would have to find the wafers.
thanks, jack :icon_biggrin:
my output chips look like this one:
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Output devices (or those which must dissipate power) are generally bolted to some kind of heatsink.
If the surface which makes contact to the heatsink has a voltage present, you should use a mica insulator between the surface of the device and the heatsink.
Best heat transfer occurs when two mating surfaces have exactly the same shape, with pefectly smooth surfaces that make perfect contact at every point. If you zoom in enough, both the transistor and heatsink have little peaks and valleys in their surface, because they're not perfectly smooth. The two surfaces would touch at the peaks but not the valleys. Heat doesn't transfer efficiently where the 2 surfaces don't come in contact (the valleys).
Thermal grease fills the imperfect mating surfaces to fill valleys and improve heat transfer. You should pretty much always use thermal grease on any device mated to a heatsink.
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Or use a silicon pad instead of the mica and grease.
Mouser and digikey both sell them. They are no mess and work great.
Brad :icon_biggrin:
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I had not heard of the 'silicon' pad? I'll take a look at those before I install. I am planning on using some old metal heatsink with the chips.
thanks amigos. :icon_biggrin:
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> chips look like this one
Look "like"?
Look real close at that picture. I *think* the metal tab is fully plastic-encapsulated. That is becoming more common. It is not universal. Look close at yours.
If the tab is metal, use the insulator!! (There is an exception when the heatsink is insulated and enclosed, but that's very rare.)
A power device on a heatsink bigger than it is pretty much "needs" grease. (What HBP said, though I'd get indifferent for postage-stamp sinks.)
The "rubber" pads are a nice alternative to mica and goo. I think in COOKing work (military power supplies), the greased mica works a bit better. For nearly all normal work, the rubber is plenty good with a lot less mess and breakage.
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yes PRR; my goof, the power trannies have exposed metal backs and tops so I'm ordering some silicon/fiber wafers to mount with. I am planning to use these power chips: BD243C and BD244C. They are not very big but, according to the data sht., will dissipate 65 watts each. The BOM calls for a 2n3055 and MJ2955 as the power chips but the ones I found all had the old style metal case which don't mount with 3 pins to the PCB? I amp building this on a PCB I found on the web from a guy in Thailand (muy loco?). Here is the schemo for it. If anyone sees a flaw in my choice of the BD243/44 power chips, please let me know prior to installing. thanks :icon_biggrin:
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> muy loco?
Si, loco.
The output stage has NO protection against thermal runaway. Finals have no series emitter resistors nor base shunt resistors.
This was crude in 1971. Inexcusable today.
The days of making an audio POWER amplifier under 70 Watts out of separate parts are long-gone, and gladly. Make a necklace out of that PCB and buy a LM3886 kit off an asian eBay supplier. IMHO.
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Oh MG! well, I will chalk this up as some pcb soldering "practice" :BangHead:
I like the necklace idea but I'm leaning towards a more useful part such as a fishing lure for Minnesota Musky. LOL after I'm done crying.
thanks amigo.
:icon_biggrin: