Byron, if you're building Bob's bipolar H-Bridge with TIP120 and
TIP125 power Darlingtons, the tab is connected to the collector in
both devices. You can easily use two heat sinks, one for each side of
the bridge since their collectors are connected. Just make sure the
sinks do not contact other circuitry. If you go the isolated route,
silicon thermal pads are generally preferred to the older mica/grease
method. I recently built myself an AMD Athalon 1.33GHz system the heat
sink is crucial to the survival of the chip. Mine is a large aluminum
assembly approved by AMD with a fan and silicon pad. AMD specifically
states to not use thermal grease.

   - Tom

At 10:31 28-08-01 -0400, Byron A Jeff wrote:
>On Tue, Aug 28, 2001 at 02:28:24PM +0100, John Walshe wrote:
>> Why don't you make use of all of that metal in the drive path?
>
>Because it's spinning. The motors are direct mounted to the wheels and
>are outboard of the platform.
>
>>Assuming the FETs/Transistors are isolated devices...
>
>Any way to make that assumption? In almost every case that tab on a TO220
>case is connected to something. I was thinking that each half of the HBridge
>could share a sink because the tabs are connected to the collectors and the
>collectors are shared.
>
>> the metal framework should provide a
>> reasonable heatsink, without too much trouble. If the above are not
isolated
>> devices then you will have to put a mica strip(or modern equivalent)
between
>> the device and the metal.
>
>Now that's an idea. Would anyone care to explain how mica gives good heat
>coupling while maintaining electrical isolation?
>
>Thanks for the info.
>
>BAJ


------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

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