I've always put a bit of heat sink compound on the rectifier unit before
mounting it to the bottom plate. I've found several power supplies
where the bridge rectifier assembly failed and upon removal, there was
no heat sink compound. Add a new bridge assembly and some heat sink
compound resolved the issues. Never had one come back.
73
Bob, K4TAX
On 4/24/2020 10:50 PM, Martin Sole wrote:
I can add perhaps another data point on the rectifier matter. Years
back my employer, one of the largest ground to air radio manufacturers
was having failures with their mainstay transmitter, a 50w carrier AM
unit. It had a large toroidal transformer and a packaged rectifier.
The rectifier was bolted to the steel sub chassis and all was fine in
regular intermittent service but in ATIS service the transmitter runs
essentially continuous, so about 160-180 PEP, and rectifiers were
failing.
Investigation showed that the steel chassis was not 100% flat and the
contact surface of the rectifier was compromised. The fix was to
insert a (really) flat aluminium bar under the rectifier and bolt that
to the chassis. The bar did much of the heat sinking and had many more
contact patches with the steel sub chassis. Even paint ridges can be a
source of reduced contact leading to overheating of the rectifier
package and those things really do need to shift some heat. A good way
to think of it is like you would a PA transistor in a 100 watt
amplifier, we appreciate how well they need to be bolted to a heat
sink and your rectifier package wants very similar assessment.
It's not so much the current that causes failure rather the heat is
not being dissipated properly from the smaller package.
Martin, HS0ZED
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