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W6KWF > TECH 07.09.90 00:00l 40 Lines 1821 Bytes #999 (0) @ WW
BID : 85845_W1XSC
Subj: PK-88 LM7805 Voltage Regulator Replacement
Path: IW8PGT<IZ3LSV<IW0QNL<SR1BSZ<SR6DWH<OK1TOR<OK0NAG<F3KT<VA2RIZ<VK2RZ<
W6RAY<KI6QCY<N6RME<N0ARY<W4XSC<W1XSC
Sent: 260906/2030z @:W1XSC.#NCA.CA.USA.NOAM [Santa Clara Co] #:85847 $:85845_W1
Documenting my mods to the AEA PK-88 Terminal Node Controller
The sizable heat sink inside of the PK-88 is used to cool the linear
LM7805 voltage regulator used to step the +12VDC input down to the 5VDC
used by the Z80 and supporting digital electronics. This means that
more than half of the power used by the PK-88 is simply heat being
dissipated by the heat sink, which can get noticeably warm.
Thanks to it no longer being the 1980s, there are now drop-in replacement
switch mode power supplies in the same TO-220 form factor as an LM7805.
Since it is a switch mode power supply, it does not rely on resistance
to dissipate the unwanted portion of the input voltage and the conversion
efficiency is north of 90%, instead of sub-40%.
Good Amazon or eBay search terms are "LM7805 Buck replacement module"
Amazon link: https://amzn.to/4gPiVtQ
Most of the modules come with the header pin orientation expecting them
to be standing up-right, so for uses like on the PK-88 where the TO-220
package has been bent over to attach to a heat sink, I desolder the
header pins the module comes with and install new straight ones pointing
straight down out the back of the PCB. When installing the module on the
main board, I leave the headers long so the module isn't touching the
heat sink at all, since it is no longer needed to keep the voltage
regulator cool. I have done this modification on three PK-88 TNCs so far
and they run much cooler and consume half as much power.
Some of these switch mode regulator modules also come with different solder
pads for other voltages so you can use the same module for 3.3V, 5V, 9V, 12V,
or any variable voltage with an on-board potentiometer, so if you don't
absolutely need the small form factor, look at the larger, more flexible,
options as well.
Kenneth, W6KWF @ W1XSC
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