RockMite Reverse-Polarity Protection Mod ------------------------------------------------------------------------ A couple nights ago while experimenting with various battery supplies for the RockMite transceiver I accidentally connected a 12v battery to the rig in reverse. This was not a good thing, but not as bad as I'd thought. Within a second the 10 ohm resistor on the op-amp Vcc started smoking and I pulled the power. Nothing was permanently damaged except the PA transistor. Looking closer at the design I see that the SA612 and microcontroller ICs have zener diodes and series 1k resistors on their power lines - built-in reverse polarity protection. But the op-amp and transistor stages are not protected. Putting a diode in series with the incoming V+ to the board would be the easy way out, but you lose about 0.6 volts in the diode, and I didn't want that to happen if I'm running a battery that's getting low. Instead, I cut the trace between the V+ pad and the op-amp and transistor stages and soldered a single strand of hookup-wire across the gap as a fuse. I then added a reversed 1N4148 to ground from the next pad on this trace and ground. Now if reverse-voltage is applied the zeners and series R protects the mixer and controller, and the reverse diode looks like a short on the other line and blows the "fuse" to protect the op-amp and transistors. Steve WD8DAS sbjohnston@aol.com --- StripMime Report -- processed MIME parts --- multipart/alternative text/plain (text body -- kept) text/html The reason this message is shown is because the post was in HTML or had an attachment. Attachments are not allowed. To learn how to post in Plain-Text go to: http://www.expita.com/nomime.html --- ------------------------------------------------------------------------ * Previous message: [NERT] Bad right angle UHF adapters <001254.html> * Next message: [NERT] RockMite protection mod <001256.html> * Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]