I had a skinny trace fuse on a CRK-10A QRP transceiver which did burn through, but not before it fried virtually everything else on the board. I wouldn't trust a skinny trace fuse in a circuit at all...period. That's sad, too, because it's a really nice rig with an outstanding receiver.
Jack, W8TEE
On Tuesday, May 22, 2018, 4:28:17 PM EDT, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:
The relay trick is good, especially if running a kW. At 10W, semiconductors are fine.
At a minimum, I'd just make it series schottky diode into the main uBitx rail with a 0.5A fuse there. I have a lot more faith in the $0.10 diode than I would in a $1 ebay relay. Still have a good spot for that idiot light.
PA-PWR fused separately at 3A, with beefy enough traces and coils that the fuse blows first when the IRF510's intrinsic diodes see reverse voltage. ? If fuses are deemed too expensive, perhaps just a skinny trace on the PCB where power enters and goes to the series schottky.? Have a big feedthrough on each end to mount a replacement fuse. The 3A PA-PWR fuse could be a thicker trace, or perhaps just a skinny wire to the power connector.
A fuse on the ground lead is not something I've ever worried about, but you give a good argument.? Again, maybe just a skinny wire (such as the 26 gauge hookup wire provided with the uBitx) from power connector ground lug? to chassis and PCB grounds is a sufficient fuse there.
On Tue, May 22, 2018 at 12:58 pm, ajparent1/KB1GMX wrote:
My prefered way uses a diode and relay.? Relay has zero voltage drop. IF the diode does not conduct, ? ? then power to the relay does not happen and power to the board never happens. When the diode/relay combo I add an "idiot led" bicolor before the relay depending on polarity red not good, green good, none means fuse or no power.?? Why go that far? Never loan gear, and bad stuff happens in a hurry.
But I still include a fuses (each lead) before the reverse polarity diode.? Why? If the negative side is case and I hook the positive to the case we get sparks and fried wires in most mobile environments.? Lets protect the source.
