Some information can be recovered from the old forum.

[John Lawton]

Hi Joe,

I do like your carrier system.

I design new products for clients and the design process is usually to create the complete schematic & pcb layout and then do most of the firmware development on the first issue pcb.

However as part of proving the schematic design before committing to the pcb, I like to bring up my chosen PIC and test out bits of the functionality that perhaps I'm not sure of first, such as testing out some of the peripherals used with a bit of test code.

So in my case I could usually use a basic breakout type board like yours that could plug into a breadboard to do this testing, or plug the PIC chip directly into the breadboard - which is more work.

Alternatively, if I needed more hardware support then I might use one of the Olimex boards.

As I've said before, I'm not an Arduino user but of course there are many of them and the plug-in shields can then be used with that design.

This all started when JohnB mentioned the lack of decent PIC24 dev boards and I realised I had some Amicus24 bare boards lying in a box. I'm currently building up one to try it out.

As I'm at a loose end work wise, (hint! hint!), I thought I would look at recreating a similar design as it is no longer available to purchase.

In conclusion it seems designers all have their own ideas on what would be a good dev board.

I'd be interested to know what users want, I'll start a new thread.

John

NEW TOPIC started: https://protoncompiler.com/index.php/topic,2104

[david]

Many years back now we used an Internal/External power switching circuit similar to that attached.   Because the internal power was 3x AA cells we had to provide reverse battery protection for the product and also over-voltage protection for the plug pack input because someone, somewhere would have a 12 volt plug pack that just happened to fit.  The circuit would normally be followed by a regulator.
The simple circuit shown should provide the power changeover and reverse battery protection but not overvoltage protection.  Of course a dual Schottky diode can also provide simple changeover switching but does have some voltage drop and no protection for reversed cells.

Cheers,
David

[John Lawton]

Thanks, interesting.

John

[top204]

That's the circuit I have used in a few projects and it works well, but one of the voltages does have a voltage drop because of the Shottky diode, but in the projects I have used it in, it has not mattered.

Because the external power has to have priority, so larger currents can be drawn from the external power in, as shown, the USB can go to the MOSFET, and the external voltage switch it off when applied, and because the external voltage will, generally, be larger than the 5 volts from the USB, the MOSFET will switch nicely. A voltage drop from the external voltage will not matter that much because it is going into a regulator anyway. I generally add a Shottky diode anyway from the external voltage input for polarity protection, so there is always a voltage drop.

I know USB is now capable of quite high currents, but I simply do not trust it with too much current in case it destroys the USB transievers in the PC. I have had this happen once, and it killed the whole motherboard because of too much current supplied by the USB, but it was a dreadful HP computer, so it wasn't a big surprise. :-)