> Date: Sat, 28 Mar 2009 22:54:52 -0500 > Subject: Re: [EE] Dangerous to interconnect PC/LAPTOP in microcontroller circuit? > From: x.solarwind.x@gmail.com > To: piclist@mit.edu > > On Sat, Mar 28, 2009 at 10:45 PM, Funny NYPD wrote: > > The PICkit 2 can only supply limited current to target chip/board (about 20~30mA). It can be used to program PIC16F and PIC18F chips without any issue. But it cannot afford most of the power-hungry chips/boards, eg. dsPIC30Fs. So an external power for target boards/dsPIC30Fs is highly recommended. > > Thanks for the reply, but I really don't see how this is related to my > question. The PICKIT2 can supply far more power than that and I know > that for sure. I hooked it up to my LED driver and powered up 3 5x7 > dot matrix displays without issues and program the device > (successfully) while the displays were on. My question is, is it safe > to connect both the laptop and desktop to my target boards? The > PICKIT2 will power the target while the MAX232 will communicate to the > desktop separately. The PICKIT2 documentation recommends the current drawn from the device be limited to 25mA or less. As the PICKit2 is configured as a low power device, the total combined current drawn from the USB port for the PICKit2 and its target must stay below 100mA. I haven't cracked one open but the limitation could be due to using a pin of the PIC18F2550 inside the PICKit2 to drive the +5V pin on the programming header. If you were drawing more, you could be pushing your luck and eventually damage the PICKit2. Best to use a small, regulated voltage output wall-wort supply, build a supply or best yet for prototyping, get your hands on a variable supply where you can also set a current limit. It will be safe to connect both your laptop and with desktop/RS-232 at the same time. This of course assumes you haven't done someting odd and tied the TX line to the +5V of the USB port. And this assumes that the chassis ground of your desktop PC isn't somehow tied to the AC hot lead. You would probably have already noticed such a situation before now. I seriously doubt you can do any harm to either of your PCs this way. Both have pretty stout power supplies of their own and the USB port in the laptop should have proper current limiting devices in it along with reverse power protection. If it doesn't, well it deserves to get blown up. Likewise, in your desktop, its supply is much stiffer than anything you can build on your little proto board save a dead short between supplies with a big lump of solder. And the serial port chips are tough little buggers. Having said all that, it is remotely possible to damage equipment while connecting them together while powered but I haven't seen a case of this happening without extenuating circumstances in many years. YMMV. Rob -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist