On Mar 24, 3:26 pm, mattschinkel <[email protected]> wrote: > Let's say your power supply is giving out 4.7v and your power supply > VDD pin is giving out 5.2v. If you connect them together, there is a > difference of 0.5v so where does this current go? >
Into the local PSU/'chips via series resistor or worse via the "improve" reset performance diode. Bad, This is why the MCLR reset capacitor/resistor from local PSU, optionally with reverse discharge diode, should connect to MCLR pin via a diode to stop the +12V or +5V from VPP going to PSU. If programmer supplies VDD/ +5V to target, rather than consume it, then you need a 3rd blocking diode or jumper so that programmer PSU and local PSU do not "fight". BTW the 7805, 78x05 series Regulator will "die" if it has large discharged capacitors unpowered on input (normal mains PSU with no mains) and you apply +5V via ICSP's vdd. Note some programmers supply +5V on VPP when not programming. A 7805 regulator *might* only be 4.75V and all will work according to spec. A programmer built in PSU *might* be 5.25V and be in spec. Note not all Schottky diodes are suitable, at some current levels some are no better or even worse than a 1N4148/1N916 if designed for speed/ isolation rather than low voltage drop. Check data sheets or even measure volts drop at desired current. Typically you want 1N400x package type designed for low voltage drop on 3.3V and 5V SMPSU rather than some signal types. I have three blocking diodes... 1 x 1N4148 between MClr cpu pin and resistor/diode/capacitor (10k, 1N4148 reversed, 100nF) reset. CPU MCLR tied direct to ICSP 1 x 1N58xx something Schottky from USB mini-B socket to VDD of CPU. For gadget powered of USB/PC 1x 1N58xx something Schottky from main power regulator to VDD when gadget powered off external Radio (8V to 16V, 8xNiMH or nominal 12V Gel or car electric (10.5V flat, 14.5V charged)) There is Microchip schematic *SOMEWHERE* that shows such diodes! But it's pretty standard stuff. -- You received this message because you are subscribed to the Google Groups "jallib" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/jallib?hl=en.
