On Mar 27, 7:43 am, vasile surducan <[email protected]> wrote: > On Thu, Mar 25, 2010 at 2:58 AM, m...@watty <[email protected]> wrote: > > copy & paste in Fixed Font, i.e. courier, > > D1 > > > PSU ---|>|--- pic VDD and ICSP VDD > > > D2 D3 > > > ------|<|---------|>|------- pic MCLR/Vpp and ICSP MCLR/Vpp > > psu _ _| | > > |__/\/\/\/\__| > > 10k | > > === 100nF > > | > > OV/GND > > > D1 is 100mV to 150mV drop Schottky > > D2,1N4184 or 1N916 etc, D3 can be either, though Microchip > > recommends Schottky > > > Not all Schottky are suitable. Check voltage drop vs current at 30mA > > Very nice drawing, and huge comment Mike, BUT both diodes aren't necessary. > When you'll have a little spare time take off D2 and replace D3 with a short > circuit and see what's happening. I have about 30 projects (some of them in > relatively mass production) working with different types of programmers in > the suggested way. > BTW, the schottky voltage drop is a matter of current flow, so you can't > define a schottky like this... :) > > friendly > Vasile Generally what you say is true. But sub 3ms power glitches are possible (maybe more easy on the Ice Cream Making machine I did electronics for!), hence D2. If no D2, then D3 isn't needed to protect PSU/Stop VDD shorting, but is still recommended by Microchip to avoid the reset C slowing the VPP rise time. My programmer has a large capactor pumped by ICL7660 and VDD switched from it to the MCLR pin, so mine would not be affect. Most Programmers would be OK.
If you look at Schottky Data sheets, some of the RF types in 1N4148 type package might have 300mV to 500mV drop at the *SAME* current as 1N58xx series Schottky (in a 1N400x type package) intended for very low loss SMPSU applications. Yes, the Schottky forward voltage varies with forward current as does regular Silicon Diode (up to 1V drop at peak currents is possible on some diodes) and not all are the same drop at same current, as they are for different applications. So it's wrong just to say "put in a Schottky diode" as at 40mA some are exactly same as common 1N4148 for voltage drop as they are maybe processed for high switch speed, or low capacitance or small PIN action or some other feature rather than low voltage drop. BTW, as example of how diodes vary, compare 1N4001 and 1N4007 (both similar, only differ in voltage rating). Due to higher isolation, bigger "gap", if you vary forward voltage of 1N4007 and 1N4001 (by current via series resistor) you will find the 1N4001 will rectify 4V RMS 1MHz, but the 1N4007 will vary the level. The 1N4001 is "just" a rectifier, but the 1N4007 due to different "doping" to get high voltage rating, works quite well as a PIN attenuator. Similarly a 1N4148 may work better as an RF switch 1MHz to 30MHz than a PIN diode or some Schottky diodes. PIN diodes work better as attenuators than on/off switches and have a minimum RF frequency they will do such. So the "100mV to 150mV drop Schottky" needs defined for a particular current, as I mentioned on the schematic I edited in Photo Editor and uploaded here: http://jallib.googlegroups.com/web/icsp_full_circuit.png -- 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.
