Roman, Microchip have an application note (AN251? or AN521?) that talks about using just an R to high-voltage. In that note that state the diodes have a 500uA rating, which is presumably a continuous rating. Bob Ammerman RAm Systems (contract development of high performance, high function, low-level software) ----- Original Message ----- From: Roman Black To: Sent: Saturday, February 10, 2001 10:39 PM Subject: Re: [PIC]: Input clamp current (was Schmitt Trigger (RB0) > Olin Lathrop wrote: > > > guaranteed limit within which the current won't be exceeded. > > > > If this is indeed true (maybe it isn't), then I don't see the problem with > > connecting a PIC pin to 170V via 5Mohm. That's only 34uA. The voltage at > > the other end of the resistor is irrelevant (as long as the resistor can > > handle it) because that voltage never shows up at the PIC. > > > > I don't see Roman's point about not wanting to rely on the protection > > diodes, as long as the relevant parameters are specified and guaranteed by > > Microchip. > > > The problem is more real-world oriented than theory oriented, > like diodes and resistors have max voltage specs. Exposing a > small physical size resistor to a large voltage does cause > failures, there is significant current leakage, (can be well > over the current expected from it's value) there is > breakdown of the resistive material (especially carbon) and > even the paint breaks down. I replace these every day in > TVs and Monitors that use resistors exposed to more than > 100v. Small resistors have a significant capacitive > reactance, you may be able to ignore this at 20v but not > on fast slewing spikes above 100v. > > Theoretically the clamp diode is forward biased, so the > voltage on its anode won't get more than 0.7v over the > PIC 5v rail. But I cannot believe this part is a diode > rated for 170v, and as a designer I don't use a low > voltage diode for 170v, forward biased or not. Think > of where these diodes are and what they are attached to. > You would be connecting 170v (+ spikes) directly to > the silicon chip inside the PIC. What voltage isolation > is that silicon chip rated for?? I'll tell you, 5v. > > The worst thing is that the 170v is not smoothed in any > way, mains spikes (which are often in the kV range) will > go straight through the bridge and the PIC will be > exposed to these scary peaks with only a leaky overspecced > 5mohm resistor to protect it? Anyone think this is > smart design? :o) > > A zener at this point will give absolute clamp especially > with a capacitor across it to help decouple the spike > energy. That system will take that punishment forever > assuming the zener is run at low average disspation. > If the zener fails they always go short, giving good > protection to the PIC. In TV's there are many sections > where a higher voltage needs to be sensed by a chip, > this solution is reliable and cheap. > > The 20mA absolute max spec for clamp diodes is typical > of modern logic chips. These are safety devices to > protect the input logic from infrequent FAULTS. They > are not a design spec to be run at 10mA continuous. > Or any other continuous "fault" current. Would you > connect the input of a 74xx series gate to 170v via > a resistor?? > > The spec is a 5v device, DON'T exceed 5v on any pin. > Using a last resort safety component for continuous > out of spec operation gets my goat. > -Roman > > PS. Olin, do Microchip provide a continuous rating > for the clamp diodes?? Or just an absolute "this is > the worst abuse before it blows" current figure?? ;o) > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu