On Sat, 17 Dec 2011 01:27:27 -0500, you wrote: >Hmmm - beware that the diode may well have an effective internal >resistance of more than 0.05 ohm, in which case, the majority of the >current will flow through your shunt even when the diode is present >during the fault condition. I don't think that there is any >alternative for protection here other than to make your overcurrent >detection faster or add enough mass so that the temperature will not >rise too much before overcurrent is detected and shut-down. By the way >- how are you shutting-down this current when there is a fault? > >Schottky diodes are fundamentally different than normal PN junction >diodes. They can have significant leakage current - I know that in the >reverse-bias condition, at high temperature, they can pass 10s or 100s >of microAmps. I would expect even worse in the slightly forward-biased >condition. These current levels, though, do not seem relevant when >your sense resistor is only 0.05 ohm. > >I searched for "Schottky diode iv curve" and it seems that they follow >the same exponential diode equation as PN junction diodes, although >with different values for the constants. This means that the forward >current is roughly exponentially-dependent on the forward voltage, so >that a given incremental increase in the forward voltage causes a >multiplicative increase in the forward current. Let's say, for >example, that a particular Schottky diode has a forward drop of 0.2V >at 1mA. Then, at 0.15V it might be 30 microAmps, and then at 0.1V it >might be 1 microAmp (a factor of 30 for every 0.05V change in the >voltage). This relationship no longer holds true, though, once the >current is around the "saturation current", which is highly >temperature dependent. > >Sean > > >On Fri, Dec 16, 2011 at 10:45 PM, Forrest Christian w= rote: >> So I'm looking for ideas, and information. >> >> I have 0.05 ohm shunt resistor which I need to protect from overcurrent >> (effectively overvoltage). =A0 It's measuring steady-state current, and >> also being used for overcurrent trip purposes. =A0In certain >> very-high-current 'dead short' situations, this poor component becomes >> less of a resistor and more of a fuse - I'd like to prevent that, >> bypassing some of the current long enough for the trip to occur - a few >> ms at most. >> >> The obvious cheap solution here is to do some sort of tvss-like device >> around the unit to bypass some of the current once the voltage across >> the resistor rises to a certain point. =A0 And looking at the curves of >> the B1100 Schottky diode which is a standard part around here, it looks >> like I might be able to just put one forward-biased across the resistor, >> and use it in sort of tvss-mode... >> >> My mental picture of a diode is that below Vf there is minimal if any >> current conducted. =A0Is this an accurate mental picture? =A0 What curre= nt >> will flow if you 'bias' a schottky diode at say 0.2V? =A0I'm not worried >> about capacitance more than what I'd call 'leakage current' (in the >> forward direction though). >> >> Is there something else I should be using instead? >> >> -forrest Schottkys get very leaky at higher temperatures, so probably not a good cho= ice. Is the current really so high that you can't use a more chunky sensing resi= stor? If you really want to bypass the shunt, a MOSFET is probably a better bet -= arranged such that it turns on quickly when the measured current exceeds a threshold, witha monos= table to hold it on for a while, limiting the current duty cycle through the resistor to the monostab= le time / the turn-on time. =20 --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .